Large Hadron Collider

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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 accelerators for protons (Linac 2) 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, intended to collide opposing particle beams, of either protons at an energy of 7 TeV/particle, or lead nuclei at an energy of 574 TeV/nucleus.

The Large Hadron Collider was built by the European Organization for Nuclear Research (CERN) with the intention of testing various predictions of high-energy physics, including the existence of the hypothesised Higgs boson[1] and of the large family of new particles predicted by supersymmetry.[2] It lies in a tunnel 27 kilometres (17 mi) in circumference, as much as 175 metres (570 ft) beneath the Franco-Swiss border near Geneva, Switzerland. It is funded by and built in collaboration with over 10,000 scientists and engineers from over 100 countries as well as hundreds of universities and laboratories.[3]

On 10 September 2008, the proton beams were successfully circulated in the main ring of the LHC for the first time.[4] On 19 September 2008, the operations were halted due to a serious fault between two superconducting bending magnets.[5] Due to the time required to repair the resulting damage and to add additional safety features, the LHC is scheduled to be operational again no sooner than September 2009.[6]

Contents

[edit] Purpose

A simulated event in the CMS detector, featuring the appearance of the Higgs boson.
A Feynman diagram of one way the Higgs boson may be produced at the LHC. Here, two quarks each emit a W or Z boson, which combine to make a neutral Higgs.

It is hypothesized that the collider will produce the elusive Higgs boson, the last unobserved particle among those predicted by the Standard Model.[7][8] 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:[9]

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."[12]

[edit] 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.[13] (see A Large Ion Collider Experiment). 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.

[edit] Design

Map of the Large Hadron Collider at CERN

The LHC is the world's largest and highest-energy particle accelerator.[14][15] 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.[16] 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.

Superconducting quadrupole electromagnets are used to direct the beams to four intersection points, where interactions between accelerated protons will take place.

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.[17] 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.[18]

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 circulated for 10 to 24 hours while collisions occur at the four intersection points.[19]

CMS detector for LHC

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.

[edit] Detectors

The Large Hadron Collider's (LHC) CMS detectors being installed.

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.[15] 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:[20]

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

[edit] Test timeline

The first beam was circulated through the collider on the morning of 10 September 2008.[21] 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.[22] 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 travelled the full length of the collider. It took less than one hour to guide the stream of particles around its inaugural circuit.[23] 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.

On 19 September 2008, a quench occurred in about 100 bending magnets in sectors 3 and 4, causing loss of approximately six tonnes of liquid helium, which was vented into the tunnel, and a temperature rise of about 100 kelvins in some of the affected magnets. Vacuum conditions in the beam pipe were also lost.[24] Shortly after the incident CERN reported that the most likely cause of the problem was a faulty electrical connection between two magnets, and that—due to the time needed to warm up the affected sectors and then cool them back down to operating temperature—it would take at least two months to fix it.[25] Subsequently, CERN released a preliminary analysis of the incident on 16 October 2008,[26] and a more detailed one on 5 December 2008.[27] Both analyses confirmed that the incident was indeed initiated by a faulty electrical connection. At most 29 magnets have been damaged in the incident and will have to be repaired or replaced during the winter shutdown.

In the original timeline of the LHC commissioning, the first "modest" high-energy collisions at a center-of-mass energy of 900 GeV were expected to take place before the end of September 2008, and the LHC was expected to be operating at 10 TeV by the time of the official inauguration on 21 October 2008.[28] However, due to the delay caused by the above-mentioned incident, the collider will not be operational again before the end of September 2009.[6] Despite the delay, LHC was officially inaugurated on 21 October 2008, in the presence of political leaders, science ministers from CERN's 20 Member States, CERN officials, and members of the worldwide scientific community.[29]

[edit] 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.[14]

[edit] Proposed upgrade

Main article: Super Large Hadron Collider

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,[30] 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 total cost of the project is expected to be €3.2–6.4 billion.[15] 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.[31] The superconducting magnets were responsible for 180 million francs (€120 million) of the cost increase. There were also further costs and delays due to engineering difficulties encountered while building the underground cavern for the Compact Muon Solenoid,[32] and also due to faulty parts provided by Fermilab.[33]

[edit] 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 Open Science Grid is used as the primary infrastructure in the United States, and also as part of an interoperable federation with the LHC Computing Grid.

The distributed computing project LHC@home was started to support the construction and calibration of the LHC. The project uses the BOINC platform, enabling anybody with an internet connection to use their computer idle time 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 of particle collisions

Main article: Safety of particle collisions at the Large Hadron Collider

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.[34] 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,[35][36][37] a conclusion expressly endorsed by the American Physical Society, the world's second largest organization of physicists.[38]

[edit] 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.[19][39] 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).[40]

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.[41][42]

[edit] Construction accidents and delays

  • On 25 October 2005, a technician was killed in the LHC tunnel when a crane load was accidentally dropped.[43]
  • 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.[44] 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.[45][46] Repairing the broken magnet and reinforcing the eight identical assemblies used by LHC delayed the startup date,[47] then planned for November 2007.
Sister project Wikinews has related news: CERN says repairs to LHC particle accelerator to cost US$21 million
  • 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.[48] The LHC is expected to be restarted at the end September 2009 with first collisions happening in October.[6] 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.[27][49]

[edit] In popular culture

The Large Hadron Collider has gained a lot of attention from outside the scientific community and its progress is followed by most popular science media. The LHC has also stirred a lot of imagination in works of fiction such as novels, tv series and video games. As is common in fiction, the portrayal is often only vaguely accurate, something that occasionally has caused concern among the public.

The novel Angels & Demons by Dan Brown, involves antimatter created at the LHC to be used in a weapon against the Vatican. In response CERN published a "Fact or Fiction?" page discussing the accuracy of the book's portrayal of the LHC, CERN, and particle physics in general.[50] 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.[51]

BBC Radio 4 commemorated the switch-on of the LHC on 10 September 2008 with "Big Bang Day".[52] Included in this event was a radio episode of the TV series Torchwood, with a plot involving the LHC, entitled Lost Souls.[53] CERN's director of communications, James Gillies, commented, "The CERN of reality bears little resemblance to that of Joseph Lidster's Torchwood script."[54]

CERN employee Katherine McAlpine's "Large Hadron Rap"[55] surpassed 4.5 million YouTube views.[56][57][58]

[edit] References

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[edit] External links

Hadron Colliders
Intersecting Storage Rings CERN, 1971–1984
Super Proton Synchrotron CERN, 1981–1984
ISABELLE BNL, cancelled in 1983
Tevatron Fermilab, 1987–present
Relativistic Heavy Ion Collider BNL, operational since 2000
Superconducting Super Collider Cancelled in 1993
Large Hadron Collider CERN, 2009–
Very Large Hadron Collider Theoretical
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European Organization for Nuclear Research (CERN)
Experiments
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Fixed target experiments at the SPS
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2008– Large Hadron Collider
ATLAS · CMS · LHCb · TOTEM · LHCf · ALICE
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