The LHC accelerator complex

The LHC and the ATLAS experiment

3.1 The Large Hadron Collider

TheLarge Hadron Collider(LHC) [87] is the largest hadron synchrotron accelerator to date, located at theEuropean Organisation for Nuclear Research(CERN) near Geneva. Its circumference is roughly 27 km and it is located approximately 100 m underground. It was designed to collide protons at

√s=14 TeV and heavy ions with√s=2.8 TeV per nucleon. The LHC has been operating in several periods with different setup and collision energy since 2010, with several planned technical stops for upgrades and maintenance. Currently, the LHC is undergoing upgrades and maintenance in preparation for the Run-III period of collisions, expected to begin in 2021.

3.1.1 The LHC accelerator complex

The LHC itself is a final collider in a series of linear and circular colliders. The overview of the accelerator complex is shown in Fig.3.1. First, the protons are obtained by stripping electrons from Hydrogen atoms and injected in the first, linear accelerator (LINAC2) and are accelerated to 50 MeV.

The protons then enter a series of circular colliders, each accelerating the particles up to a specific energy, theProton Synchrotron Booster(PSB) to 1.4 GeV, theProton Synchrotron(PS) to 26 GeV, and theSuper Proton Synchrotron(SPS) to 450 GeV. The SPS is the final accelerator before the LHC, with a circumference of 7 km. Half of the protons from SPS enter LHC in one direction and the other half in opposite direction. The oppositely-moving protons are accelerated in separate acceleration tubes.

Both proton beams are grouped intobunches, where each bunch contains the order of 1×10¹¹protons.

The bunches are accelerated using radio-frequency (RF) cavities in a single region of the accelerator.

The proton beam trajectory within the accelerator is contained by super-conducting niobium-titanium magnets operating at the temperature of 1.9 K, capable of generating magnetic field of 8.3 T. Dipole magnets are used to bend the trajectory within the LHC, and additional quadrupole magnets are used to focus the beam. Further corrections to the beam trajectory are achieved by higher multipole magnet systems.

There are four collision points along the LHC, where caverns with four detectors are installed. In these spots, the beam pipes are connected and the opposite beams are squeezed using focusing magnets and crossed using deflecting magnets to produce the collisions. The four detectors correspond to four main experiments using collisions from the LHC. Eth ATLAS (A Toroidal LHC AparatuS) and CMS (Compact Muon Solenoid) are the two largest, general-purpose detectors for testing SM predictions as well as searching for BSM physics. LHCb (LHC beauty) is a specialised experiment using a forward detector that focuses on physics of B-hadrons to study CP-violating processes. ALICE (A Large Ion

3.1. The Large Hadron Collider

Fig. 3.1: The accelerator complex at CERN [88]..

Collider Experiment) is another specialised experiment focused on studying quark-gluon plasma and hadronization processes in heavy ion (lead) collisions.

Other than unprecedented collision energy of the LHC, the other very important characteristic of a collider is its instantaneous luminosity, a quantity that describes the number of collisions per unit area and unit time:

L= fN_cn₁n₂

A . (3.1)

The luminosity depends on the revolution frequency f of the proton beam, the number of bunches in the beamN_c, the number of particles per bunch in the colliding bunchesn₁,n₂and the overlapping area of the bunches A. The product of integrated luminosity over the period of collisions and the cross-section of a particular process gives us the prediction of how many times that process occurred during the time the LHC was colliding the beams. The operation of the LHC was divided into several periods of data-taking by the experiments.

In 2011, the LHC operated at √s = 7 TeV and both ATLAS and CMS experiments recorded approximately 5 fb⁻¹of data. In 2012, the operation of LHC was restarted at√s=8 TeV, with ATLAS and CMS collecting approximately 20 fb⁻¹. These data taking periods concluded the Run-I period of data taking.

In summer of 2015, the LHC resumed operation after major upgrades for the Run-II period of data taking, achieving√s=13 TeV. During years 2015 to 2018, the Run-II dataset recorded by ATLAS and CMS reached almost 140 fb⁻¹. The large size of the dataset was achieved thanks to the outstanding instantaneous luminosity of the LHC that peaked during 2017 and 2018 data at over 2×10⁻³⁴cm⁻²s⁻¹,

3. The LHC and the ATLAS experiment

surpassing the original design by a factor of two. A visualisation of the cumulation of the integrated luminosity is shown in Fig.3.2.

Month in Year

Jan Apr Jul Oct

]-1 Delivered Luminosity [fb

0

Fig. 3.2: Visualisation of the cumulative luminosity vs the day delivered to ATLAS during stable beams forppcollisions [89].

The large luminosity reached by LHC means that a single bunch crossing leads to multiple pp interactions, characterised by distribution of mean number of interactions per bunch crossinghµi, shown in Fig.3.3. This poses additional challenges in the reconstruction of theppcollisions due to in-time and out-of-time pile-up. In-time pile-up leads to signals in the ATLAS detector from multiple ppcollisions in a single bunch crossing. Out-of-time pile-up is caused by limited read-out time of certain detector systems, which can be higher than spacing between individual bunches (25 ns) in the LHC. This means that slower detector systems can produce signals originating from more than one bunch crossing.

0 10 20 30 40 50 60 70 80

Mean Number of Interactions per Crossing 0 /0.1]-1 Recorded Luminosity [pb

Online, 13 TeV

ATLAS

∫

> = 13.4

Fig. 3.3: The distribution of mean number of interactions per bunch crossinghµifor the individual data-taking periods in Run-II [89].