Lhc:large Hadron Collider

Large Hadron colliders •

What Is Large Hadron colliders? The LHC is exactly what its name suggests - a large collider of hadrons. Strictly, LHC refers to the collider; a machine that deserves to be labelled ‘large’, it not only weighs more than 38,000 tonnes, but runs for 27km (16.5m) in a circular tunnel 100 metres beneath the Swiss/French border at Geneva. However, the collider is only one of three essential parts of the LHC project. The other two are: the detectors, which sit in 4 huge chambers at points around the LHC tunnel and the GRID, which is a global network of computers and software essential to processing the data recorded by LHC’s detectors. The LHC’s 27km loop in a sense encircles the globe, because the LHC project is supported by an enormous international community of scientists and engineers. Working in multinational teams, at CERN and around the world, they are building and testing LHC equipment and software, participating in experiments and analysing data. The UK has a major role in leading the project and has scientists and engineers working on all the main experiments.

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Who is involved? The LHC project includes 111 nations in designing, building and testing equipment and software, participating in experiments and analysing data. It is a remarkably harmonious international collaboration in which the UK has a leading role. British scientists and engineers have prominent roles in construction, management and experimental teams and the UK makes a significant contribution to the LHC budget. Over the 13 year construction period (1994 to 2006 inclusive) the total UK contribution for the detectors, GriddPP (materials and staff effort) and collider was £511M. This includes the UK’s annual CERN subscription over this period. This is less than the price of one pint of beer per UK adult per year. The total cost to the UK of participating in the LHC project will be £108M per year, including £82M per year as its national subscription to CERN’s on-going annual budget of approximately £455M. The subscription of member countries to the CERN 2

budget is linked to their GDP. Non-member countries are also involved in, and contribute to, experiments. The cost of the LHC project (machine and personnel) is £2.1bn, or £3.5bn if the infrastructure costs, incurred during the construction phase, and the costs of computing, GRID, early running etc are included. The cost of the LHC is mainly paid for by the 20 members of CERN, with significant contributions from the 6 observer nations.

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Purpose It is theorized that the collider will produce the elusive Higgs boson, the last unobserved particle among those predicted by the Standard Model. The verification of the existence of the Higgs boson would shed light on the mechanism of electroweak symmetry breaking, through which the particles of the Standard Model are thought to acquire their mass. In addition to the Higgs boson, new particles predicted by possible extensions of the Standard Model might be produced at the LHC. More generally, physicists hope that the LHC will enhance their ability to answer the following questions: •

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Is the Higgs mechanism for generating elementary particle masses in the Standard Model indeed realised in nature? If so, how many Higgs bosons are there, and what are their masses? Are electromagnetism, the strong nuclear force and the weak nuclear force just different manifestations of a single unified force, as predicted by various Grand Unification Theories? Why is gravity so many orders of magnitude weaker than the other three fundamental forces? Is Supersymmetry realised in nature, implying that the known Standard Model particles have supersymmetric partners? Are there additional sources of quark flavour violation beyond those already predicted within the Standard Model? Why are there apparent violations of the symmetry between matter and antimatter? What is the nature of dark matter and dark energy? Are there extra dimensions, as predicted by various models inspired by string theory, and can we detect them?

Of the possible discoveries the LHC might make, only the discovery of the Higgs particle is relatively uncontroversial, but even this is not considered a certainty. Stephen Hawking said in a BBC interview that "I think it will be much more exciting if we don't find the Higgs. That will show something is wrong, and we need to think again. I have a bet of one hundred dollars that we won't find the Higgs." In the same interview Hawking mentions the possibility of finding superpartners and adds that "whatever the LHC finds, or fails to find, the results will tell us a lot about the structure of the universe."

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As an ion collider The LHC physics program is mainly based on proton–proton collisions. However, shorter running periods, typically one month per year, with heavy-ion collisions are included in the program. While lighter ions are considered as well, the baseline scheme deals with lead ions. This will allow an advancement in the experimental program currently in progress at the Relativistic Heavy Ion Collider (RHIC). The aim of the heavy-ion program is to provide a window on a state of matter known as Quark-gluon plasma, which characterized the early stage of the life of the Universe

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Design The LHC is the world's largest and highest-energy particle accelerator. The collider is contained in a circular tunnel, with a circumference of 27 kilometres (17 mi), at a depth ranging from 50 to 175 metres underground. The 3.8 m wide concrete-lined tunnel, constructed between 1983 and 1988, was formerly used to house the Large Electron-Positron Collider. It crosses the border between Switzerland and France at four points, with most of it in France. Surface buildings hold ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants. The collider tunnel contains two adjacent parallel beam pipes that intersect at four points, each containing a proton beam, which travel in opposite directions around the ring. Some 1,232 dipole magnets keep the beams on their circular path, while an additional 392 quadrupole magnets are used to keep the beams focused, in order to maximize the chances of interaction between the particles in the four intersection points, where the two beams will cross. In total, over 1,600 superconducting magnets are installed, with most of each weighing over 27 tonnes. Approximately 96 tonnes of liquid helium is needed to keep the magnets at their operating temperature of 1.9 K, making the LHC the largest cryogenic facility in the world at liquid helium temperature. Once or twice a day, as the protons are accelerated from 450 GeV to 7 TeV, the field of the superconducting dipole magnets will be increased from 0.54 to 8.3 teslas (T). The protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV (2.2 μJ). At this energy the protons have a Lorentz factor of about 7,500 and move at about 99.9999991% of the speed of light. It will take less than 90 microseconds (μs) for a proton to travel once around the main ring – a speed of about 11,000 revolutions per second. Rather than continuous beams, the protons will be bunched together, into 2,808 bunches, so that interactions between the two beams will take place at discrete intervals never shorter than 25 nanoseconds (ns) apart. However it will be operated with fewer bunches when it is first commissioned, giving it a bunch crossing interval of 75 ns. Prior to being injected into the main accelerator, the particles are prepared by a series of systems that successively increase their energy. The first system is the linear

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particle accelerator LINAC 2 generating 50 MeV protons, which feeds the Proton Synchrotron Booster (PSB). There the protons are accelerated to 1.4 GeV and injected into the Proton Synchrotron (PS), where they are accelerated to 26 GeV. Finally the Super Proton Synchrotron (SPS) is used to further increase their energy to 450 GeV before they are at last injected (over a period of 20 minutes) into the main ring. Here the proton bunches are accumulated, accelerated (over a period of 20 minutes) to their peak 7 TeV energy, and finally circulated for 10 to 24 hours while collisions occur at the four intersection points. The LHC will also be used to collide lead (Pb) heavy ions with a collision energy of 1,150 TeV. The Pb ions will be first accelerated by the linear accelerator LINAC 3, and the Low-Energy Ion Ring (LEIR) will be used as an ion storage and cooler unit. The ions then will be further accelerated by the PS and SPS before being injected into LHC ring, where they will reach an energy of 2.76 TeV per nucleon. 1.

Detectors Six detectors have been constructed at the LHC, located underground in large caverns excavated at the LHC's intersection points. Two of them, the ATLAS experiment and the Compact Muon Solenoid (CMS), are large, general purpose particle detectors. A Large Ion Collider Experiment (ALICE) and LHCb have more specific roles and the last two TOTEM and LHCf are very much smaller and are for very specialized research. The BBC's summary of the main detectors is: •

ATLAS – one of two general purpose detectors. ATLAS will be used to look for signs of new physics, including the origins of mass and extra dimensions.

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CMS – the other general purpose detector will, like ATLAS, hunt for the Higgs boson and look for clues to the nature of dark matter.

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ALICE – will study a "liquid" form of matter called quark-gluon plasma that existed shortly after the Big Bang.

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LHCb – equal amounts of matter and anti-matter were created in the Big Bang. LHCb will try to investigate what happened to the "missing" anti-matter.

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How does the LHC work? The LHC accelerates two beams of atomic particles in opposite directions around the 27km collider. When the particle beams reach their maximum speed the LHC allows them to ‘collide’ at 4 points on their circular journey. Thousands of new particles are produced when particles collide and detectors, placed around the collision points, allow scientists to identify these new particles by tracking their behaviour. The detectors are able to follow the millions of collisions and new particles produced every second and identify the distinctive behaviour of interesting new particles from among the many thousands that are of little interest. As the energy produced in the collisions increases researchers are able to peer deeper into the fundamental structure of the Universe and further back in its history. In these extreme conditions unknown atomic particles may appear.

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Expected results Once the supercollider is up and running, CERN scientists estimate that if the Standard Model is correct, a single Higgs boson may be produced every few hours. At this rate, it may take up to three years to collect enough data to discover the Higgs boson unambiguously. Similarly, it may take one year or more before sufficient results concerning supersymmetric particles have been gathered to draw meaningful conclusions.

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Proposed upgrade The LHC is still new, but its successor - the International Linear Collider (ILC) – is already being discussed. So why build two high energy colliders that operate on the same principles? The LHC is a ‘discovery’ machine, a general purpose tool that will open up new areas of physics and demonstrate the existence, or not, of predicted new laws and particles. The ILC is a precision instrument that will allow scientists to explore in detail the discoveries made by the LHC. The ILC is still at the planning stage, no location for the machine has been agreed and much feasibility testing has to be conducted before the construction phase.

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Cost The total cost of the project is expected to be €3.2–6.4 billion. The construction of LHC was approved in 1995 with a budget of 2.6 billion Swiss francs (€1.6 billion), with another 210 million francs (€140 million) towards the cost of the experiments. However, cost over-runs, estimated in a major review in 2001 at around 480 million francs (€300 million) for the accelerator, and 50 million francs (€30 million) for the experiments, along with a reduction in CERN's budget, pushed the completion date from 2005 to April 2007. The superconducting magnets were responsible for 180 million francs (€120 million) of the cost increase. There were also engineering difficulties encountered while building the underground cavern for the Compact Muon Solenoid, in part due to faulty parts loaned to CERN by fellow laboratories Argonne National Laboratory, Fermilab, and KEK.

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Safety of particle collisions 7

The upcoming experiments at the Large Hadron Collider have sparked fears among the public that the LHC particle collisions might produce doomsday phenomena, involving the production of stable microscopic black holes or the creation of hypothetical particles called strangelets. Two CERN-commissioned safety reviews have examined these concerns and concluded that the experiments at the LHC present no danger and that there is no reason for concern, a conclusion expressly endorsed by the American Physical Society, the world's second largest organization of physicists.

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Operational challenges The size of the LHC constitutes an exceptional engineering challenge with unique operational issues on account of the huge energy stored in the magnets and the beams. While operating, the total energy stored in the magnets is 10 GJ (equivalent to one and a half barrels of oil or 2.4 tons of TNT) and the total energy carried by the two beams reaches 724 MJ (about a tenth of a barrel of oil, or half a lightning bolt). Loss of only one ten-millionth part (10−7) of the beam is sufficient to quench a superconducting magnet, while the beam dump must absorb 362 MJ, an energy equivalent to that of burning eight kilograms of oil, for each of the two beams. These immense energies are even more impressive considering how little matter is carrying it: under nominal operating conditions (2,808 bunches per beam, 1.15×1011 protons per bunch), the beam pipes contain 1.0×10-9 gram of hydrogen, which, in standard conditions for temperature and pressure, would fill the volume of one grain of fine sand. On 10 August 2008, computer hackers defaced a website at CERN, criticizing their computer security. There was no access to the control network of the collider.

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Construction accidents and delays •

On 25 October 2005, a technician was killed in the LHC tunnel when a crane load was accidentally dropped.

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On 27 March 2007 a cryogenic magnet support broke during a pressure test involving one of the LHC's inner triplet (focusing quadrupole) magnet assemblies, provided by Fermilab and KEK. No one was injured. Fermilab director Pier Oddone stated "In this case we are dumbfounded that we missed some very simple balance of forces". This fault had been present in the original design, and remained during four engineering reviews over the following years. Analysis revealed that its design, made as thin as possible for better insulation, was not strong enough to withstand the forces generated during pressure testing. Details are available in a statement from Fermilab, with which CERN is in agreement. Repairing the broken magnet and reinforcing the eight identical assemblies used by LHC delayed the startup date, then planned for November 2007. 8

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Problems occurred on 19 September 2008 during powering tests of the main dipole circuit, when an electrical fault in the bus between magnets caused a rupture and a leak of six tonnes of liquid helium. The operation was delayed for several months.The LHC is expected to be restarted at the end September 2009 with first collisions happening in October. It is currently believed that a faulty electrical connection between two magnets caused an arc, which compromised the liquid-helium containment. Once the cooling layer was broken, the helium flooded the surrounding vacuum layer with sufficient force to break 10-ton magnets from their mountings. The explosion also contaminated the proton tubes with soot.

Latest News from the LHC LHC inauguration at CERN, 21 October 2008 Swiss President Pascal Couchepin and French Prime Minister François Fillon were joined by science ministers from CERN’s Member States and around the world to inaugurate the Large Hadron Collider as planned on the 21st October 2008. CERN explains what happened to the LHC and the timetable for repair CERN has confirmed that a fault in an electrical connection between two magnets caused the incident on the 19th September, which has temporarily shut down preparation of the LHC for experiments. The fault led to mechanical damage and a release of liquid helium, which contributed to further damage to the affected sub-sector of the machine. All of the safety systems operated as expected and no one was put at risk. CERN has the spares and resources available to complete replacement and repair during the scheduled CERN-wide, maintenance shutdown over winter. Checks and modifications will ensure that similar failures do not occur elsewhere once the LHC restarts in Spring 2009. What had been an exceptionally smooth early commissioning phase for the LHC, following first injection of proton beams on the 10th September , was halted by a technical failure on the 19th September . The first few days of commissioning beams in the LHC had encountered some technical problems, which had been resolved . However, the damage caused by failure of an electrical connection (during a test) has required that the affected sector be warmed up for repair. The worldwide LHC Computing Grid is an essential element of the LHC project, responsible for the analysis and management of the more than 15 million Gigabytes of data flowing from the LHC every year. The LHC Grid combines the power of more than 140 computer centres across a 33 country collaboration.

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The launch of the LHC project was be covered live by international broadcasters and followed by an audience estimated (unofficially) to be close to 1 billion. UK media organisations were at CERN and at a simultaneous media event in London. The extensive preparations for the start of LHC experiments have included exhaustive safety assessments, including the potential risk of creating new particles, black holes etc.

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