The Large Hadron Collider at CERN

The Large Hadron Collider [61] is a circular proton-proton, proton-heavy ion and heavy ion-heavy ion collider. It has a circumference of roughly 27 km and is located roughly 100 m underground at CERN. Its construction was approved in 1994 and started in 2000, after the Large Electron Positron Collider (LEP) had been shut down. Making use of LEP’s infrastructure the LHC was accelerating protons for the first time in September 2008. The first proton-proton collisions took place roughly 14 months later as after only 9 days of running the machine was severely damaged in 2008 due to a faulty connection.

3.1.1 Design and Experiments

Before being injected into the LHC itself, the protons are run through a series of pre-accelerators.

Hydrogen atoms are stripped off their electrons and are accelerated to 50 MeV in the LINAC2.

In the Proton Synchroton Booster the energy per proton is raised to 1.4 GeV, after which the protons are accelerated to an energy of 25 GeV in the Proton Synchrotron. The last step in the chain of pre-accelerators is the Super Proton Synchroton, from which the protons are injected into the LHC at an energy of 450 GeV. The acceleration chain is shown in Figure 3.1.

Using radio frequency (RF) cavities, the LHC can accelerate protons to energies up to 7 TeV by design. The proton trajectories are bent around the circle by superconducting cryodipole magnets which provide a magnetic field of 8.33 T. These are made of Nb-Ti and operate at a nominal temperature of 1.9 K. The RF cavities and a number of multipole magnets focus the beam. The LHC has been designed to store and accelerate 2808 bunches of protons separated into a clockwise and a counter-clockwise beam, with roughly 10¹¹protons per bunch. At each of the eight interaction points, where the beams can be brought to collision, a bunch crossing can appear at the minimal distance of 25 ns. It was estimated that on average there would be about 20 proton-proton interactions per bunch crossing, which would correspond to an instantaneous luminosity of 10³⁴ cm⁻²s⁻¹. The total cross section for proton-proton collisions at a centre of mass energy of 14 TeV was estimated to be 40 mbarn for elastic scattering and 60 mbarn for inelastic scattering.

The LHC was also designed to collide heavy ions. Pb⁸²⁺ ions can be accelerated to energies up to 2.76 TeV/nucleon. The maximum instantaneous luminosity in ion mode was estimated to

Figure 3.1: A schematic view of the CERN accelerator complex [62]. The chain of LHC pre-accelerators is included: Starting at the LINAC2, the protons are accelerated by the Booster, the PS the SPS before they are finally injected into the LHC. The four large-scale experiments are also shown in the sketch.

be 7.0·10⁷ cm⁻²s⁻¹, with 592 bunches of ions in the ring and a bunch spacing of 100 ns.

With its design specifications, the LHC can provide high energy hadron collisions, raising the centre of mass energy E_CM by a factor of roughly 7 compared to previous experiments. It is expected to cover a wide range of the parameter space of extensions to the SM as well as the SM itself, in particular the mass of the Higgs boson. In addition it provides the possibility to test the SM predictions in an energy regime that was not accessible under laboratory conditions so far. With the heavy ion collisions at even higher energies, it is also expected to provide new insights into the physics of the quark-gluon plasma.

There are two multipurpose detectors at the LHC, ’A Toroidal LHC Apparatus’ (ATLAS) [63]

and ’Compact Muon Solenoid’ (CMS) [64]. Although using different technologies and putting emphasis on different aspects of particle detection, ATLAS and CMS have a similar structure and pursue a very similar physics program. Apart from the search for physics beyond the SM both experiments aim for precise measurements of SM physics, such as top-pair and single top production cross sections, the top quark properties, the search for the Higgs boson and measure-ment of its properties, or the measuremeasure-ment of production cross sections for electroweak bosons in association with jets. Particles are detected using an inner tracker, different calorimeters and muon chambers, as shown in Figure 3.2. For a more complete overview of techniques and methods for particle detection, see for instance [65,66]. For the precise determination of parti-cle momenta both experiments use sophisticated magnet systems. The ATLAS detector will be described in more detail in the second part of this chapter.

There are two more large scale experiments at the LHC with a focus on different physics questions. The LHCb detector [67] is a highly asymmetric detector with a rich agenda in b-physics, aiming at gaining insights into CP violation. With a focus on heavy ion b-physics, the ALICE detector [68] has been build to study the structure and behaviour of the quark-gluon plasma, mainly.

Figure 3.2: This schematic illustrates how the onion shell structure of modern multipurpose detectors allows for the reconstruction of nearly all known (semi)stable particles [69]. Dashed lines denote invisibility of particles in the respective detector parts. As shown, an electron leaves hits in the tracker and creates an electromagnetic shower in the electromagnetic calorimeter. If such a shower is found in the calorimeter without an associated track, it most likely originates from a photon. Charged hadrons can be identified by entries in the electromagnetic and hadronic calorimeters with an associated track in the inner detector, while neutral hadrons only deposit their energy in the hadronic calorimeter. This is exemplarily shown for a proton and a neutron, here. Muons, which are stable on the detector scale, are the only particles which are expected to leave tracks in the muon chambers. However, it may happen that the content particles of a jet are not fully stopped in the calorimeter and cause hits in the muon chambers, too (punch through). Of course, as shown here, muons can also be seen by the inner detector, such that for precise muon reconstruction the combined information of the inner tracker and the muon system may be used. The picture also shows a neutrino, which escapes the detector without leaving a significant amount of energy in the calorimeters, or hits in the tracking detectors.

In addition to the larger experiments, the LHC is used as an accelerator for smaller ex-periments as well. Located close to the ATLAS detector with the goal of improving hadron interaction models for cosmic ray studies there is the LHCf detector [70]. The TOTEM detector has the goal to measure the total proton-proton cross section using a luminosity independent method [71]. The MoEDAL experiment searches for magnetic monopoles and semi-stable mas-sive particles [72].

3.1.2 Operation in 2010-2012

Operation in p-p mode in 2010 and 2011 After a short commissioning phase with beam energies of 450 GeV and 1.05 TeV, the LHC ran at 3.5 TeV/beam in 2010 and 2011. In 2010, a

relatively small dataset was collected, resulting in a total of roughly 35 pb⁻¹for physics analyses.

The total luminosity could be measured with a final uncertainty of 3.4% in 2010 [73]. This data has not been used for the results presented in this thesis.

In 2011, the instantaneous luminosity could be significantly enhanced, yielding a dataset of about 5.25 fb⁻¹, as shown in Figure 3.3(a) with an uncertainty of 3.7% [74]. The data is split into different periods during which the beam and detector conditions are roughly stable. A summary of the periods in 2011 is given in Table 3.1.

Day in 2011 28/02 30/04 30/06 30/08 31/10

]-1Total Integrated Luminosity [fb

0 1 2 3 4 5 6

7 ATLAS Online Luminosity ^{s = 7 TeV} LHC Delivered

ATLAS Recorded Total Delivered: 5.61 fb-1 Total Recorded: 5.25 fb-1

(a) 2011

Day in 2012 26/03 31/05 06/08 11/10 17/12

]-1Total Integrated Luminosity [fb

0 5 10 15 20 25

30 ATLAS Online Luminosity ^{s = 8 TeV} LHC Delivered

ATLAS Recorded Total Delivered: 23.3 fb-1 Total Recorded: 21.7 fb-1

(b) 2012

Figure 3.3: Performance of the LHC and the ATLAS detector in 2011 and 2012

Operation in p-p mode in 2012 In 2012 the centre of mass energy was increased to 8 TeV, corresponding to a beam energy of 4 TeV. A total of 21.7 fb⁻¹ was collected, as shown in figure 3.3(b). The integrated luminosity could be determined with an uncertainty of 3.6%. A summary of the periods in 2012 is also given in Table 3.1.

period 2011 integrated luminosity [pb⁻¹] 2012 integrated luminosity [pb⁻¹]

A 9 910

B 18 5594

C - 1643

D 186 3598

E 53 2863

F 160

-G 572 1404

H 287 1655

I 416 1149

J 240 2941

K 685

-L 1,625 983

M 1,184 14

Table 3.1: ATLAS data taking periods in 2011 and 2012