Exploring the Large Hadron Collider - the Detectors: The World Machine Clearly Explained (essentials) 3658332921, 9783658332921

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Michael Hauschild

Exploring the Large Hadron Collider–the Detectors The World Machine Clearly Explained

essentials

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Michael Hauschild

Exploring the Large Hadron Collider—The Detectors The World Machine Clearly Explained

Michael Hauschild Geneva, Switzerland

ISSN 2197-6708 ISSN 2197-6716  (electronic) essentials ISSN 2731-3107 ISSN 2731-3115  (electronic) Springer essentials ISBN 978-3-658-33292-1 ISBN 978-3-658-33293-8  (eBook) https://doi.org/10.1007/978-3-658-33293-8 This book is a translation of the original German edition „Neustart des LHC: die Detektoren“ by Hauschild, Michael, published by Springer Fachmedien Wiesbaden GmbH in 2018. The translation was done with the help of artificial intelligence (machine translation by the service DeepL. com). A subsequent human revision was done primarily in terms of content, so that the book will read stylistically differently from a conventional translation. Springer Nature works continuously to further the development of tools for the production of books and on the related technologies to support the author. © Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2022 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Responsible Editor: Lisa Edelhaeuser This Springer imprint is published by the registered company Springer Fachmedien Wiesbaden GmbH part of Springer Nature. The registered company address is: Abraham-Lincoln-Str. 46, 65189 Wiesbaden, Germany

What You Can Find in This essential

• Here we go!—The bumpy start of the LHC • People and particles!—Large collaborations and large detectors • What is the particle doing inside the detector?—How to see particles

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Preface

The world machine, the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research near Geneva, is the largest particle accelerator in the world. The first ideas and concepts for the LHC were already made in the early 1980s. From these beginnings, however, it took more than a quarter of a century until the LHC was finally completed, a ring-shaped particle accelerator with a circumference of 27 km, 100 m below ground. When particle beams circulated in the LHC for the first time on September 10, 2008, the excitement among scientists was boundless. The launch of the LHC with live transmission from the LHC control room was in the top news media worldwide. The physicists were in each other’s arms. Only a few days later, on September 19, 2008, came the great disillusionment. It happened during a test: one of over 10,000 cable connections could not withstand the stress of the high electric current and melted. No one was hurt, but the LHC was massively damaged and it took more than a year until it was finally possible to resume operations in November 2009. In the accident investigations, the cable connections turned out to be a potential weak point. It would have taken far more than a year to check and repair or even renew all connections. CERN’s management therefore decided to operate the LHC at half power for the time being in order not to put too much stress on the connections. But even half the energy was enough to announce the discovery of a new elementary particle on July 4, 2012 using the two large particle detectors ATLAS and CMS. And the LHC continued to run. In March 2013, the physicists from ATLAS and CMS were finally certain that the newly discovered particle was indeed the long-sought Higgs particle.

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More than 50 years ago, in 1964, theoretical physicists Robert Brout, François Englert, and Peter Higgs, among others, published ideas on how elementary particles can obtain mass, that is, become heavy. One consequence of their theories is the existence of a new particle, the Higgs particle, named after Peter Higgs. For a long time, this particle was searched for at various particle accelerators and detectors around the world, until the physicists finally found it at the LHC. Brout had already died in 2011 and could not live to see the triumph, but Englert and Higgs were awarded the Nobel Prize in Physics in autumn 2013, with great jubilation and sympathy from the physicists involved at CERN. But this is not the end of research at the LHC, it is only the beginning. The newly discovered Higgs particle must be measured, its properties determined and compared with the theoretical predictions. More new particles may just be waiting to be found in the next few years, and every newly discovered particle could trigger a revolution in the understanding of our world and the universe. In 2013 and 2014, the LHC and the particle detectors have, therefore, been made fit for the new challenges. The 2-year break was used to eliminate all weak points in the cable connections, to install new security systems and to improve the detectors in order to unravel even more of nature’s secrets with now higher energy. As more than 6 years earlier, the first circulating particle beams were eagerly awaited in March 2015 when the LHC was put back into operation. Two months later, on June 3, 2015, the first collisions took place with almost twice the energy as before: 13 TeV, comparable to the energy of two colliding mosquitoes, but highly concentrated on two tiny particles, and once again a new world record. The world machine is running again! In the years to come, particle physicists will look even more intensively than before into their collected data from countless collisions to see if there are any indications of new particles and new phenomena beyond the so-called Standard Model. This essential is part of a series on the LHC’s relaunch in spring 2015, in which you will follow the bumpy launch of the LHC in 2008, get to know the experiments at the LHC, and understand how particle detectors work. In other essentials of this series you will learn more about the beginnings of CERN, one of the most fascinating research centers ever, its history, its people, and its accelerators. You will learn how particle accelerators work and how the first ideas were used to build the LHC, the world machine of today. You will be

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on the spot when a new particle was finally discovered, the long-sought Higgs boson, and learn about future projects. And you will learn more about the theory behind the Higgs and the Standard Model, and the theoretical approaches beyond the Standard Model. Michael Hauschild CERN, Geneva Switzerland

Contents

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The Start. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 The Very First Beam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 The Accident . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 The LHC Inauguration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4 The Higgs Plan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2 The Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1 The Marriage Market. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.2 The Decision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.3 Collaborations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3 Particle Detectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.1 Can Particles Be Seen?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.2 Momentum and Energy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.3 The Onion-skin Principle. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.4 Cathedrals of Science. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4 Outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

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The Start

Summary The year 2008 was supposed to be the year of the world machine. Almost 25 years had passed since the basic concept of a large proton-proton collider was developed at a 1-week workshop in Lausanne in March 1984. It was one thing to build the LHC, test the magnets, install them in the tunnel and connect them to form a ring. But now the LHC also had to be put into operation, an extremely complex system in which the most diverse elements had to interact to finally form a functioning accelerator. In addition to the magnets, these are large refrigeration plants that have to cool the magnets to a temperature of 1.9 K, distributed over a length of 27 km. The vacuum system must provide a sufficiently good vacuum in the 10−10 mbar range, ten times better than on the moon. The accelerating sections need high-frequency to accelerate the particles, the position of the particle beam within the vacuum tube must be monitored by a system of hundreds of beam monitors, and finally, all data must be fed into a software system that monitors and controls the LHC and all its subsystems. To this end, a new, large control room was built on the French site of CERN in Prevessin, which brought together the control rooms of the pre-accelerators previously scattered throughout CERN. When the first accelerators were built at CERN, there was no control and management by computers and software and the control rooms were full of walls with switches and panel meters, which had to be very close to the accelerator because of the short cable lengths required. Meanwhile, as with other large-scale facilities, control is carried out exclusively by software and the control room can be located practically anywhere as long as there is a reliable and fast data connection. The new CERN Control Center was inaugurated in 2006 in time for the LHC start and consists of four clo-

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2022 M. Hauschild, Exploring the Large Hadron Collider—The Detectors, essentials, https://doi.org/10.1007/978-3-658-33293-8_1

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verleaf-shaped areas, each of which responsible for one part of the CERN accelerator complex, as well as for the technical infrastructure such as the power and cooling water supply. The LHC control area is one of the four cloverleafs of the CERN Control Center and should soon become the focus of attention of both the people at CERN and the world public.

1.1 The Very First Beam To enable particle beams to circulate in the LHC, two new tunnels, each about 2.7 km long, had to be built beforehand from the last pre-accelerator, the super proton synchrotron (SPS) from the 1970s, all the way to the LHC. Since the LHC is located deeper than the SPS, it was also necessary to build both tunnels 70 m deep. The more than 700 normal-conducting magnets for the beam transport were built by the Budker Institute in Novosibirsk in Siberia and corresponded to a large part of the contributions in kind that Russia had promised to make to the LHC. The first of the two transfer lines was completed well before 2008 and was successfully tested in autumn 2004. For the first time, protons reached the LHC tunnel, even though they were stopped there immediately. The second transfer line was tested 3 years later in autumn 2007. The first half of 2008 was then characterized by more and more extensive testing of all systems. There was no prototype of the LHC on which valuable experience could have been gained. The LHC was its own prototype. In mid-2007, the start was expected to take place in May 2008, but it became increasingly clear that such a complex system required more preparation time, and the possible start date shifted further toward summer and beyond, to the displeasure of the then CERN Director General Robert Aymar, whose term of office would expire at the end of 2008. At the end of July, all eight sectors of the LHC were at operating temperature of 1.9 K and only a few necessary tests were still pending. At the beginning of August, the start of the LHC was finally set for September 10, 2008. They wanted to dare to switch on the world machine and invited the world to be there live. While until then it was mainly rocket launches or the moon landings that were broadcast worldwide as milestones of science and technology, it was now the start of a particle accelerator, an absolute premiere and a challenge for the media and the press office at CERN. In contrast to a rocket, which catapults itself into the depths of space with a great roar, visible to everyone in the sky, invisible particles were to orbit the LHC for the first time, made visible only by the electronic displays on the monitors.

1.1  The Very First Beam

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On September 10, 2008, more than 300 journalists gathered in and around the Globe of Science and Innovation, which has become CERN’s landmark since its 50th anniversary in 2004, and a dedicated satellite link sent images from the control room to Eurovision throughout the day, which were then forwarded to 450 television stations, complemented by a live webcast for everyone via additionally rented servers. All four surviving predecessors of the acting Director General, during whose terms of office the LHC took shape for more than 20 years, were in the control room and followed the events of the next few hours with great interest (see Fig. 1.1). At 8 a.m. the pre-accelerators were ready to send the first protons to the LHC. One had to be careful, only a single particle bunch of a few billion protons was to be injected into the LHC. This quantity was just enough for the beam control instruments along the ring, but small enough to prevent damage if the beam got out of control, which was always to be expected with a machine that was being put into operation for the first time. Under no circumstances should more than 3 billion protons be injected during the first tests. If more than 3.8 billion protons were to be injected and lost into one of the superconducting magnets in an uncon-

Fig. 1.1   Media rush at the CERN Control Center (CCC) on September 10, 2008. (© 2008 CERN)

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trolled manner, it had been calculated that a quench would occur, a spontaneous transition from the superconducting to the normal state. The magnet would not suffer any immediate damage, but would be heated up to such an extent that it would take hours to cool it down again and have the LHC ready for operation again. Any stress on the magnets and associated delays in the schedule should be avoided. Later on, however, under controlled conditions, almost 2 × 3000 bunches of particles with more than 100 billion protons each in both directions, that is, a 6.5 × 1014 total of more than 200,000 times as many protons, would be circulating in the LHC. The live broadcast from the control room began at 9 a.m. with the welcoming address and an introduction by the commentator, an animation about the LHC and short scenes from the meeting of the responsible accelerator physicists that had just taken place to discuss the planned schedule. This was followed shortly afterward by statements from the LHC project leader, the Welsh Lyn Evans, and from Director General Robert Aymar, for whom the commissioning of the LHC was to be the crowning achievement of his term of office, which would end in a few months’ time. Then it was the turn of experts. At 9.35 a.m. the metal block at the end of the transfer tunnel from the SPS to the LHC was removed and the protons were able to make almost a quarter turn in the LHC until they hit a retracted fluorescent screen and were made visible like the electron beam in a picture tube. There were no surprises to be feared on these first few kilometers, as a first test with protons over this distance had already been carried out 4 weeks earlier. The position of the beam deviated by only 3 mm from the target position after 4 km. An almost perfect machine! After optimizing the beam position with the help of the countless small correction magnets along the ring, it went into unknown territory for the first time. Step by step, the protons were guided further through the ring, monitored by more than 500 beam position monitors, which record the signal of the passing beam via four electrodes. The beam continued to travel, as could be seen on the retracted light screens at the end of each section, and after less than an hour it was done! To the enthusiastic cheers of all those present in the LHC control room and in the control rooms of the four LHC detectors, where the particle physicists observed the passage of the beam through their detectors, the protons had completed a first orbit around the LHC at 10.26 a.m. In the afternoon of the same day, the second beam also managed to circle the LHC in less than an hour, and it did so several times. In the evening, the champagne bottles lined up in the control room, because it was a long-standing tradition at CERN to share and celebrate important breakthroughs or new records with colleagues.

1.2  The Accident

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The start of the LHC was in the top news worldwide, just a few days before the collapse of the major US bank Lehman Brothers on September 15, 2008, which put the global financial crisis in the spotlight for a long time. The Google homepage showed the LHC, 2,500 TV reports were seen by hundreds of millions of viewers, 6,000 press articles were written, often as headline on the first page, and taxi drivers discussed the start of the LHC with their passengers. CERN was the new NASA!

1.2 The Accident The euphoria about the successful start continued. Within just a few days after the first orbit, the protons circulated in the LHC for many hours. A success that even the most experienced accelerator physicists didn’t dare to dream of. The next step was to see whether the protons could also be accelerated to high energy. Now it came down to the magnets that had to hold the high current for this. Every single magnet had been thoroughly tested before installation, but now it was up to the interplay of all the elements of the LHC. Even before the first beam was injected, the magnets had already been successfully tested in seven of eight sectors for an energy of 5.5 TeV. Only sector 3–4 was still missing and this test was to be repeated on the morning of September 19, 2008. For this purpose, the current was slowly increased in small steps. After a little more than 15 min the nominal value of 9,300 A corresponding to 5.5 TeV should have been reached when something completely unexpected happened: Around 11.18 a.m., shortly before the end of the test at a current of 8,700 A, the displays went crazy and within only a few seconds it triggered countless alarm signals which promised nothing good. Everyone involved realized that something dramatic must have happened, a malfunction that nobody had expected! What was worrying was the fact that the oxygen content in the affected tunnel sector dropped rapidly within a very short time and remained practically at zero for several hours. This could only mean that an enormous amount of liquid helium, which was supposed to remain in a closed circuit to cool the magnets, had evaporated and completely displaced the normal air. The LHC tunnel had become an area hostile to life, but harmed no one in it because during every operation of the LHC, whether in magnet tests without a beam or in regular beam operation, no people are allowed to be in the tunnel. When, after several hours, a small squad of CERN’s own fire brigade equipped with oxygen supplies carefully entered the tunnel, the picture was frightening: the

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power was off, cold fog was in the air, and ice had formed in many places on the magnets, some of which were torn from their anchorages. It was only a few days later, when the safety conditions in the tunnel allowed it, a team of experts was able to inspect the damage more closely and was getting an overview. The devastation extended over a distance of 700 m and the complex connections between the magnets were destroyed in many places (see Fig. 1.2). It was not possible to think of resuming operations for the next few months, but initially only a few people knew this. On the very same day, rumors of a serious accident involving massive damage to the LHC were circulating at CERN, and the media, which had reported on the successful start of the LHC just a short time before, also took up the subject with some gratitude. A first press release from CERN the following day did not give any indication that the investigation of the causes and repair would take more than a whole year. In fact, the remaining term of office of Director General Robert Aymar until the end of 2008 was a period of decreed silence, because in contrast to CERN’s otherwise open atmosphere, where everyone felt like members of a big family, none of the people directly involved in the investigations at the LHC were allowed to report their work and results to the outside world, and this also affected colleagues in the experiments, who were left in the dark. A strange, completely atypical atmosphere spread at CERN. In the weeks that followed the LHC accident, initially referred to by CERN management as an “incident”, was the dominant topic in every coffee round. What might have happened, or who knew about it, why did the Director General hold back conspicuously and when would the LHC continue to proceed? It took 4 weeks before a first interim report was available from the investigation commission set up shortly after the accident, which consisted of both CERN members and external experts, such as from the Fermi National Accelerator Laboratory (Fermilab) in the USA, near Chicago, IL. According to this, the suspected cause was a faulty cable connection between two LHC magnets, which had not withstood the high current load in the final phase of the test and triggered a chain of fatal events. In contrast to the superconducting cables in the magnets, which show no electrical resistance when cooled appropriately, the cable connections are not superconducting. This is due to the nature of the cables, whose wires consist only in the core of countless fine, only 7 μm thin filaments of a superconducting niobium-titanium alloy, but which are surrounded by far more normal state conducting copper. The copper not only serves to mechanically stabilize the filaments, but also has the task of absorbing

1.2  The Accident

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Fig. 1.2   Damage to the LHC magnets after the accident on September 19, 2008. (© 2008– 2015 CERN)

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the high discharge currents that occur briefly during a quench.1 This is because, although a superconductor is a perfect conductor in the superconducting state, it is a poor normal state conductor, which would melt in a very short time in this case. In addition, two cables are connected by ordinary normal state conducting solder, so that a very small but nevertheless present electrical resistance causes low heating during operation. Normally, the resistance of the connection is only a tiny 0.2 n, a million times lower than in power cords used to connect electrical devices in every household. However, at a current of up to 11,850 A, even this tiny resistor causes a power loss of 0.03 W, corresponding to the typical power of a light-emitting diode used as a standby indicator in many televisions and other multimedia devices. This small amount of heat is normally dissipated without any problems by the liquid helium that circulates around the power connection. However, as was shown after evaluation of all previous tests and in laboratory experiments, the faulty connection must have had a 1,000-fold higher resistance in the range of 200 n. This could only be explained by the almost complete absence of solder, which should actually have been found by the subsequent quality control. As is so often the case in serious accidents, two or more serious faults had to come together to be fatal.

1.3 The LHC Inauguration Nevertheless, a few days after the publication of the first interim report on the cause of the accident, the long-planned official LHC inauguration took place on October 21, 2008, to which the heads of state or government of the CERN member countries were invited in advance. Almost 20 years earlier, in autumn 1989, CERN held its last major celebration to mark the start of an accelerator, the Large Electron Positron Collider (LEP), for which today’s LHC tunnel was initially built and which was in operation until the end of 2000 before it had to make way for the LHC. At that time, the Swedish King Carl XVI Gustaf and the French President François Mitterrand as well as the President of the Swiss Confederation were the highest-ranking guests who, together with other representatives of the

1 See

also in the essential “Exploring the Large Hadron Collider - CERN and the accelerators”.

1.3  The LHC Inauguration

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respective countries and a further 1,500 guests, inaugurated CERN’s then flagship to specially composed music. As in those days, a similar number of participants followed the inauguration of the new LHC collider, although the Swedish king and the French president stayed away this time. In contrast to the LEP inauguration of an accelerator that was already running and producing its first results, this time only a collider in dire need of repair could be inaugurated. A real celebratory mood did not come up, even though the organizers came up with some musical accompaniment, such as the Symphony Orchestra Orchestre de la Suisse Romande and the video recording of the Welsh choir Morriston Orpheus Choir in honor of the long-time LHC project leader, the Welshman Lyn Evans (see Fig. 1.3). Following the official part, the LHCfest for all CERN employees and guest scientists became very modern when Katherine McAlpine (also known as alpinekat), a US-American science journalist, performed the LHC rap with her troupe, which within a short time made 8 million clicks on YouTube.

Fig. 1.3   LHC project manager Lyn Evans during the LHC inauguration on October 21, 2008. (© 2008 CERN)

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1.4 The Higgs Plan Before two proton beams would again circulate in the LHC, as announced in the rap, and even be brought into collision, it would still take another year. In spring 2009, the final report of the investigation commission was published, in which the course of the accident and the damage were documented in detail. According to the report, the damaged electrical connection heated up so much that it melted and opened, creating a huge electric arc between the now separated ends. In the heat of the discharge, the immediate surroundings and especially the insulation of the liquid helium from the outside world evaporated. About six tons of helium escaped, two tons of which within the first few minutes, turned into gas and caused a rapid increase of air pressure in the tunnel, which even ripped open safety doors and completely displaced the air. A pressure wave also spread inside the LHC magnets, which, with an overpressure of up to 20 bars, crushed, damaged and ripped from their mountings 53 magnets over a total distance of 750 m, 37 of which were so heavily damaged that they had to be completely replaced. Added to this were soot and dust in the vacuum tubes that had spread from the damaged area. All this by the failure of a single power connection. And the subsequently evaluated data from the earlier tests of all LHC sectors pointed to further bad connections, which, although not as serious as the destroyed one, would still rule out operation of the LHC at the planned beam energy of 7 TeV. For operation at the highest energies, it was necessary to replace all the connections with an improved system, as the existing technology proved to be a potential weak point. In addition, precautions had to be taken to ensure that such an accident could not be repeated under any circumstances. Only with luck there were just enough replacements for the damaged magnets. For this reason, a new sensitive monitoring system for the connections was to be installed, which had already been planned but had not yet been done, along with a large number of safety valves in order to release the evaporating helium in a controlled manner in case of emergency. To tackle all the necessary work immediately would cause a considerable delay in the physics programme of several years, but would then allow an LHC with full energy from the start. Another option was a two-stage plan: first, all the immediate damage from the accident would be repaired and the new monitoring system installed. However, all the work that would involve a complete warm-up of the ring would be postponed. Only some of the new safety valves could be installed and the cable connections could not be renewed at first. The LHC could then be operated with only half the beam energy so as not to put too much stress

1.4  The Higgs Plan

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on the connections, but would be ready for operation again only after 1 year. After a few years at half energy, all cable connections would then be renewed in a stop of several years and the safety valves that are still missing would be retrofitted. The new CERN management under the German Director General Rolf Heuer, who took over on January 1, 2009, now had to decide: should the LHC first be completely refurbished before going hunting for Higgs with the highest energy, but with a long delay of several years? Or should one start operation as soon as possible at only half the beam energy, while gaining valuable operating experience and hoping that the LHC would deliver enough collisions to discover the long sought-after Higgs particle with only half the energy? Heuer decided to follow the two-stage plan in order to deliver the first results from the LHC as soon as possible, because after its initial glimmering up, physicists were eagerly awaiting the opportunity to finally put their detectors into operation with real collisions after many years and to advance into energy ranges never explored before. Even half the energy would still be three and a half times higher than that of the previous world record holder, the Tevatron Collider at Fermilab, about 50 km west of Chicago, IL. As would later become apparent, this was exactly the right strategy, crowned by the discovery of the Higgs particle in the summer of 2012.

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The Experiments

Summary In addition to a powerful accelerator, it was equally important to develop the best possible particle detectors to measure the collision products and record their data. In order to make the LHC competitive with the then planned Superconducting Super Collider (SSC) in the USA in the 1980s, a significantly higher collision rate was to largely compensate for the disadvantage of the LHC’s lower energy. However, the enormously high collision rate placed unprecedented demands on the detectors, which at that time still had to be developed. In the years before, no one had any idea of the later, premature end of the SSC in October 1993. The accelerator was largely the responsibility of CERN, but the development of the detectors was a worldwide undertaking, and in the late 1980s groups of particle physicists from many universities and research centers, CERN’s guest scientists, formed to develop new concepts and detector technologies for the LHC. More than 1,000 physicists were already involved in the development of detectors for the LHC in this early phase. After several years of research and development, the time was ripe for further steps. The CERN Council held a special meeting on December 19, 1991, to which high-ranking scientific representatives from the member countries and from other countries interested in the LHC were invited. It was agreed that the LHC would provide a rich scientific output and would become CERN’s next project well into the next century. This was the actual start of the LHC project, even though formal approval was still 3 years away. Now, suitable concepts were needed to determine which individual detector elements a large LHC particle detector should consist of. It is no surprise that the respective groups had very different ideas about the perfect LHC detector.

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2022 M. Hauschild, Exploring the Large Hadron Collider—The Detectors, essentials, https://doi.org/10.1007/978-3-658-33293-8_2

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2  The Experiments

In total, the LHC provided room for four large detectors in underground caverns. Only two of these caverns were reserved for large multipurpose detectors whose main task was to discover new phenomena at the highest energies such as the Higgs particle. As in the SppS collider with the two detectors UA1 and UA2, two detectors at the LHC were also intended to mutually confirm possible discoveries and results. In addition, there were to be two dedicated detectors, one of them to investigate collisions of heavy lead nuclei, which apart from protons could also be accelerated in the LHC. The other dedicated detector was to investigate the b- and anti-b-quarks produced in large quantities at the LHC, in particular to answer questions about the difference between matter and antimatter.

2.1 The Marriage Market In March 1992, a few months after the CERN Council had given the go-ahead, more than 650 physicists from almost 30 countries gathered for a few days in Évian-les-Bains (France), the famous, idyllically situated spa resort on Lake Geneva, to discuss possible detector concepts. However, physicists are usually not very interested in the spa life, but take the opportunity to formulate or reject new ideas with their colleagues in an informal atmosphere away from the offices and detector test stands, to find out common interests and, in general, to make connections toward future cooperation. This is the marriage market in which the upcoming large collaborations are formed, the unions of many particle physicists who later jointly plan, build, and operate a large particle detector. In Évian, four groups of physicists presented initial ideas (Expression of Interest—EoI) for large multipurpose detectors at the LHC, although it was clear from the outset that there could only be two detectors. The groups called themselves ASCOT (Apparatus with Super Conducting Toroids), CMS (Compact Muon Solenoid), EAGLE (Experiment for Accurate Gamma, Lepton and Energy measurements) and L3P, where the last group did not choose a meaningful name, but wanted to extend the already existing detector L3 at the Large Electron Positron Collider (LEP) for the future LHC, so that its name was simply supplemented by the “P” for protons to L3P. After the presentations of all groups it became clear that ASCOT and EAGLE were two very similar concepts, both based on large so-called toroidal magnet systems. It was therefore not surprising that both groups joined together a short time later to increase their chances to actually build one of the two planned multipurpose detectors.

2.1  The Marriage Market

15

Now an attractive name for the unified concept was still missing and to keep the physicists on, a competition was announced with ten bottles of champagne as the prize. It has long been a tradition at CERN to celebrate extraordinary events, records, and awards with champagne. The countless bottles of champagne lined up in the CERN accelerator control room bear witness to new accelerator energy records, the first circulating beams or the highest collision rates achieved. And as happened so often before, the new name was born on the terrace of CERN-Restaurant 1 when three physicists from the Max Planck Institute of Physics in Munich sat together there in the summer of 1992. The future name should reflect basic ideas of ASCOT and EAGLE and it should be a name from Greek mythology. So they came up with ATLAS, and afterward they thought about what this name should stand for as an abbreviation. Finally, A Toroidal LHC ApparatuS was chosen and so the toroidal magnet system of the two earlier concepts is found in its name (see Fig. 2.1). Although the deadline for the name had actually already passed, ATLAS prevailed over 50 other proposed names and the champagne bottles went to Munich.

Fig. 2.1   Schematic diagram of the ATLAS detector, with a volume of 46 × 25 × 25m3 the world’s largest particle detector of this kind. (© 1998 CERN)

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2  The Experiments

The two other groups that had presented detector ideas in Évian, CMS, and L3P, could not agree on a common concept, leaving three more elaborated proposals that were submitted to the newly founded LHC Experiments Committee (LHCC) for review on October 1, 1992. In each of the three drafts (Letter of Intent—LoI), the physical objective and, as a result, the detector design with the essential structure of the main components and their capabilities were described on more than one hundred pages, as well as the data readout, because especially the expected amount of data far exceeded the possibilities for data processing in the early 1990s. There was also a first preliminary cost estimate: each detector should be in the range of 400 million Swiss francs. If, according to this first estimate, one takes into account the inflation rate of 20% in Switzerland in the 15 years up to completion, the final construction costs of 550 million Swiss francs were not very far off.

2.2 The Decision The LHCC Committee is composed of a number of mainly external experts, not based at CERN, who now had the task of reviewing the proposals, in particular the feasibility, estimated performance, schedule, and cost estimate of the detectors. After 6 months of intensive work, the committee finally made a recommendation to the CERN Director General in April 1993 to pursue only the ATLAS and CMS detector proposals. Since the target of keeping the cost of a detector within the range of 300 million Swiss francs was clearly exceeded by both groups, the recommendation was made under the condition that a slimmed-down, less expensive version be developed first, which was to be completed later. The L3P group’s proposal was not to be pursued any further, but many of the former L3P physicists switched to CMS in the following years, whose concept still had the greatest similarities in order not to be left out of the LHC project. Parallel to the discussions in the CERN Council about the LHC accelerator, which was also approved for cost reasons in December 1994 with initially only two thirds of the required magnets and only 2 years later with all the magnets, the particle physicists had to further refine their detector proposals. The next step was the description of the technical implementation (technical proposal), which took place at the same time as the approval of the LHC. This was followed by a series of detailed technical descriptions (technical design reports) of the individual detector components. Over a period of several years, the construction plans for the two large LHC multipurpose detectors were produced on 9,600 pages, divided into 24 volumes.

2.3 Collaborations

17

In addition, the LHC was to have two dedicated detectors that would provide answers to specific questions, in addition to the hoped-for discovery of the Higgs particle and other new particles beyond the Standard Model of particle physics. ALICE (A Large Ion Collider Experiment) focuses on the investigation of collisions of heavy lead nuclei (heavy ions), which the LHC can also bring to previously unattained high energies. The structure of protons and neutrons and their constituents, the quarks and gluons, which are bound in the nuclei, is broken up during the collision for an extremely short time, creating a plasma of free quarks and gluons. To investigate the properties of this new state of matter is the main task of ALICE. The other dedicated detector, called LHCb, is designed primarily to study the bottom quark and its antiparticle, the antibottom quark, both of which are produced at the LHC at an extraordinarily high rate. In addition to the combinations of bottom quarks with other quarks, which manifest themselves in new, usually rarely occurring particles, the question of the differences between quarks and antiquarks, that is, between matter and antimatter, is of particular interest. Apart from the opposite charge of antiparticles, one should not expect any differences in production and decays compared to ordinary particles. And yet there must be a tiny difference that is responsible for our existence and for the very existence of our universe as we know it. Immediately after the big bang, pure energy should have formed as much matter as antimatter, which immediately afterward radiated back into pure energy, if, indeed, matter and antimatter do not differ slightly. This tiny difference must have ensured that a little more matter was left over from the annihilation, of which we and the universe ultimately consist. LHCb wants to track down this difference and thus shed light on a fundamental question. After a long review by the LHCC committee, the construction of the ATLAS and CMS detectors was finally approved in principle by the Director General on December 7, 1995 after a long discussion in the Research Board of CERN. The maximum cost was to remain limited to 475 million Swiss francs for each detector. A few years later, the approvals for the two dedicated detectors ALICE and LHCb followed. Construction could begin!

2.3 Collaborations Detectors for measuring particle collisions at high energies are very large and extremely complex. No single institute of a university is able to build and finance such a detector alone. For this reason, particle physicists have learned over the decades to join together in ever larger so-called collaborations, in which all the

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institutes and research centers involved jointly build and finance the detector. The three proposals for the two planned LHC multipurpose detectors were submitted in 1992 by a total of almost 2,000 physicists, including 840 physicists at ATLAS as the largest group. The listing of the names in alphabetical order alone required between four and six pages of paper. And the collaborations continued to grow. Especially after the end of the SSC in the USA in October 1993, many American particle physicists who had previously wanted to do research at the SSC decided to focus on the European side of the Atlantic and join one of the collaborations at the LHC. The earlier migration of European physicists to the USA was thus reversed. Since 1985, the particle accelerator with the world’s highest energy of almost 2 TeV, the Tevatron,1 has been located in the USA at the Fermi National Accelerator Laboratory or Fermilab for short, not far from Chicago, IL. But the future now lay in Europe. Between 1993 and 1996, the total number of guest scientists at CERN skyrocketed from 4,000 to over 5,500, largely due to the increase of US particle physicists. After a further large increase from 2005 onward, the two major collaborations ATLAS and CMS finally reached a number of 3,000 scientists each, one third of whom are PhD students. The physicists of the ATLAS collaboration come from 174 institutes in 38 countries, and the same applies to the CMS collaboration of the same size. Together with the two somewhat smaller collaborations ALICE and LHCb, about 8,000 physicists are involved in the LHC physics programme, which is more than half of the estimated 15,000 particle physicists worldwide. Each collaboration is headed by a spokesperson who is elected by the representatives of the participating institutes, usually for a period of 2 years. However, unlike the Director General of CERN, similar organizations or an industrial company, the spokesperson of the collaboration has no discretionary power or authority to give directions. Who would also want to or be able to give instructions to 174 independent institute bosses? Although the institutes are linked together in a collaboration through a Memorandum of Understanding (MoU), this is no contract and is not legally binding. The collaboration is based on the common scien-

1 The

Tevatron was a proton-antiproton collider in which protons and antiprotons collided with each other from 1985 to 2011, initially at 1.6 TeV, later at 1.8 TeV and from 2001 to 2011 at 1.96 TeV. In 1995, the last, still unknown and by far the heaviest sixth quark, the top quark, was discovered there by the two collaborations CDF and D-Zero. After the launch of the LHC with its significantly higher energy and collision rate, the Tevatron was no longer competitive and was shut down on September 30, 2011.

2.3 Collaborations

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tific goal, which has the highest priority. As with any association, there may be differences of opinion and disputes, but these are resolved by consensus as far as possible. The spokesperson therefore has a much more moderating and mediating function than that of a CEO. The data collected with the detector is available for analysis by all members of the collaboration without restrictions. As in the very first submitted letters of intent, publications list all members of the collaboration alphabetically, which caused problems for some publishers in their first publications when they were suddenly confronted with a list of 3,000 authors. The order of the authors, which in other scientific publications is extremely important and decisive for the further career, is completely irrelevant in the large collaborations. In the end, all members of the collaboration contributed in one way or another to the published result. Even if the contribution of the individual is not visible to the outside world, especially not through the order of authors in publications, the visibility of an individual doctoral student is ensured internally through lectures, conference participations, and the assumption of responsibilities. For this reason, reviews by other collaboration members play a much more important role in applications than publication lists. This model of scientific collaboration may seem surprising at first sight, but it has proven to be extremely successful, which has also attracted the interest of sociologists who have examined it in several studies.

3

Particle Detectors

Summary Particles elude our five senses, they are invisible and inaudible, we cannot smell them, taste them, or feel them. Evolution has not equipped us and all other living beings for the perception of particles, because there was no need for this on earth over the past millions of years. Particle detectors complement our missing senses. Most particle detectors are based on the principle of ionization or on the emission of light.

3.1 Can Particles Be Seen? Particles elude our five senses, they are invisible and inaudible, we cannot smell them, taste them, or feel them. Evolution has not equipped us and all other living beings for the perception of particles, because there was no need for this on earth over the past millions of years. Only under very specific circumstances, is it literally possible to perceive particles with the naked eye. Astronauts are exposed to much higher radiation in space than under the Earth’s shielding atmosphere and regularly report sporadic flashes of light that they see in total darkness with their eyes closed. High-energy particles from the Sun or cosmic rays penetrate the vitreous body of the eye and produce so-called Čerenkov radiation, which was first observed by the Soviet Russian physicist Pavel Čerenkov (1904–1990) during his doctoral thesis in 1934 and for which he and two other Soviet Russian physicists were awarded the 1958 Nobel Prize in Physics. Čerenkov radiation always occurs when electrically charged particles move faster than the local speed of light in a medium, for example in water, where

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2022 M. Hauschild, Exploring the Large Hadron Collider—The Detectors, essentials, https://doi.org/10.1007/978-3-658-33293-8_3

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the speed of light is only about 75% of the vacuum speed of light of almost 300,000 km/s. This causes a “superluminal flash”, which corresponds to the supersonic bang of an airplane flying at supersonic speed and also produces the blue glow in the water basin of a nuclear reactor. Certain particle detectors are also based on the emission of Čerenkov radiation, in which the radiation and thus the particles are detected by sensitive photodiodes or a photomultiplier tube (PMT), a special vacuum tube for multiple amplification of the weak electrical signal. Whether photodiodes or PMTs, in both detectors the light is converted into an electrical signal of electrons via the photoelectric effect.1 In addition to Čerenkov radiation, particles can also stimulate the outer electrons of atoms to a higher orbit by a small transfer of energy, which then fall back to their original orbit after a short time while emitting so-called scintillation light. This light can also be captured by photodetectors and used to detect particles. However, electrical signals can also be generated by particles directly by ionization without the detour of light and photoelectric effect. Similar to scintillation light, particles transfer energy to the outer electrons, but the amount of energy is sufficiently high that the electrons escape from the atoms. What remains is a positively charged atomic remnant, the ion, and the free electron, which moves in a gas, a liquid, or a solid by means of an externally applied electrical voltage and thereby, together with many other electrons, generates a measurable electrical current. Most particle detectors are based on the principle of ionization or on the emission of light by charged particles. Non-charged, electrically neutral particles such as neutrons initially elude direct measurement, but sometimes interact with matter to form charged particles, which in turn trigger measurable signals. Neutrinos are an exception, their interaction with matter is extremely weak and therefore extremely difficult to detect. Solar neutrinos, which are produced by nuclear fusion inside the sun, penetrate to the outside and strike the earth and us at a rate of 80 billion neutrinos per cm2. And this happens day and night, because even at night the solar neutrinos shine through the earth to us, which is practically transparent to them. But no more than a few tens of neutrinos manage to react with the atoms of our body in the course of a human lifetime.

1 See

also section “Quarks, waves and particles” of the essential “Exploring the Large Hadron Collider - CERN and the accelerators”.

3.2  Momentum and Energy

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Similarly, a certain type of new, as yet undiscovered and still hypothetical so-called SUSY particles would be detected in the LHC detectors.2 If they did not decay at the collision point immediately after their generation, they would leave the detector like neutrinos without further interaction and escape undetected. And yet there is a way to track down these particle refugees as well and to expand into regions beyond the Standard Model of elementary particle physics.

3.2 Momentum and Energy The key to discover unknown particles that do not interact with the detector lies in the precise measurement of momentum and energy. Energy cannot be generated or destroyed, only conversions from one form of energy to another are possible, such as the potential energy of a storage reservoir, which is converted into kinetic energy when the water flows off, and is ultimately converted into electrical energy for our power grid in the turbines of a power plant. Even the total momentum of a system cannot change without external influences and forces. Conservation of energy and momentum is a fundamental principle of physics and of our universe and this is exactly what particle physicists make use of: the energy and momentum of the protons before the collision are known and must correspond to3 the energy and momentum sum of all particles after the collision. Lack of energy indicates particles that have penetrated the detector without interaction, either the known neutrinos or new, unknown particles that are revealed by the lack of energy. The missing momentum even allows to determine the flight direction of the invisible particle.

2 More

about SUSY particles in the essential “Exploring the Large Hadron Collider - the discovery of the Higgs boson”. 3 Because of the structure of protons made up of quarks and gluons, this statement is only partially correct. Since in a collision it is not the protons as a whole that collide with each other but only their constituent parts, neither the two collision partners nor their energy and momentum are known. However, there is no momentum component transversal (perpendicular) to the beam and motion direction, so the sum of all transverse momentum must also disappear after the collision. For practical reasons, instead of the momentum, which can only be measured for charged particles, the measured energies and directions of all particles are used and vectorially summed. This results in the so-called transverse energy of the collision.

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3.3 The Onion-skin Principle The main task of a particle detector at the LHC is to identify all the particles produced in a collision and to measure their energy, momentum, and mass as precisely as possible. It is sufficient to determine only two of these quantities, because energy, momentum, and mass are all linked together.4 Together with the sign of the electric charge, the type of particle and its characteristic properties are thus unambiguously determined. No single detector can perform this task alone; instead, a whole collection of different detectors and technologies is needed to measure every collision that occurs. Since the collision particles can escape in all directions, the detector must be arranged as hermetically as possible and without gaps around the point of collision, except for the small space occupied by the accelerator’s beam pipe. An onion-skin-shaped structure of the entire detector with four main components has proved to be the best solution: tracking detector, calorimeter, magnet system, and muon detector (see Fig. 3.1). The innermost shell is the tracking detector, which consists of several layers of detector material. In each layer, electrically charged particles leave a signal, usually by ionization, whose location is determined with accuracies down to a few micrometers. By combining the measured locations, the track of the particle can be followed in the detector. A magnetic field forces the particles onto curved paths from which the momentum5 can be determined. By extrapolating the particle tracks to the collision point, which is located in a vacuum inside the beam pipe, the exact primary interaction point within the colliding particle bunch can be determined. The point of decay of a particle can also be found, in case an unstable particle decays on its way to and through the tracking detector. Tracking detectors use different technologies, depending on the requirements for the precision of the space points to be measured, the speed of data readout and resistance to radiation. Even before the LHC there were tracking detectors with the required precision, but what was completely new at the LHC were the challenges of speed and radiation hardness, which were to reach up to a hundred

4  The

relativistic quadratic momentum describes the relationship between energy E, � 2 = m2. , and mass m: E 2 − p momentum p 5 In a homogeneous magnetic field B, the particles move on a helix. The radius of the helix r gives the transverse momentum component pt [GeV/c] = 0.3 × B[T] × r[m], the angle of the track to the beam axis  gives the longitudinal momentum component pl = pt / tan , which together form the total momentum p: p2 = p2t + p2l .

3.3  The Onion-skin Principle

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Fig. 3.1   Section through a part of the CMS detector with the different onion-skin-shaped detector elements: Tracking detector (Silicon Tracker), Electromagnetic Calorimeter, Hadron Calorimeter, Superconducting Solenoid, and Iron return yoke interspersed with Muon chambers. Depending on the particle type, different detector layers are penetrated to detect the particles. (© 2004 CERN, for the benefit of the CMS Collaboration)

times higher than the detectors existing until then. An intense and long research and development phase was necessary to achieve these goals, and gradually the understanding of the origin and prevention of radiation damage has increased in particular. The innermost layers of the tracking detectors at the LHC consist of silicon pixels, similar to those found in the camera sensors of today’s smartphones. With about 80 million pixels in the tracking detectors, the number of pixels in camera sensors is exceeded several times, but the main difference is the 40 million collisions per second in the LHC that can be recorded by the tracking detectors, as opposed to only 30–60 frames per second in the camera sensors, and the extremely high radiation hardness. Ordinary camera sensors would only be able to operate for a few hours near the collision point before first failures due to radiation damage would appear.

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Fig. 3.2   Installation of the ATLAS silicon pixel detector. (ATLAS experiment © 2007 CERN)

Silicon pixel detectors are very complex (see Fig. 3.2), the integrated electronics for amplifying the weak signals as the particles pass through the silicon material and for further processing require quite some electrical power and, more importantly, sufficient cooling. In addition, a light but stable mechanical support must ensure that the position of the pixel detectors does not change during operation. Any shift of only a few micrometers would immediately result in an incorrectly determined curvature of the particle tracks and thus in an incorrect momentum measurement. While tracking detectors should be made of lightweight materials in order to influence the path of the particles as little as possible, exactly the opposite is true for calorimeters. Here, the energy of all particles should be absorbed and measured, and this works best with the heaviest possible materials such as iron, lead or tungsten, whose atomic nuclei contain many protons and neutrons and have a high electrical charge. Depending on the type of particle, the measuring principle of the calorimeter is different. The three light particle types, the photon, electron and its antiparticle, the positron, are collected in the inner calorimeter layer, the electromagnetic cal-

3.3  The Onion-skin Principle

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Fig. 3.3   Crystals of lead tungstate (PbWO 4 ) for measuring the energy of light particles (photon, electron, positron) in the CMS electromagnetic calorimeter. (© 2008 CERN)

orimeter. All heavy particles with strong interaction, the hadrons, are collected in the outer calorimeter layer, the hadron calorimeter.6 Two different concepts are used for the electromagnetic calorimeter in ATLAS and CMS. In CMS, the heavy absorber material, high-purity, transparent crystals of lead tungstate (PbWO4), is also used to detect the shower particles, which generate scintillation light in the crystal, which is read by fast photodiodes (see Fig. 3.3). ATLAS, on the other hand, uses a series of alternating layers of lead as passive absorber material and active detection layers of liquid argon, in which the shower particles generate electric charge by ionization. In both cases, more energy of the incident particle means more shower particles and more light or more electric charge.

6 Light

particles produce a particle shower due to bremsstrahlung and pair production near heavy nuclei. Hadrons trigger an interaction with the nucleus, resulting in a shower of predominantly hadronic particles. The number of shower particles, which can be determined by photodetectors or ionization, is directly proportional to the energy of the initial particle.

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The outer layer of the LHC detectors are the muon detectors. Muons are neither light particles nor hadrons, so they penetrate the calorimeter practically unhindered and are detected in the outer detector layer, essentially a tracking detector exclusively for muons. Depending on the design principle, the iron return yoke of the magnet system, which is required to form the magnetic field, forms part of the muon detector and is thus used twice.

3.4 Cathedrals of Science LHC detectors are true cathedrals of science! The largest of the underground caverns measures 50 m in length at a height of just under 30 m, into which the ATLAS detector just fits. Two large shafts 18 m and 12 m in diameter lead down from the surface to a depth of 100 m. With these dimensions, it is no wonder that the ATLAS cavern played a role in the first scene of the famous Hollywood blockbuster Illuminati, even though the control room in the film was moved directly next to the underground detector for dramaturgy reasons. The ATLAS detector is one of the largest particle detectors of all, but with only 7,000 t rather a lightweight due to its open construction principle. As early as 1998, while the Large Electron Positron Collider (LEP) was still operating in today’s LHC tunnel, work began on the two shafts that would lead to the future ATLAS cavern. After the end of LEP in November 2000, it was also possible to excavate the cavern, which was ceremonially inaugurated in the summer of 2003 in the presence of the President of the Swiss Confederation, Pascal Couchepin, because the ATLAS cavern is the only one of the four large LHC detectors on Swiss soil. A little later, the construction of the detector began with the installation of the eight large coils of the toroidal magnet, which are over 22 m long (see Fig. 3.4). At 12,500 t, the CMS detector is significantly heavier than ATLAS. It even exceeds the weight of the Eiffel Tower and is located opposite the ATLAS detector on the other side of the LHC ring. For CMS, the Compact Muon Solenoid, emphasis was put on a large coil with a high magnetic field, which requires considerably more iron and thus a higher weight but with smaller detector dimensions of only 21 m in length and 15 m in diameter. When the magnet is switched on, there are indeed enormous forces at work, which shrink the detector by 16 mm when the iron is compressed and squeezed by the magnetic forces. Due to its compactness, the average density of CMS is significantly higher than that of ATLAS. If the two large LHC detectors were to be packed up water-

3.4  Cathedrals of Science

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Fig. 3.4   The eight coils of the ATLAS toroidal magnet shortly after the installation of the ATLAS detector began in November 2005, with part of the calorimeter in the center ready for retraction. The other detector elements were installed later. (© 2005 CERN)

tight and thrown onto Lake Geneva, ATLAS would float and CMS would sink, a fact that can make both ATLAS and CMS physicists smile. The work for the CMS cavern was much more difficult than for ATLAS. Test drillings revealed the remains of a Roman villa from the fourth century, together with pottery, tiles, and coins from the port of Ostia near Rome, Lyon, and London. The necessary securing of the archaeological site delayed the start of the work by 6 months at first. Water ingress during the excavations led to further delays. Before the large access shaft to the later underground cavern could be driven further into the depths, the soil around the shaft had to be frozen using a ◦ ring of cooling pipes filled with liquid nitrogen at −195 C. Only the 3 m thick ice wall protected the shaft from further water ingress. Because of these unexpected difficulties, the cavern was not ready for installation of the CMS detector until February 2005, much later than originally planned and actually too late to have the detector ready in time for the planned LHC launch. However, CMS made a virtue out of this emergency and it was decided that, unlike ATLAS, the CMS detector would not be assembled all in its parts in the cavern, but would be set up on the surface in five large and a few smaller sec-

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tions. The prefabricated sections, weighing up to 2,000 t, were then to be lowered into the cavern and connected with the help of a powerful crane. This procedure would make the detector setup independent of the excavation work and make up for lost time. In addition, the individual sections could be extensively tested before final installation. In November 2006, the first section of the CMS detector was on its way down, held by 55 individual steel cables, each 15.7 mm thick. All work was completed in January 2008 after a little more than a year (see Fig. 3.5), only a few months

Fig. 3.5   Lowering the last section of the CMS detector into the underground cavern. (© 2008 CERN, for the benefit of the CMS Collaboration)

3.4  Cathedrals of Science

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before the planned LHC start. The LHC detectors were ready, even though at that time nobody suspected that the start of data collection and the hunt for the Higgs particle would be delayed by more than a year due to the LHC accident in September 2008.

4

Outlook

Summary In this essential you learnt how the LHC after a long planning, whose beginnings date back to 1984, was finally put into operation in September 2008, when protons circulated in the LHC for the first time under the eyes of the world public. Only a few days later the LHC was massively damaged in an accident. Despite a delay of more than a year and with only half the collision energy, the data were nevertheless sufficient to announce the discovery of a new particle from the ATLAS and CMS collaborations on July 4, 2012, with the Nobel Prize in Physics in 2013 awarded to the two theoretical physicists François Englert and Peter Higgs as a preliminary high point. The restart of the world machine after a break of more than 2 years in spring 2015 marks the beginning of new research at the LHC. The Higgs particle must be further evaluated and compared with the theoretical predictions. Thanks to an energy almost twice as high as before, new particles in particular may only be waiting to be discovered in the next few years, such as SUSY particles as possible candidates for dark matter in the universe. Each newly discovered particle could trigger a revolution in the understanding of our world and the universe. The LHC, and later the High Luminosity LHC from 2027 onward, will run until 2037 or even beyond. New accelerators, which will run well into the second half of the twenty-first century, are already being seriously discussed. A milestone is the updated European Strategy for Particle Physics, which has been adopted by the CERN Council in June 2020, setting the direction of particle physics in the coming decades. The first part of this essential series [6] deals with the origins, history, and successes of CERN. You will learn about the basic features of the Standard Model, how particle accelerators work, and get an overview of the LHC with its

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2022 M. Hauschild, Exploring the Large Hadron Collider—The Detectors, essentials, https://doi.org/10.1007/978-3-658-33293-8_4

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technological challenges and construction. The third part of the essential series [7] deals with the role of the Higgs particle, which was finally discovered on July 4, 2012 after a long hunt, the relaunch of the LHC at higher energy and the possible future projects of particle physicists. In two further essentials of this series [8, 9] you will also learn more about the background of the Higgs particle, the Standard Model and New Physics beyond the Standard Model from the perspective of a theoretical physicist.

What You Learned from This essential

• The LHC at CERN near Geneva was designed back in the 1980s and finally commissioned for the first time in September 2008 after 25 years. It is the largest and by far the most powerful particle accelerator in the world. • The particles created in the collisions at the LHC are detected by large detectors built and operated by collaborations of over 3,000 physicists from all over the world. • The LHC detectors measure the momentum and energy of all the particles created in the collisions in the LHC. They have an onion-skin-shaped structure, with a tracking detector for measuring momentum as the innermost shell, followed by a calorimeter for measuring energy. • The detectors were assembled in large underground caverns and were completed a few months before the planned LHC start.

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2022 M. Hauschild, Exploring the Large Hadron Collider—The Detectors, essentials, https://doi.org/10.1007/978-3-658-33293-8

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References

1. „The LHC sees its first circulating beam“. CERN Courier 19. September 2008. http:// cerncourier.com/cws/article/cern/35864. 2. McAlpine, Katherine (Alpinekat). Large hadron rap. YouTube. 28 July 2008. https:// www.youtube.com/watch?v=j50ZssEojtM. 3.  Boisot, Max H. 2011. Collisions and collaboration: The organization of learning in the ATLAS experiment at the LHC. Oxford: Oxford University Press (ISBN 9780199567928). 4.  Merali, Zeeya. 2010. Physics: The large human collider. Nature 464 (7288):482– 484.https://doi.org/10.1038/464482a 5. Kolanoski, Hermann, und Norbert Wermes. 2016. Teilchendetektoren Grundlagen Und Anwendungen. Berlin: Springer (ISBN 978-3-662-45349-0, eBook ISBN 978-3-66245350-6). 6. Hauschild, Michael. 2016. Neustart des LHC: CERN und die Beschleuniger. Wiesbaden: Springer Fachmedien (ISBN 978-3-658-13478-5, 978-3-658-13479-2). 7. Hauschild, Michael. 2018. Neustart des LHC: Die Entdeckung des Higgs-Teilchens . Wiesbaden: Springer Fachmedien (ISBN 978-3-658-23085-2, 978-3-658-23086-9). 8. Knochel, Alexander. 2016. Neustart des LHC: Das Higgs-Teilchen und das Standardmodell. Wiesbaden: Springer Fachmedien (ISBN 978-3-658-11626-2, 978-3-65811627-9). 9. Knochel, Alexander. 2016. Neustart des LHC: Neue Physik. Wiesbaden: Springer Fachmedien (ISBN 978-3-658-13906-3, 978-3-658-13907-0).

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