The Large Hadron Collider

The Large Hadron Collider (LHC) is the largest and most powerful particle collider in the world and was constructed at CERN between 1998 and 2008 [67]. The 27 km long circular collider is located approx. 100 mbeneath the France–Switzerland border near Geneva.

For proton-proton collisions the designed centre-of-mass energy is√

s= 14 TeV, while it is currently operated with 13 TeV. To reach these high energies, the protons are

3. The ATLAS experiment at CERN

accelerated with electrical fields by several pre-accelerators (see Figure3.1).

All protons originate from a hydrogen gas cylinder where hydrogen atoms are stripped of their electrons leaving protons. First, the protons gain energy by a linear accelerator, the Linac 2, to an energy of50 MeV. After this, the protons are injected into circular accelerators, starting with the Booster. A circular accelerator uses magnets to bend particles with the Lorentz force on a circle so that particles can be accelerated multiple times from the same electrical field. The Booster consists of four superimposed synchrotron rings and boosts protons to 1.4 GeV. This machine splits the beam into bunches and increases the beam intensity. Next in the chain is the PS, one of the oldest accelerators at CERN, which is still in use today. The accelerator reaches proton energies of25 GeV and injects the beam into the SPS. This synchrotron is the largest pre-accelerator reaching an energy of450 GeVand prepares the proton bunches for the LHC. The beam is injected

Figure 3.1.: The CERN accelerator complex. Protons are accelerated by traversing the Linear accelerator 2 (Linac 2), the Proton Synchrotron Booster (Booster), the Proton Synchrotron (PS), the Super Proton Synchrotron (SPS), and finally are injected into the Large Hadron Collider (LHC). The accelerating process for ions starts with the Linear accelerator 3 (Linac 3) and continues with the Low Energy Ion Ring (LEIR), PS, SPS, and finally LHC. In addition to the LHC accelerator chain, there are further experimental sites, e.g. the Antiproton Decelerator (AD) that slows down antiprotons so that they can be combined with positrons to form neutral antihydrogen. © CERN

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3.2. The Large Hadron Collider clockwise and counterclockwise in two separated beam pipes of the LHC. The protons are further accelerated to 6.5 TeV and can be brought to collision at four interaction points.

The strong magnetic field needed to contain particles with such high energies on their design orbit can only be generated by superconducting magnets. This is a phenomenon where the electrical resistance drops to exactly zero. Therefore, electrical currents can flow without emitting heat and very high currents can be reached, both of which are crucial properties for generating strong magnetic fields. Indeed, superconductivity only appears below a certain temperature, thus, the magnets of the LHC need to be cooled to a temperature of 1.9 K using liquid helium.

In addition to proton-proton collisions, the LHC can also collide (heavy-)ions, such as lead nuclei. The starting point for these is Linac 3. Prior to acceleration, atoms are evaporated in an oven, which removes some of their electrons, the rest of which are removed during acceleration. In a second step, atoms are transformed into “bare” nuclei, which are easier to accelerate than whole ions. After reaching an energy of 4.2 MeV, particles are injected into the LEIR accelerator where the beam is split into shorter bunches and further accelerated to 72 MeV. After this, the beam is passed to the PS, where the acceleration chain is the same as for protons.

When particles collide at one of the four interaction points, they transfer their energy into mass, E =mc², and create new particles. These particles can be measured by large particle detectors located at each of the four interaction points of the LHC, where the ATLAS [68], CMS [69], LHCb [70], and ALICE [71] detectors operate. In addition, three smaller detectors TOTEM, MoEDAL, and LHCf share the interaction point with one of the larger experiments (or are located close to it) and perform specialised research such as cross-section measurements or the search of magnetic monopoles.

The ATLAS and CMS experiments are multi-purpose particle detectors with a near4π coverage in solid angle. Both experiments discovered the Higgs boson in 2012 leading to the 2013 Nobel Price for Physics, which was jointly awarded to Peter Higgs and François Englert for their theoretical work [1,2]. The main difference between the two detectors is that CMS uses all-silicon detectors for its inner tracker and is more compact than ATLAS.

To compensate for the smaller dimensions, the CMS magnetic system can create a higher magnetic field and, therefore, increase the curvature of the charged particles to achieve a similar momentum resolution. The ATLAS experiment will be described in detail in Section 3.3. Of the remaining experiments, LHCb specialises in the measurement of b quarks to understand the matter-antimatter asymmetry in the universe and ALICE specialises in the study of heavy-ion collisions. When heavy-ions collide in the LHC, a quark gluon plasma is formed. This is a state of matter where quarks can move freely around what is hypothesised to have existed a few milliseconds after the Big Bang, and can also be found in neutron stars. At lower energies, quarks are confined in groups of at least two due to their interaction via the strong force.

After an extensive test period, first collisions (with low energies) were recorded in 2009.

The first data taking period (Run 1) started in 2011 with a centre-of-mass energy of

√s= 7 TeV, which increased to 8 TeV in 2012. After this initial run, the LHC entered Long Shutdown 1 (LS1) where further tests and upgrades where performed. Due to a

3. The ATLAS experiment at CERN

better understanding of the LHC, collisions at 13 TeV were reached for Run 2, which lasted from 2015 until 2018. Run 3 is planned to start in 2021, which will explore the full potential of the LHC with√

s= 14 TeV.