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LHC

Birth of a Giant

At long last, the Large Hadron Collider (LHC) will be activated this summer. Its goal is to uncover the ultimate components of the universe and the laws that govern them. Journey to the heart of this incredible machine, a 20-year project that numerous CNRS teams helped make a reality.

lhc illustr location

© CERN, illustration P. Mouche

Located at a depth of 100 meters below the surface, the LHC was set up inside the concrete ring which used to house the LEP, the former electron collider. The ring is 27 kilometers long.




The headquarters of the European Council for Nuclear Research (CERN) near Geneva is busy as ever. In just a few weeks, the largest and most powerful machine ever built–the Large Hadron Collider,1 or LHC–will be operational.
The level of excitement is not surprising given what is at stake: For nearly 15 years, thousands of researchers and engineers from all over the world–including those from 11 labs belonging to CNRS’ IN2P32–have been working to build this huge machine, a ring 27 kilometers long buried 100 meters deep below the Franco-Swiss border. The LHC is today the largest particle accelerator in the world.
It is expected to pave the way for fundamental discoveries on the ultimate building blocks of matter and the laws that govern the universe. “With the LHC, we’re at last going to be able to test experimentally some of the physical theories put forward over the past 40 years,” enthuses Abdelhak Djouadi, from Orsay’s LPT.3 To do this, the physicists have pulled out all the stops. Thousands of billions of either protons or heavy lead ions will collide head-on at speeds near that of light, with a total energy of 14,000 billion electronvolts (14 TeV).4 Such collisions will for an instant generate temperatures a hundred thousand times greater than those found in the Sun’s core. For the particles to reach such extreme energies, the heart of the machine must be kept near absolute zero and in a nearly perfect vacuum: a temperature of -271.3°C (1.9 Kelvin) and a pressure of 10-13 atmosphere, the conditions found in deep space. But just why are such extreme conditions necessary, and what are the theories physicists want to test?

structure du lhc

© CERN

The structure of the LHC contains the two beam vacuum tubes, as well as close to 9000 magnets and a cryogenic system.



Beyond the Limits of Physics
Modern physics has long been exploring the realm of the infinitely small. Step by step, theories and experiments have enabled us to understand what matter is made up of, and how its ultimate building blocks–the so-called elementary particles– interact. Today, particles and interactions come together in what is known as the Standard Model of Particles, a sort of operating manual for physics that nonetheless has several missing links. An important one, gravity, for example, has yet to find a place in the model. Neither does the model explain why the universe is made up of matter rather than antimatter, though there is no doubt about the latter’s existence since physicists routinely handle antiparticles. Furthermore, some parts of the model are just purely theoretical. This is the case for the Higgs boson, a particle whose existence was put forth in 1964 by the physicists François Englert, Robert Brout, and Peter Higgs. At the time, no one had succeeded in explaining an undisputed fact, namely that elementary particles have a mass (except for photons and gluons). The invention of the Higgs boson solved the problem: The universe is thought to be filled with an ocean of Higgs particles (a “field”) with which most particles interact. The stronger the interaction, the higher the mass of the particle in question.
In order to fill in the gaps in the Standard Model, physicists have had no hesitation in exploring uncharted theoretical waters. “In physics we are always seeking perfection,” observes Djouadi. “As its name suggests, the Standard Model is only a model, and it’s not very predictive. So we’re on the lookout for a more all-encompassing theory.” A theory which would work on all energy scales, even the most extreme ones found in the very first instants of the universe, which is not the case for the Standard Model. There are a number of approaches. One of these, supersymmetry, postulates the existence of a host of new massive particles linked to the elementary particles already known. And although this theory already has a lot of physicists excited, others are exploring even more novel avenues, such as extra dimensions of space-time. Higgs bosons, supersymmetry, the disappearance of antimatter, other dimensions in space-time: Physicists aren’t short of ideas when it comes to explaining our universe and its tumultuous beginnings. The LHC will take center stage in proving these theories.

Lord of the Rings

First thought up in the mid 1980s and officially approved by CERN in 1994, the machine will hurl bunches of protons5 with huge energies (7 TeV) at each other. The energy resulting from these head-on collisions (14 TeV) should be enough to create a host of particles, both known and unknown, including the famous Higgs boson and even supersymmetric particles, if they exist. In concrete terms, the LHC is made up of a ring-shaped tube about one meter in diameter and 27 kilometers long. Inside are two vacuum tubes along which bunches of protons travel in opposite directions. The tubes are surrounded by around 9000 superconducting magnets.6 These generate a powerful magnetic field of 8.3 teslas–nearly 175,000 times more intense than that of the Earth–which is used to guide the protons along their paths. A liquid helium cryogenic system is used to cool everything down to a temperature of -271.3 °C. The protons are injected in bunches into the LHC by a series of increasingly powerful accelerators. When working flat out, each tube in the LHC will contain 2808 bunches each made up of one hundred billion protons traveling at 99.999 % the speed of light. This means they will travel round the ring more than 11,000 times in a single second. After operating for approximately ten hours, and having traveled round hundreds of millions of times, the bunches, slowed down by the collisions, will be removed and replaced by fresh bunches of protons.
Teams from IPNO7 were responsible for drawing up the approximately 800 blueprints needed for the production of certain parts of the rings, the short straight sections, which house the magnets used to focus the beams. Indeed, the machine is not completely circular: it is made up of eight arc-shaped sections and another eight straight sections. They also selected and calibrated the 7000 thermometers needed to monitor the cryogenic system. “We had to design a specific calibration station,” explains IPNO’s Jean-Pierre Thermeau. “In all, it took us five years to calibrate all the thermometers from 27°C to -271.3°C. His colleague Gilles Belot administered the industrial monitoring of the 1262 vacuum chambers for the dipoles–the electromagnets used to bend the path of the beams around the ring–and the 848 heat shields that insulate the system. He also supervised their assembly in the underground tunnel, expert work that earned him CERN’s “Golden Hadron Award of Best LHC Inspector.”

Four Titans to Decipher Matter

lhc cryogenic

© CERN

The LHC's cryogenic system keeps the superconducting magnets used to guide the protons at a chilly -271.3 °C.


Moving bunches of protons around at huge speeds is one thing, but getting them to meet is another. At four points around the ring, the two tubes join up and magnets called quadrupoles focus the beams to make them collide. Forty million collisions between proton bunches should take place per second. It will then be up to the scientists to collect the particles formed, analyze them, and look for the trace of a Higgs boson or a supersymmetric particle. In fact, it will actually not be possible to observe any of these new theoretical particles directly. It is only the particles produced by their disintegration–two photons, four electrons or muons of a given energy–and they alone that can be detected by scientists. It is for this purpose that four experiments–Atlas, CMS, LHCb, and Alice–have been set up at the LHC’s four collision points.8
All these experiments, the result of a collaboration between IN2P3, the French Atomic Energy Agency (CEA), CERN and all the other countries taking part, are based on the same principle: determining the path and energy of all the particles produced by a collision in order to get back to the short-lived particle from which they originate. The latter may either be as yet unknown or may carry information about the environment into which it was born. Analysis of this particle explosion is handled by a stack of sub-detectors. The closest to the collision point is a tracker, which is used to identify the charged particles by observing their paths. Then there are electromagnetic calorimeters, which measure the energy of photons and electrons, and hadronic calorimeters, which measure the energy of hadrons. The set-up is completed by a muon detector.
For each experiment, it proved necessary to design systems able to observe millions of particles per second, radiation-resistant electronics, and ultra-rapid data acquisition systems. This was a Herculean task that took the thousands of researchers involved several years to accomplish. “When it was decided to build the LHC, the technologies we needed didn’t exist,” explains Yves Sirois, a researcher at LLR9 in charge of the CMS experiment for CNRS. “We had to develop them ourselves, and hope that the components we needed would follow.”

The Giant Atlas

atlas

© CERN, ATLAS

ATLAS, CMS, and ALICE are in the form of large cylinders located at three of the points where the proton beams collide. The particles created during these collisions are tracked down by the successive layers of detectors that make up each experiment.



The most impressive experiment is Atlas (A Toroidal LHC Apparatus), quite a giant indeed as the entire detector is nearly 46 meters long and 25 meters in diameter. Its aim is to hunt down the famous Higgs boson and test physics beyond the Standard Model.
Its liquid argon electromagnetic calorimeter, which measures the energy of photons and electrons, was invented at LAL in Orsay.10 It is made up of a stack of metal plates and electrodes, submerged in liquid argon. Every particle traveling through it removes some of the argon atoms’ electrons that are then collected by the electrodes, forming an identifiable signal. However, for the machine to be able to respond in a few tenths of a nanosecond, it had to be given a totally novel shape: the plates and the electrodes were folded like an accordion and stacked up, forming a cylinder. “This structure also ensures that there is no blind zone inside the detector,” explains Daniel Fournier, in charge of the Atlas experiment for CNRS and the device’s co-inventor. “It was a real mechanical and electronic challenge because, in the end, we will have to process nearly 200,000 readout channels.” Many CNRS teams have taken part in the development and assembly of the different components of the ATLAS experiment,11 such as the hadronic calorimeter, the pre-sampler, used to precisely determine the energy of the particles at the moment they enter the calorimeter, and lastly, the key component of ATLAS’ tracker, an 82 million-pixel detector.

A Heavy Higgs Hunter
The LHC’s other big experiment is CMS (Compact Muon Solenoid). Though much smaller than Atlas, it is also much heavier, weighing in at 11,000 tons compared to its big brother’s mere 7000 tons. This mass is mainly due to its highly compact internal magnet, which generates a magnetic field 80,000 times more intense than that of Earth. CMS is also aimed at uncovering the Higgs boson and supersymmetry particles, but to do this it uses technologies that are different and complementary to those used by ATLAS. Its electromagnetic calorimeter, for instance, is made up of lead tungstate crystals, which when struck by photons or electrons emit light. This light is then collected by devices such as avalanche photodiodes. The various technologies and parts required to build the CMS, from the lead tungstate crystals to the tracker, came out of pioneering labs, many of which are from or associated to CNRS.12 “We were in charge of making one third of the 288 silicon ‘petals’ that make up the tracker,” explains Pierre Van Hove from IPHC.13 “We also had to guarantee their quality: This detector has to last 15 years with no more than 0.5% of defects.”

b for Beauty Quarks
LHC’s two other collision points are taken up by the LHCb and ALICE experiments. Although they may also be able to observe events that are caused by novel physics and even uncover the Higgs boson, their job is mainly to elucidate very specific phenomena. For instance, LHCb was designed to study specific particles–hadrons, made up of so-called bottom quarks–and the fine variations in behavior between them and their antiparticles. The aim is to better understand the subtle mechanism that upsets the balance between matter and antimatter, with the hope of understanding why antimatter, rather than matter, disappeared from the universe. “Bottom hadrons will be emitted very close to the beam of protons,” explains Marie-Hélène Schune, from LAL. “This is why LHCb is not cylinder-shaped like the other three experiments. It was sufficient to place the detectors on one side of the collision point.” Five French laboratories14 took part in building the mechanical structure and the whole electronic chain of the calorimeters. They were also in charge of the major components of the first level triggering system.
Indeed, and this is valid for each of the four experiments, most of the 40 million collisions that take place every second produce nothing of interest, making it pointless to record all these events. Selection systems, based on the nature and energy of the particles detected, have therefore been installed. They will be the ones telling the computer systems which events should be recorded so that they can subsequently be made available to the physicists.

ALICE in particle land
Unlike the other three experiments, Alice will mainly be looking at collisions between lead ions, which will only begin next year. The violence of the collision between these heavy ions will be so great that a new state of matter–a plasma of quarks and gluons–should appear for a fraction of a second, as it probably did in the first instants of the universe. By studying the particles produced by the collision, especially muons and certain hadrons, the researchers will find out if a plasma of this kind has really been produced, and will be able to determine some of its characteristics. Such plasma may still exist today in the cores of particular stars called neutron stars. In this case as well, teams from IN2P3/CNRS15 have been very much involved in the design and production of several sub-detectors, including the muon spectrometer, which has over a million detection channels, and part of the silicon tracker and its electronics. Some groups are working on the development of an electromagnetic calorimeter which should be installed after LHC’s launch.
Of course, the work of all these teams with LHC is far from over. After taking part in setting up the instruments in the underground tunnel, many CNRS scientists are carrying out final tests, measuring in particular the response of the detectors to cosmic rays coming from space. Others, meanwhile, are already looking to the future, and are starting to study the technical solutions needed to turn LHC into an even more powerful superLHC.
But most are already putting the last touches to their equations, in high anticipation of the LHC’s first results–a momentous event in the history of physics and a further step towards understanding our universe.

Fabrice Demarthon

>> The LHC online, general information and latest news:
http://lhc-france.fr
http://public.web.cern.ch/public/

 >> For information on each experiment:
ATLAS: http://atlas.ch
CMS: http://cms.cern.ch
LHCb: http://lhcb.web.cern.ch
ALICE: http://aliceinfo.cern.ch

LHC’s Computing Grid
Once LHC is up and running smoothly, some 15 million gigabytes of data will be produced every year. To cope with this massive influx, CERN and its partners have set up the LHC Computing Grid, an international data processing infrastructure that incorporates thousands of computers and storage resources. In fact, the LHC data will be processed in around 200 sites all over the planet. The first center, known as Tier 0 and located at CERN, will acquire all the data produced by the detectors, perform a first processing, and make a backup. It will then send the data to the 11 major national centers, known as Tier 1, one of which is the IN2P3/CNRS Computing center (CCIN2P3) in Lyon. The center currently has 8200 processors and a storage capacity of 8 million gigabytes, which is expected to double every year. “These centers, available 24 hours a day, will ensure data preservation and will carry out further processing, before sending the derived data on to Tiers 2 and 3, scattered all over the world, where the actual physics analysis will be carried out,” explains Fabio Hernandez, deputy director of CCIN2P3. Meanwhile, IN2P3’s eleven labs, which are already heavily involved in the LHC experiments, are currently ramping up local and regional computing centers connected to the LCG grid.
FD
Contact: Fabio Hernandez, fabio@in2p3.fr

Notes :

1. Hadrons are subatomic particles made up of quarks and gluons (protons and ions are hadrons, for example).
2. Institut national de physique nucléaire et de physique des particules.
3. Laboratoire de physique théorique (Université Paris-Sud / CNRS).
4. The electronvolt is a unit for measuring energy. In particle physics, it is also used to refer to a particle's mass. 1 teraelectronvolt (1 TeV) = one thousand billion electronvolts. 1 gigaelectronvolt
(1 GeV) = 1 billion electronvolts.
5. Only a very small portion of time will be dedicated to sending heavy lead ions through the machine.
6. Superconducting magnets are made up of coils in which electric current flows without any resistance. They require very low temperatures to be able to work.
7. Institut de physique nucléaire d'Orsay (CNRS / Université Paris-Sud).
8. Two smaller experiments, Totem and LHCf, have been installed near CMS and Atlas.
9. Laboratoire Leprince-Ringuet (CNRS / Ecole Polytechnique).
10. Laboratoire de l'accélérateur linéaire (CNRS / Université Paris-Sud).
11. Laboratoire de physique nucléaire et des hautes énergies (LPNHE) (CNRS / Universités Paris-VI and VII); Laboratoire d'Annecy-le-Vieux de physique des particules (LAPP) (CNRS / Université de Savoie); Centre de physique des particules de Marseille (CPPM) ( CNRS / Université Aix-Marseille-II) ); Laboratoire de physique corpusculaire (LPC Clermont) (CNRS / Université de Clermont-Ferrand-II); and the Laboratoire de physique subatomique et cosmologie (LPSC) (CNRS / Université Grenoble-I / Institut National Polytechnique de Grenoble).
12. LAPP, LLR and IPNL (Institut de physique nucléaire de Lyon: CNRS / Université Lyon-I).
13. Institut pluridisciplinaire Hubert Curien (CNRS / Université Strasbourg-I).
14. CPPM, LAL, LAPP, LPC Clermont, and LPNHE.
15. IPHC, LPC Clermont, IPNL, IPNO, LPSC, and Laboratoire Subatech (CNRS / Ecole des Mines de Nantes / Université de Nantes).

Contacts :

Didier Lacour,
lacour@in2p3.fr
Jean-Marie Brom,
jean-marie.brom@ires.in2p3.fr
Daniel Bloch,
daniel.bloch@ires.in2p3.fr
Christian Kuhn,
christian.kuhn@ires.in2p3.fr
Pierre VanHove,
pierre.vanhove@ires.in2p3.fr
Yves Sirois,
yves.sirois@in2p3.fr
Serge Kox,
kox@in2p3.fr
Bernard Ille,
b.ille@ipnl.in2p3.fr
Guy Wormser,
wormser@lal.in2p3.fr
Daniel Fournier,
daniel.fournier@cern.ch
Marie-Helene Schune,
schunem@lal.in2p3.fr
Tomas Junquera,
junquera@ipno.in2p3.fr
Gilles Belot,
belot@ipno.in2p3.fr
Jean-Pierre Thermeau,
thermeau@ipno.in2p3.fr
Bruno Espagnon,
espagnon@ipno.in2p3.fr
Yannis Karyotakis,
yannis.karyotakis@lapp.in2p3.fr
Francois Vazeille,
vazeille@clermont.in2p3.fr
Ginés Martinez,
Gines.Martinez@subatech.in2p3.fr
Renaud Le Gac,
legac@cppm.in2p3.fr
Sylvain Tisserand,
tisseran@cppm.in2p3.fr
Abdelhak Djouadi,
abdelhak.djouadi@th.u-psud.fr


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