Large Hadron Collider

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The accelerator chain
of the Large Hadron Collider (LHC)
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
LHC preaccelerators
p and Pb Linear accelerator
for proton and Lead
(not marked) Proton Synchrotron Booster
PS Proton Synchrotron
SPS Super Proton Synchrotron

The Large Hadron Collider (LHC) is a particle accelerator and collider located at CERN, near Geneva, Switzerland (46°14′00″N, 6°03′00″E). Currently under construction, the LHC is scheduled to begin operation (at reduced energies) in November 2007. The LHC is expected to become the world's largest and highest energy particle accelerator in 2008, when commissioning at 7 TeV is completed. The LHC is being funded and built in collaboration with over two thousand physicists from 34 countries, universities and laboratories.

The collider is contained in a 27 km circumference tunnel located underground at a depth ranging from 50 to 150 metres [1]. The tunnel was formerly used to house the LEP, an electron-positron collider. The 3 metre diameter, concrete-lined tunnel actually crosses the border between Switzerland and France at four points, although the majority of its length is inside France. The collider itself is located underground, with many surface buildings holding ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants.

The collider tunnel contains two pipes enclosed within superconducting magnets cooled by liquid helium, each pipe containing a proton beam. The two beams travel in opposite directions around the ring. Additional magnets are used to direct the beams to four intersection points where interactions between them will take place.

The protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV. It will take around 90 microseconds for an individual proton to travel once around the collider. Rather than continuous beams, the protons will be "bunched" together into approximately 2,800 bunches, so that interactions between the two beams will take place at discrete intervals never shorter than 25 nanoseconds apart. When the collider is first commissioned, it will be operated with fewer bunches, to give a bunch crossing interval of 75 nanoseconds. The number of bunches will later be increased give the final bunch crossing interval of 25 nanoseconds.

Prior to being injected into the main accelerator, the particles are prepared through a series of systems that successively increase the particle energy levels. The Proton Synchrotron (PS) consists of two linear accelerators generating 50 MeV; the Proton Synchrotron Booster (PSB) produces 1.4 GeV; and the Proton Synchrotron Ring (PSR) 26 GeV. The Low-Energy Injector Ring (LEIR) will be used as an ion storage and cooler unit. The Antiproton Decelerator (AD) will produce a beam of anti-protons at 2 GeV, after cooling them down from 3.57 GeV. Finally the Super Proton Synchrotron (SPS) can be used to increase the energy of protons up to 450 GeV.

Six detectors are being constructed at the LHC. They are located underground, in large caverns excavated at the LHC's intersection points. Two of them, ATLAS and CMS are large, "general purpose" particle detectors. The other four (LHCb, ALICE, TOTEM, and LHCf) are smaller and more specialized.

The LHC can also be used to collide heavy ions such as lead (Pb) with a collision energy of 1,150 TeV.

The size of the LHC constitutes an exceptional engineering challenge with unique safety issues. While running, the total energy stored in the magnets is 10 GJ, and in the beam, 725 MJ. Loss of only 10−7 of the beam is sufficient to quench a superconducting magnet, while the beam dump must discharge an energy equivalent to a considerable quantity of explosives.

Contents

[edit] Research

A Feynman diagram of one way the Higgs boson may be produced at the LHC. Here, two quarks emit a W or Z boson which combine to make a neutral Higgs.
Enlarge
A Feynman diagram of one way the Higgs boson may be produced at the LHC. Here, two quarks emit a W or Z boson which combine to make a neutral Higgs.

Physicists hope to use the collider to enhance their ability to answer the following questions:

[edit] LHC 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 (Pb) ions.[3] This will allow an advancement in the experimental programme currently in progress at the Relativistic Heavy Ion Collider (RHIC).

[edit] LHC proposed upgrade

After some years of running, any particle physics experiment typically begins to suffer from diminishing returns. This is because the statistical precision achievable in the presence of background by any given experiment scales with the square root of the run time. 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,[4] 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.

[edit] Cost

The construction of LHC was originally approved in 1995 with a budget of 2.6 billion Swiss francs, with another 210 million Swiss francs towards the cost of the experiments. However, cost overruns, estimated in a major review in 2001 at around 480 million Swiss francs in the accelerator, and 50 million Swiss francs for the experiments, along with a reduction in CERN's budget pushed the completion date out from 2005 to April 2007.[5] 180 million Swiss francs of the cost increase has been the superconducting magnets. There were also engineering difficulties encountered while building the underground cavern for the Compact Muon Solenoid.[6] The estimated final price tag is expected to be about US$ 8 billion.

[edit] LHC@Home

Main article: LHC@home

LHC@Home, a distributed computing project, 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.

[edit] Safety concerns

As with the Relativistic Heavy Ion Collider (RHIC), people both inside and outside of the physics community have voiced concern that the LHC might trigger one of several theoretical disasters capable of destroying the Earth or even the entire Universe. These include:

CERN performed a study to investigate whether such dangerous events as micro black holes, strangelets, or magnetic monopoles could occur.[8] The report concluded, "We find no basis for any conceivable threat." For instance, it is not possible to produce microscopic black holes unless certain untested theories are correct. Even if they are produced, they are expected to evaporate almost immediately via Hawking radiation and thus to be harmless. Perhaps the strongest argument for the safety of colliders such as the LHC comes from the simple fact that cosmic rays of much higher energies than the LHC can produce have been bombarding the Earth, Moon and other objects in the solar system for billions of years with no such effects.

However, some people remain concerned about the safety of the LHC. As with any new and untested experiment, it is not possible to say with utter certainty what will happen. John Nelson at Birmingham University stated of RHIC that "it is astonishingly unlikely that there is any risk - but I could not prove it."[9] In academia there is some question of whether Hawking radiation is correct.[10]

RHIC has been running since 2000 and has generated no hint of Earth-destroying effects.

[edit] Construction

On October 25, 2005, a technician was killed in the LHC tunnel when a crane load was accidentally dropped. [1] [2]

[edit] See also

CMS detector for LHC
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CMS detector for LHC

[edit] Notes and references

  1. ^ Symmetry magazine, April 2005
  2. ^ "...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
  3. ^ Ions for LHC
  4. ^ PDF presentation of proposed LHC upgrade
  5. ^ LHC Cost Review to Completion, CERN 2001
  6. ^ Toni Feder. CERN Grapples with LHC Cost Hike. Retrieved on 2006-06-12.
  7. ^ Dimopoulos, S. and Landsberg, G. Black Holes at the Large Hadron Collider. Phys. Rev. Lett. 87 (2001).
  8. ^ Blaizot, J.-P. et al. Study of Potentially Dangerous Events During Heavy-Ion Collisions at the LHC. (PDF)
  9. ^ Jonathan Leake:Big Bang machine could destroy Earth, Sunday Times
  10. ^ Adam D. Helfer: General Relativity and Quantum Cosmology

[edit] External links

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