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 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 a particle accelerator and collider located at CERN, near Geneva, Switzerland (46°14′N 6°03′E / 46.233°N 6.050°E / 46.233; 6.050). Currently under construction, the LHC is scheduled to begin operation in May 2008. The LHC is expected to become the world's largest and highest energy particle accelerator. The LHC is being funded and built in collaboration with over two thousand physicists from thirty-four countries, universities and laboratories.
When activated, it is hoped that the collider will produce the elusive Higgs boson — often dubbed the God Particle — the observation of which could confirm the predictions and 'missing links' in the Standard Model of physics, and explain how other elementary particles acquire properties such as mass. The verification of the existence of the Higgs boson would be a significant step in the search for a Grand Unified Theory which seeks to unify three of the four fundamental forces: electromagnetism, the strong force, and the weak force. The Higgs boson may also help to explain why the remaining force, gravitation, is so weak compared to the other three forces.
Technical design
The collider is contained in a circular tunnel with a circumference of 26.659 kilometres (16.5 mi), at a depth ranging from 50 to 175 metres underground. The tunnel was formerly used to house the LEP, an electron-positron collider.
The 3.8 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 ninety 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 twenty-five nanoseconds apart. When the collider is first commissioned, it will be operated with fewer bunches, to give a bunch crossing interval of seventy-five nanoseconds. The number of bunches will later be increased to give a final bunch crossing interval of twenty-five 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 first system is the linear accelerator Linac2 generating 50 MeV protons which feeds the Proton Synchrotron Booster (PSB). Protons are then injected at 1.4 GeV into the Proton Synchrotron (PS) at 26 GeV. Finally the Super Proton Synchrotron (SPS) can be used to increase the energy of protons up to 450 GeV.
The LHC can also be used to collide heavy ions such as lead (Pb) with a collision energy of 1,150 TeV. The 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 are then further accelerated by the Proton Synchrotron (PS) and Super Proton Synchrotron (SPS).
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 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 of the beam is sufficient to quench a superconducting magnet, while the beam dump must absorb an energy equivalent to a typical air-dropped bomb. For comparison, 725 MJ is equivalent to the detonation energy of approximately 157 kg (347 pounds) of TNT, and 10 GJ is about 2.5 tons of TNT.
Research
When in operation, about seven thousand scientists from eighty countries will have access to the LHC, the largest national contingent of seven hundred being from the United States. Physicists hope to use the collider to test various grand unified theories such as Lisi's Exceptionally Simple Theory and enhance their ability to answer the following questions:
- Is the popular Higgs mechanism for generating elementary particle masses in the Standard Model realised in nature? If so, how many Higgs bosons are there, and what are their masses?
- Will the more precise measurements of the masses of baryons continue to be mutually consistent within the Standard Model?
- Do particles have supersymmetric ("SUSY") partners?
- Why are there apparent violations of the symmetry between matter and antimatter?
- Are there extra dimensions, as predicted by various models inspired by string theory, and can we "see" them?
- What is the nature of dark matter and dark energy?
- Why is gravity so many orders of magnitude weaker than the other three fundamental forces?
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 programme. While lighter ions are considered as well, the baseline scheme deals with lead (Pb) ions. This will allow an advancement in the experimental programme currently in progress at the Relativistic Heavy Ion Collider (RHIC).
Proposed upgrade
After some years of running, any particle physics experiment typically begins to suffer from diminishing returns. 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 construction of LHC was originally approved in 1995 with a budget of 2.6 billion Swiss francs (currently about 1.7 billion euro), with another 210 million francs (€140 m) towards the cost of the experiments. However, cost over-runs, estimated in a major review in 2001 at around 480 million francs (€300 m) in the accelerator, and 50 million francs (€30 m) for the experiments, along with a reduction in CERN's budget pushed the completion date out from 2005 to April 2007. 180 million francs (€120 m) 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.
LHC@Home
Main article: LHC@homeLHC@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.
Safety concerns and assurances
As with previous particle accelerators, people both inside and outside 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. This has raised controversy as to whether any such risks outweigh the potential benefits of constructing and operating the LHC.
Though the standard model predicts that LHC energies are far too low to create black holes, some nonstandard theories lower the requirements, and predict that the LHC will create tiny black holes, with potentially devastating consequences. The primary cause for concern is that Hawking Radiation - a postulated means by which any such black holes would dissipate before becoming dangerous, remains entirely theoretical. In academia, the theory of Hawking Radiation is considered plausible, but there remains considerable question of whether it is correct.
Other disaster scenarios typically involve the following theoretical events:
- Creation of strange matter that is more stable than ordinary matter
- Creation of magnetic monopoles that could catalyze proton decay
- Creation of a strangelet
CERN has pointed out that the probability of such events is extremely small. One argument for the safety of colliders such as the LHC states that if the Earth were in danger of any such fate, the Earth and Moon would have met that fate billions of years ago due to their constant bombardment from space by very high energy cosmic rays such as protons and other particles, which are millions of times more energetic than anything that could be produced by the LHC.
Quantum calculations presented in the CERN report predict that:
- Any black holes created by the LHC are not expected to be stable and will not accrete matter.
- Any monopoles that could catalyse the decay of matter will quickly exit the Earth.
Construction accidents and delays
On October 25, 2005, a technician was killed in the LHC tunnel when a crane load was accidentally dropped.
On March 27, 2007, there was an incident during a pressure test involving one of the LHC's inner triplet magnet assemblies provided by Fermilab and KEK. No people were injured, but a cryogenic magnet support broke. 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 copies used by LHC, in addition to a number of other small delays, caused a postponement of the planned November 26, 2007 startup date to May 2008.
See also
Notes and references
- New start-up schedule for world's most powerful particle accelerator
- Symmetry magazine, April 2005
- "...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
- Ions for LHC
- PDF presentation of proposed LHC upgrade
- Maiani, Luciano (16 October 2001). "LHC Cost Review to Completion". CERN. Retrieved 2001-01-15.
- Feder, Toni (2001). "CERN Grapples with LHC Cost Hike". Physics Today. 54 (12): 21. Retrieved 2007-01-15.
{{cite journal}}: Unknown parameter|month=ignored (help) - American Institute of Physics Bulletin of Physics News, Number 558, September 26, 2001, by Phillip F. Schewe, Ben Stein, and James Riordon
- Dimopoulos, S. and Landsberg, G. Black Holes at the Large Hadron Collider. Phys. Rev. Lett. 87 (2001).
- Adam D. Helfer: General Relativity and Quantum Cosmology
- Tiny Black Holes - Physicist Dave Wark of Imperial College, London reporting for NOVA scienceNOW
- Blaizot, J.-P. et al. Study of Potentially Dangerous Events During Heavy-Ion Collisions at the LHC. (PDF)
- Hewett, JoAnne (25 October 2005). "Tragedy at CERN" (Blog). Cosmic Variance. Retrieved 2007-01-15. author and date indicate the beginning of the blog thread
- "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) - LHC Magnet Test Failure
- Updates on LHC inner triplet failure
- "The God Particle". www.bbc.com. Retrieved 2007-05-22.
- "CERN announces new start-up schedule for world's most powerful particle accelerator" (Press release). CERN. 2007-06-22. Retrieved 2007-07-01.
External links
- LHC - The Large Hadron Collider webpage
- Challenges in Accelerator Physics
- LHC UK webpage
- US LHC webpage
- UK Science Museum, London Exhibition supported by the Science and Technology Facilities Council
- The Alice experiment
- Compact Muon Solenoid (CMS) Main Page
- Compact Muon Solenoid Page (U.S. Collaboration)
- Energising the quest for 'big theory'
- LCG - The LHC Computing Grid webpage
- The Large Hadron Collider ATLAS Experiment - Virtual Reality (VR) photography panoramas (requires QuickTime)
- LHC startup plan. Includes dates, energies and luminosities
- Seed short film - Lords of the Ring
- symmetry magazine LHC special issue
- BBC Horizon, The six billion dollar experiment
- New Yorker: Crash Course. The world’s largest particle accelerator (ca. 6 500 words)
- NYTimes: A Giant Takes On Physics’ Biggest Questions (ca. 4 300 words)
- Beam Parameters and Definitions. The chapter of the LHC Technical Design Report (TDR) that lists of all the beam parameters for the LHC.
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