List of Tables
1.2 Experiments at the LHC
There are four large scale experiments at the LHC plus several smaller specialized experiments. ATLAS[11] and CMS[10], described in more detail in chapter 2, are the two largest, general purpose experiments. Both have a rich physics program involving measurements of cross-sections of a wide range of processes, searches for exotic particles and particle resonances, topological measurements of particle production, CP violation, Super Symmetry and of course, the Higgs boson and its possible variants.
ATLAS is the largest detector at the LHC, measuring 44 m in length and having a 25 m diameter. The inner detector, comprised of a Silicon Pixel Tracker, Silicon Microstrip Trackers (SCT) and Transition Radiation Trackers (TRT), is immersed in a 2T field generated by a thin superconducting solenoid. Liquid Argon electro-magnetic calorimeters cover the barrel region (|η|<3.2)¶ and a scintillator based hadron calorimeter covers the region η < 1.7. Liquid Argon forward calorime-ters measure electromagnetic and hadronic energy in the pseudorapidity range 1.5 <|η| <4.9. The entire calorimeter is surrounded by the muon spectrometer which includes triggering chambers with 1.5 - 4ns timing resolution. Three large superconducting endcap toroids arranged in an 8-fold azimuthal symmetry around the calorimeters provide a toroidal magnetic field for muon momentum analysis.
ATLAS is a large but open structure, designed to minimize multiple scattering effects which deteriorate the muon momentum resolution. A full, concise descrip-tion of the ATLAS detector can be found in [11].
Located at point 2, ALICE is the only dedicated Heavy Ion physics detector at the LHC. It is designed to investigate the physics of strongly interacting matter and quark-gluon plasma generated in the collisions of lead (Pb) nuclei. It also operates during the nominal proton-proton (pp) collisions to provide reference data for the heavy-ion program and to search for other strong-interaction processes that will complement data from the other LHC experiments. The ALICE detector dimen-sions are 16 m×16 m×26 m and has a total weight of approximately 10,000 tonnes.
¶See section 6.2 for explanation of pseudorapidity.
Chapter 1. Introduction to the LHC & CMS detector 9 The central barrel of ALICE measures hadrons, electrons and photons using an In-ner Tracker System made from high resolution silicon pixel detectors, silicon drift detectors, silicon strip detectors, a cylindrical Projection Chamber, Time-of-Flight Ring Imaging Cherenkov, Transition Radiation detectors and finally two electromagnetic calorimeters, providing an impressive array of particle detection, tracking and identification techniques. The forward regions (2.5≤ |η| ≤4) contain the muon spectrometers which measure the trajectories of pairs of muons, partic-ularly from the decay of J/ψ and ψ’. ALICE employs the use of two magnets. A 0.5 T solenoid magnet houses the central detectors and provides the bending power for momentum measurements of high pT particles. A separate dipole magnet is located 7 m from the interaction point and forms part of the muon spectrometer, extending the horizontal magnetic field beyond the reach of the solenoid magnet [7].
The LHCb detector is a single-arm spectrometer with a forward angular coverage [8]. Its interaction region is displaced by 11.25 m from the center of the cavern, to-wards IR7 to allow optimum use of the pre-existing cavern which originally housed the DELPHI experiment at LEP. LHCb is designed to run at a lower luminosity than other experiments with L . 5×1032cm−2s−1 and maintaining the average number of visible collisions per bunch crossing to approximately one. Due to the nature of LHCb’s physics goals, the design of the detector was driven by the need for a hardware-based trigger with short latency capable of selecting the decays of b-hadrons. This b-tagging method needs to work quickly and is achieved through the use of a silicon vertex detector. A dipole magnet at the center of the LHCb experiment is employed for measuring the momentum of charged particles. Un-like CMS, the magnet design does not use superconducting coils and provides an integrated field of 3.6 Tm over a region of 2.5 - 7.95 m from the interaction region.
Summary
The experiments at the LHC will cover a staggeringly wide range of cutting-edge topics in the field of particle physics research, all of which will have a direct bearing on our understanding of the birth and evolution of the Universe. The physics goals of ALICE will bring a better understanding of the Strong interaction and in-turn explain how nuclei formed in the initial microseconds of the Universe’s creation by
attempting to extract the quarks and gluons from the colliding nuclei and breaking the apparent rule of confinement. LHCb hopes to uncover new physics in regards to matter/anti-matter asymmetry through yet undiscovered sources of CP viola-tion. Currently known examples of CP violation (eg. K0/K0) can be explained¯ by assuming two different interaction pathways, involving weak and strong inter-action effects. However, the mechanism cannot account for the magnitude of CP invariance required to cause the observed abundance of matter over anti-matter.
ATLAS and CMS are both targeting a wide range of topics at the leading edge of particle physics including the search for the Higgs Boson, Dark Matter, Dark Energy & Super Symmetry (SUSY). These searches will continue in parallel with countless other analyses which aim to refine and better understand previous mea-surements.
The experimental discovery and measurements of the underlying mechanisms of these phenomena would have a huge impact on our understanding of particle physics and mark the success of the LHC program. But even if some of these proposals are proven to be incorrect, it would only serve to correct the current course of High Energy Physics and yet again allow physicists to make refinements to the already highly successful Standard Model.