The Large Hadron Collider

The Large Hadron Collider (LHC)[195–197]began operating at the end of 2009. It is a two-ring superconducting accelerator which is able to accelerate protons or heavy ions. As it is focussed on proton-proton (pp) collisions in this thesis, the experimental setup of the LHC is described based on the acceleration and collision of protons. The choice of protons is motivated by the fact that these baryons form a stable beam, that they can be produced in large numbers and - in comparison to, for example, electrons - that the energy loss due to synchrotron radiation is small. The latter argument is particularly important when a higher energy reach is the experiment’s main motivation.

Also based on this argument, the LHC superseded the Large Electron Positron Collider (LEP)[198] at CERN, which had been built in the same tunnel that hosts the LHC today, lying approximately 100 m under ground. In 2010 and 2011, the centre-of-mass energy of the LHCpp collisions was ps=7 TeV. One year later, in 2012, the LHC collided protons atp

s=8 TeV. The analysis presented in this thesis uses data which was collected by the ATLAS detector[199–201]at the latter centre-of-mass energy which is why the following description of the LHC is based on this data-taking period.

Since 2015, after a shutdown period of two years, the LHC is operating at a centre-of-mass energy ofp

s=13 TeV while the design energy of the LHC isp

s=14 TeV at a luminosity of 10³⁴cm⁻²s⁻¹.

1European Organisation for Nuclear Research, name originating from: Conseil Européen pour la Recherche Nucléaire.

The LHC, with a circumference of 27 km, is not built as a perfect circle but is composed of eight arcs and eight straight sections, so called “insertions”. The former contain the dipole bending magnets, the insertions consist of a long straight section with two transition regions at both ends, and two

“dispersion suppressors”. Four of these insertions host different LHC detectors while the others also fulfil purposes of injection, beam dumping or beam cleaning, for instance.

Four main detector experiments located at the straight sections record the resulting LHC particle collisions: ATLAS and CMS[202]as the so-called multipurpose high-luminosity detectors, LHCb (Large Hadron Collider beauty)[203], an asymmetric detector with a main focus onB-physics, and ALICE (A Large Ion Collider Experiment)[204]which concentrates on analysing the quark-gluon plasma in heavy ion collisions in order to study conditions comparable to those shortly after the Big Bang. A further small angle scattering experiment is TOTEM (TOTal Elastic and diffractive cross-section Measurement)[205]. Two additional experiments located at the LHC are LHCf (Large Hadron Collider forward)[206]and MoEDAL (Monopole and Exotics Detector at the LHC)[207]. The proton beams which enter the LHC are preaccelerated in a long injection chain of older, already existing and smaller rings or linear accelerators located at CERN. These were upgraded to fulfil all LHC requirements. The LHC as well as this chain of preaccelerators including the four main experiments are sketched in Fig. 3.1.

Figure 3.1:The LHC and its accelerator chain with the four main detector experiments at their interaction points within the framework of the entire CERN accelerator complex [208].

The protons of the LHC beam originate from the ionisation of hydrogen atoms and are initially accelerated to 50 MeV in a linear collider called LINAC2. During the first acceleration, radio frequency cavities are used to split the protons into bunches. These protons are then transferred to the Proton Synchrotron Booster where they reach an energy of 1.4 GeV. The Proton Synchrotron (PS), which is the oldest accelerator of the complex and put into operation almost 60 years ago, accelerates

3 . 1 T H E L A R G E H A D R O N C O L L I D E R

the protons further to an energy of 25 GeV after leaving the booster. The PS was also used in the past to provide beams to other experiments such as a neutrino beam to the bubble chamber Gargamelle which led to the discovery of weak neutral currents in 1974. Following the PS, in the Super Proton Synchrotron (SPS) with a circumference of 7 km the protons gain an energy of 450 GeV, which is the injection energy of the LHC. The SPS on its own was also used for important particle physics experiments - the experiments UA1 and UA2, which ran at the SPS, discovered theW andZbosons in 1983. Bunches of protons from the SPS enter the separate beam pipes in both opposite directions around the LHC ring. With the help of radio frequency cavities situated inside the beam pipe and providing an ultrahigh vacuum of 10⁻¹⁰mbar, these bunches are accelerated simultaneously. Once the protons reached their final energy of 4 TeV (equivalent top

s=8 TeV) in 2012 and of 6.5 TeV since 2015, they collide at the different interaction points of the LHC where the beam pipes cross.

These are directly located at the centre of the main detectors introduced above.

The proton bunches consist of about 10¹¹particles each. The design luminosity value that amounts to 10³⁴cm⁻²s⁻¹is reached by up to 2,808 bunches with a bunch crossing every 25 ns. This number corresponds to at least 20 inelastic collisions per bunch crossing on average, depending on how the beam is focussed. In 2012, the bunch spacing was mainly 50 ns while 25 ns was reached recently atp

s=13 TeV.

In order to keep the beams on their path within the LHC rings, 1,232 superconducting dipole magnets with a magnetic field of up to 8.6 T were installed. In total, 392 quadrupole magnets are responsible for correcting the position of the beams and their focusing. These guidance magnets as well as the acceleration cavities both rely on superconducting technologies. The temperature to which the dipole magnets are cooled down amounts to about 1.9 K. The high central field strengths are realised by the usage of superfluid helium.

In 2012, the LHC deliveredpp collision data corresponding to an integrated luminosity of about 22.8 fb⁻¹ [209], out of which 21.3 fb⁻¹ were recorded by the ATLAS detector. The integrated luminosity inpp collisions in 2012 as a function of time is plotted in Fig. 3.2. The uncertainty in this luminosity was determined to beδL/L =±1.9 %[210]. The recorded dataset is discussed more thoroughly in Sec. 6.1.

Figure 3.2:The total integrated luminosity in 2012 versus days inppcollisions delivered by the LHC (green) and recorded by the ATLAS detector (yellow) [209].