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Large Hadron Collider
(LHC)

Plan of the LHC experiments and the preaccelerators.
LHC experiments
ATLAS	A Toroidal LHC Apparatus
CMS	Compact Muon Solenoid
LHCb	LHC-beauty
ALICE	A Large Ion Collider Experiment
TOTEM	Total Cross Section, Elastic Scattering and Diffraction Dissociation
LHCf	LHC-forward
MoEDAL	Monopole and Exotics Detector At the LHC
FASER	ForwArd Search ExpeRiment
SND	Scattering and Neutrino Detector
LHC preaccelerators
p and Pb	Linear accelerators for protons (Linac 4) and lead (Linac 3)
(not marked)	Proton Synchrotron Booster
PS	Proton Synchrotron
SPS	Super Proton Synchrotron

The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator complex, intended to collide opposing beams of protons (one of several types of hadrons) with very high kinetic energy. Its main purpose is to explore the validity and limitations of the Standard Model, the current theoretical picture for particle physics. It is theorized that the collider will confirm the existence of the Higgs boson. This would supply a crucial missing link in the Standard Model and explain how other elementary particles acquire properties such as mass.

The LHC was built by the European Organization for Nuclear Research (CERN), and lies underneath the Franco-Swiss border between the Jura Mountains and the Alps near Geneva, Switzerland. It is funded by and built in collaboration with over eight thousand physicists from over eighty-five countries as well as hundreds of universities and laboratories. The LHC is operational and is presently in the process of being prepared for collisions. The first beams were circulated through the collider on 10 September 2008, and the first high-energy collisions are expected to take place after 6-8 weeks.

Although there have been questions concerning the safety of the Large Hadron Collider in the media and even through the courts, the consensus in the scientific community is that there is no conceivable threat from the LHC particle collisions.

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 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 tesla (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 99.999999% of the speed of light. It will take less than 90 microsecond (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 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 stored 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 Injector 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.

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 so-called general purpose detectors. Atlas will be used to look for signs of new physics, including the origins of mass and extra dimensions.

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

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

• 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.

Purpose

When in operation, about seven thousand scientists from eighty countries will have access to the LHC. 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:

• 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? See also Hierarchy problem.
• Is Supersymmetry realised in nature, implying that the known Standard Model particles have supersymmetric partners?
• Will the more precise measurements of the masses and decays of the quarks continue to be mutually consistent within the Standard Model?
• Why are there apparent violations of the symmetry between matter and antimatter? See also CP-violation.
• 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."

As an ion collider

The LHC physics programme is mainly based on proton–proton collisions. However, shorter running periods, typically one month per year, with heavy-ion collisions are included in the programme. While lighter ions are considered as well, the baseline scheme deals with lead ions. This will allow an advancement in the experimental programme currently in progress at the Relativistic Heavy Ion Collider (RHIC). The aim of the heavy-ion programme 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.

Test timeline

The first beam was circulated through the collider on the morning of 10 September 2008. CERN successfully fired the protons around the tunnel in stages, three kilometres at a time. The particles were fired in a clockwise direction into the accelerator and successfully steered around it at 10:28 local time. The LHC successfully completed its first major test: after a series of trial runs, two white dots flashed on a computer screen showing the protons traveled the full length of the collider. It took less than one hour to guide the stream of particles around its inaugural circuit. CERN next successfully sent a beam of protons in a counterclockwise direction, taking slightly longer at one and a half hours due to a problem with the cryogenics, with the full circuit being completed at 14:59.

The first high-energy collisions are expected to take place 6-8 weeks after the start of LHC commissioning on September 10. In the 2008 run, however, the LHC will operate at a reduced energy of 10 TeV. The winter shut-down (starting likely around end of November) will then be used to train the superconducting magnets, such that the 2009 run will start at the full 14 TeV design energy.

Expected results

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

Proposed upgrade

After some years of running, any particle physics experiment typically begins to suffer from diminishing returns; each additional year of operation discovers less than the year before. The way around the diminishing returns is to upgrade the experiment, either in energy or in luminosity. A luminosity upgrade of the LHC, called the Super LHC, has been proposed, to be made after ten years of LHC operation. The optimal path for the LHC luminosity upgrade includes an increase in the beam current (i.e., the number of protons in the beams) and the modification of the two high-luminosity interaction regions, ATLAS and CMS. To achieve these increases, the energy of the beams at the point that they are injected into the (Super) LHC should also be increased to 1 TeV. This will require an upgrade of the full pre-injector system, the needed changes in the Super Proton Synchrotron being the most expensive.

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 and Fermilab.

David King, the former Chief Scientific Officer for the United Kingdom, has criticised the LHC for taking a higher priority for funds than solving the Earth's major challenges; principally climate change, but also population growth and poverty in Africa.

Computing resources

The LHC Computing Grid is being constructed to handle the massive amounts of data produced by the Large Hadron Collider. It incorporates both private fiber optic cable links and existing high-speed portions of the public Internet, enabling data transfer from CERN to academic institutions around the world.

The distributed computing project LHC@home was started to support the construction and calibration of the LHC. The project uses the BOINC platform to simulate how particles will travel in the tunnel. With this information, the scientists will be able to determine how the magnets should be calibrated to gain the most stable "orbit" of the beams in the ring.

Safety issues

Safety of particle collisions

The upcoming experiments at the Large Hadron Collider have sparked fears among the public that the LHC particle collisions might produce doomsday phenomena, including dangerous microscopic black holes and strange matter. 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.

Operational safety

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) 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, 1.15×10 protons per bunch), the beam pipes contain 1.0×10 gram of hydrogen, which, in standard conditions for temperature and pressure, would fill the volume of one grain of fine sand.

Construction accidents and delays

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

In popular culture

The Large Hadron Collider was featured in Angels & Demons by Dan Brown, which involves dangerous antimatter created at the LHC used as a weapon against the Vatican. CERN published a "Fact or Fiction?" page discussing the accuracy of the book's portrayal of the LHC, CERN, and particle physics in general. The movie version of the book has footage filmed on-site at one of the experiments at the LHC; the director, Ron Howard, met with CERN experts in an effort to make the science in the story more accurate.

CERN employee Katherine McAlpine's "Large Hadron Rap" surpassed two million YouTube views on 10 September 2008.

BBC Radio 4 commemorated the switch-on of the LHC on 10 September 2008 with "Big Bang Day". Included in this event was a radio episode of the TV series Torchwood, with a plot involving the LHC, entitled Lost Souls. CERN's director of communications, James Gillies, commented, "The CERN of reality bears little resemblance to that of Joseph Lidster's Torchwood script."

References

Notes

1. ^ CERN Communication Group (2008). "CERN FAQ — LHC: the guide" (PDF). Geneva: CERN. p. 44. Retrieved 2008-09-12. {{cite web}}: Unknown parameter |month= ignored (help)
2. ^ Achenbach, Joel (2008-03-01). "The God Particle". National Geographic Magazine. National Geographic Society. ISSN 0027-9358. Retrieved 2008-02-25.
3. "The Z factory". European Organization for Nuclear Research.
4. ^ "LHC commissioning with beam". CERN.
5. ^ Operational challenges of the LHC. 53 Microsoft PowerPoint slides.
6. Paul Rincon (9 September 2008). "Cern collider ready for power-up". BBC. Retrieved 2008-09-09.
7. "...in the public presentations of the aspiration of particle physics we hear too often that the goal of the LHC or a linear collider is to check off the last missing particle of the Standard Model, this year’s Holy Grail of particle physics, the Higgs boson. The truth is much less boring than that! What we’re trying to accomplish is much more exciting, and asking what the world would have been like without the Higgs mechanism is a way of getting at that excitement." -Chris Quigg, Nature's Greatest Puzzles
8. Randall, Lisa. "Extra Dimensions and Warped Geometries.] Science. Vol. 296, 24 May 2002" (PDF).
9. Stephen Hawking interviewed by the BBC on the switch-on of the Large Hadron Collider. news.bbc.co.uk. Retrieved on 10 September 2008.
10. "Ions for LHC".
11. "Success for 'Big Bang' experiment". BBC.
12. "First beam in the LHC - accelerating science". CERN. Retrieved 2008-09-10.
13. timesonline.co.uk, Scientists cheer as protons complete first circuit of Big Bang machine
14. "Putting Memory to Work". Fermilab.
15. "PDF presentation of proposed LHC upgrade" (PDF).
16. Maiani, Luciano (16 October 2001). "LHC Cost Review to Completion". CERN. Retrieved 2001-01-15.
17. Feder, Toni (2001). "CERN Grapples with LHC Cost Hike". Physics Today. 54 (12): 21. doi:10.1063/1.1445534. Retrieved 2007-01-15. {{cite journal}}: Unknown parameter |month= ignored (help)
18. Amostitle, Jonathan (8 September 2008). "'Climate crisis' needs brain gain". BBC.
19. Boyle, Alan (2 September 2008). "Courts weigh doomsday claims". Cosmic Log. msnbc.com.
20. Blaizot JP, Iliopoulos J, Madsen J, Ross G, Sonderegger P, Specht H (2003). Study of Potentially Dangerous Events During Heavy-Ion Collisions at the LHC. CERN. Geneva. CERN-2003-001.
21. Ellis J, Giudice G, Mangano ML, Tkachev I, Wiedemann U (LHC Safety Assessment Group) (5 September 2008). "Review of the Safety of LHC Collisions". ''Journal of Physics G: Nuclear and Particle Physics. 35, 115004 (18pp). doi:10.1088/0954-3899/35/11/115004. CERN record. arXiv:0806.3414.
22. "The safety of the LHC". CERN 2008 (CERN website).
23. "Statement by the Executive Committee of the DPF on the Safety of Collisions at the Large Hadron Collider" issued by the Division of Particles & Fields (DPF) of the American Physical Society (APS)
24. "Challenges in accelerator physics".
25. "Beam Parameters and Definitions" (PDF).
26. "Message from the Director-General" (Press release) (in English and French). CERN. 26 October 2005. Retrieved 2007-01-15.{{cite press release}}: CS1 maint: unrecognized language (link)
27. "Fermilab'Dumbfounded'by fiasco that broke magnet".
28. "LHC Magnet Test Failure".
29. "Updates on LHC inner triplet failure".
30. "The God Particle". www.bbc.com. Retrieved 2007-05-22.
31. "Angels and Demons". CERN. Retrieved 2008-07-16.
32. Perkins, Ceri. "ATLAS gets the Hollywood treatment". ATLAS e-News. CERN. Retrieved 2008-07-16.
33. "YouTube - Large Hadron Rap". Youtube.com. Retrieved 2008-09-09.
34. "Large Hadron Rap links and lyrics".
35. "Rap about world's largest science experiment becomes YouTube hit". Telegraph. Last Updated: 2 September 2008
36. Bogo, Jennifer (1 August 2008). "Large Hadron Collider Rap Teaches Particle Physics in 4 Minutes". Popular Mechanics.
37. "BBC - Radio 4 - Big Bang Day". BBC. 2008-09-10. Retrieved 2008-09-11.
38. "Programming for Big Bang Day on BBC Radio 4". BBC Press Office. Retrieved 2008-08-11.
    "Radio 4 - Big Bang Day". BBC. Retrieved 2008-09-10.
    Donovan, Paul (2008-09-07). "The BBC has Big Bang to rights". The Sunday Times. Retrieved 2008-09-11.
39. Gillies, James. "CERN in Science-Fiction". BBC Radio 4 website. BBC. Retrieved 2008-09-11.

Bibliography

• Energising the quest for 'big theory'
• symmetry magazine LHC special issue August 2006, special issue December 2007
• BBC Horizon, The six billion dollar experiment
• New Yorker: Crash Course. The world’s largest particle accelerator.
• NYTimes: A Giant Takes On Physics’ Biggest Questions.
• Large Hadron Collider, Ingenia magazine
• Birth of a Giant, CNRS magazine
• Why a Large Hadron Collider? Seed Magazine interviews with physicists.

External links

• Official website
• Lyndon Evans and Philip Bryant (editors) (2008-08-14), "LHC Machine", Journal of Instrumentation, 3 (S08001): S08001, doi:10.1088/1748-0221/3/08/S08001 {{citation}}: |author= has generic name (help). Full documentation for design and construction of the LHC and its six detectors (1600 p).
• Official Timeline of LHC Milestones
• TED talks (video) - Brian Cox: What really goes on at the Large Hadron Collider
• Overview of the LHC at CERN's public webpage
• ATLAS Experiment - Virtual Reality photography panoramas

• Ain
• Canton of Geneva
• Large Hadron Collider
• Particle physics facilities
• E-Science