Large Hadron Collider

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The Large Hadron Collider (LHC) is a particle collider at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland. It is a proton-proton collider, holding the record for highest energy collisions produced by a collider experiment. The LHC is a synchrotron accelerator 27 km (17 mi) in circumference, sitting up to 175 m below ground on the Franco-Swiss border. The collider is running at 7 TeV (Terra-electron-volts) Center-of-Mass (COM) energy as of March 14 2011, and will achieve 14 TeV COM collisions once running at full design energy (predicted for 2014). The LHC physics program has many goals including the search for the Higgs Boson, the last unobserved particle predicted by the Standard Model, as well as investigation of the validity of theories “beyond” the Standard Model such as Supersymmetry, Technicolor, and Extra Dimensions. In addition to proton-proton collisions, the LHC can collide Lead ions, producing conditions comparable to those present immediately after the Big Bang.


Before the LHC, CERN was home to the Large Electron-Positron Collider (LEP), which occupied the same tunnel that is now home to the LHC. Running from 1989 to 2000, LEP set the energy record for a lepton collider with collisions above 200 GeV. Construction of the LHC began immediately upon the decommissioning of LEP. Between Summer 2001 and November 2006, over 1600 superconducting magnets were installed in the tunnel. The detectors were assembled at the same time, beginning with LHCb in 2003. The detectors were slowly assembled in their respective caverns between 2003 and 2008.

On September 10th 2008 the first proton bunches were circulated in the main ring. The beam circulated for three day while the machine was carefully monitored and tested. On September 19th (during a short period without beam), a helium enclosure was punctured due to the quench of a superconductor during electrical testing of the dipole magnets. This caused a rapid pressure and temperature increase inside the magnets, damaging several hundred meters of magnets and destroying vacuum conditions [3]. Beam was not able to circulate again until November 2009. Research officially began in March 2010, with 7 TeV COM collisions. The Collider ran at this energy until the end of October 2010.

Detectors & Collaborations

Detectors collect the data from a collider experiment by observing the particles exiting from the collision. At the LHC, there are four detectors: ALICE, ATLAS, CMS, and LHCb. The largest detectors are ATLAS (A Toroidal Lhc ApparatuS) and CMS (Compact Muon Solenoid), which are ‘multi-purpose’ detectors designed to perform a wide range of analyses. They observe proton-proton collisions to improve previous measurements and observe any new phenomena. These detectors are composed of layers of sub-detector elements. The inner detector consists of various tracking components. Particles leaving the collision interact with the tracking elements providing information about the path the particle followed. This part of the detector is immersed in a magnetic field so that charged particles will follow a bent path characteristic of their charge and momentum. The next section contains the calorimeters (both hadronic and EM). These measure the energy deposited by the particles. The outermost section is the muon spectrometer, also immersed in a magnetic field, which observes the tracks of muons which will generally pass undetected through the inner parts of the detector.

File:0610006 01-A5-at-72-dpi.jpg
The ATLAS detector during assembly. The toroidal magnets, muon chamber, and inner detector are visible. Note the people on the bottom left for scale. (ATLAS Experiment © 2011 CERN)

ALICE (A Large Ion Collider Experiment) and LHCb (LHC beauty) are smaller, more specialized detectors. The ALICE detector is devoted entirely to observing heavy ion collisions, which occur during periods when the LHC switches from proton collisions to lead ion collisions. LHCb is focused on studying bottom quark (or “beauty” quark) physics.

Each detector has an associated collaboration. These international collaborations are each comprised of thousands of physicists who study the data collected by the detector and publish the results.

Standard Model Higgs

One of the main objectives of the ATLAS and CMS collaborations is observation of the scalar Higgs boson. As of March 2011, existence of the Higgs has been excluded in the mass ranges mH<114 GeV (LEP), 156-175 GeV (Tevatron), and mH>185 GeV (indirect electroweak measurements). Thus, the LHC will look for a Higgs in the mass ranges 114-156 GeV and 175-185 GeV.[4]

At the LHC, Higgs bosons will be produced mostly by gluon- gluon fusion (see figure). The leading order (LO) diagram is at the loop level via top quarks, with an LO cross-section of roughly 20 pb (for 14 TeV collisions) [1]. Higher level corrections to the gluon fusion production cross-section are substantial, and the NNLO cross section is roughly 45 pb (√s=14 TeV ) [2]. A smaller contribution to the total production cross-section comes from weak boson fusion (WW/ZZ->H), with σWeak~ 4 pb. Once operating at design luminosity (~100 fb-1/year), the LHC will thus produce about 500 Standard Model Higgs per year, assuming one exists.

The branching ratios of the Higgs decay are very mass dependent, even within the mass range not yet excluded. For a light Higgs (mH<135 GeV), the dominant decay is to a bottom quark pair. This is not an ideal channel for a Higgs search at the LHC, as the quarks will hadronize into jets, for which the background is extremely large. Decays to weak boson pairs are much less likely, however the final state of such decays (leptons or leptons + ET) is much easier for the detectors to disentangle from the background. This decay is even more accessible for a heavier Higgs (mH>135 GeV), were H->WW becomes the dominant decay mode.