Wing Sheung Chan

28 The Large Hadron Collider and the ATLAS detector 2.1. The CERN accelerator complex and the LHC The European Organization for Nuclear Research, also known as CERN, is a world-leading institute for frontier particle physics research. Among other experiments, it maintains and operates the world’s most powerful particle accelerator and collider, the Large Hadron Collider (LHC). 2.1.1. The CERN accelerator complex Located at the French-Swiss border close to Geneva, CERN’s main site is the home to many particle accelerators built and used throughout the 66-year history of CERN. Although many of these accelerators are no longer the most energetic accelerators as they once were, physicists capitalise on them by using them to pre-accelerate particles before feeding them into the LHC. These accelerators form a chain that accelerates particles in successive stages of increasingly higher energies. This chain of accelerators is known as the CERN accelerator complex. Figure 2.1 provides a schematic representation of the CERN accelerator complex during the second operational run (Run 2) of the LHC. When the complex is operating for proton-proton ( pp ) collisions, protons are first extracted from hydrogen atoms by ionisation and injected into the linear accelerator LINAC 2 for initial acceleration. After accelerated to 50MeV , the protons are then directed into the Proton Synchrotron Booster (PSB or BOOSTER) where they are further accelerated to 1 . 4 GeV . Following that, the proton beam enters the Proton Synchrotron (PS) and subsequently the Super Proton Synchrotron (SPS), which drive the beam energy up to 25 GeV and 450 GeV per proton respectively. Finally, the protons are split into two beams which are then fed into the LHC, where they are accelerated in opposite directions up to the maximum energy of 6 . 5 TeV . 2.1.2. The Large Hadron Collider The LHC is a circular collider with a circumference of 26 . 7 km located 50–170 m un- derground. It uses radiofrequency (RF) cavities to accelerate, dipole magnets to steer, and quadrupole magnets to focus charged particle beams. All of these components are maintained in a superconducting state when operating to avoid energy dissipation due to electrical resistance. After the beams have reached the desired energy, they are made to collide at one of the four designated collision points where particle detectors are situated. To accelerate the charged particles, oscillating electromagnetic fields are supplied to the 16 RF cavities (eight per beam) on the LHC. These RF cavities are superconducting chambers made in specific shape and size that allow the electromagnetic field inside to resonate and build up at a designated frequency. When timed correctly, the oscillating field in a RF cavity allows charged particles that pass through to experience a forward electric force only. They also work as a modulator as charged particles with energies