The Z boson

The weak interaction is mediated via two kind of vector bosons, a chargedW^±and a neutralZ. TheW was known already from its role in nuclear decays, being a mediator of the interaction for beta decay. The Z being neutral and very similar to the photon was more difficult to observe.

First evidence came from neutrino elastic scattering in 1973 at the Gargamelle bubble chamber at CERN. A phenomenon that was explained as the interaction of neutrinos with electrons with the exchange of a Z boson. The actual discovery of the particle was achieved at the SPS p¯p collider at CERN in the early 1980s. The current world average of the mass and the decay width are

m_Z = 91.1876±0.0021 GeV and Γ_Z = 2.4952±0.0023 GeV, (2.26) respectively [4].

The main production mechanism is annihilation of a quark-antiquark pair. The (anti)quark can be a valence or a sea (anti)quark depending on the colliding particles. In hadron colliders, it is impossible to distinguish between Z boson and an off-shell photon production, γ^∗, thus henceforth they are considered as one. The Feynman diagram of the leading order process³ is shown in Fig. 2.5(a). In the Feynman diagram a quark–antiquark pair annihilates via γ^∗/Z to a lepton pair. Higher order corrections come from QCD initial and final state radiation or QCD Compton scattering (q +g → γ^∗/Z +q), shown in Fig. 2.5(b)-(f). The total cross section is calculated from the sum of all possible processes up to next-to-next-to-leading order diagrams. The cross sections for leading order (LO), leading order (NLO) and next-to-next-to-leading order (NNLO) accuracy are shown in Table 2.2. TheZ boson decays to leptons (ee, µµ, τ τ, νν) and hadrons. The partial decay widths are shown in Table 2.3.

The NNLO QCD corrections of the theoretical prediction for the cross section times branch-ing ratio forpp collisions at √

s= 7 TeV is estimated at [12]. The value is found to be σ^{N N LO}_γ∗/Z→ll= 0.96±0.05 nb for 66 < m_ll < 116 GeV.

3The cross section of a process is the sum of the contributing Feynman diagrams. If only tree-level diagrams are considered the calculation is referred to as leading order, while if one-loop diagrams are taken into account it is called next-to-leading order, for two-loop diagrams next-to-next-to-leading order and so on.

12

Table 2.2: Central values for the production cross sections of σ×BR(γ^∗/Z → ll) process at leading order (LO), next-to-leading order (NLO) and next-to-next-to-leading order (NNLO) accuracy [11].

order inαs σ(γ^∗/Z →ll) [nb]

LO 0.758

NLO 0.938

NNLO 0.964

Table 2.3: Main decay modes of theZ boson [4].

decay mode Branching Fraction (Γ_i/Γ) [%]

e⁺e⁻ 3.363 ±0.004

µ⁺µ⁻ 3.366 ±0.007

τ⁺τ⁻ 3.367 ±0.008

invisible 20.00 ±0.06

hadrons 69.91 ±0.06

Chapter 3

Experimental Setup

The experimental setup that physicists use to look into the world of the elementary particles is a particle accelerator combined with a simpler or more complicated detector. That is exactly what it is done for the discovery of the last missing particle in the Standard Model, the Higgs boson. The Large Electron Positron (LEP) collider at CERN in the late eighties and nineties and the TeVatron at Fermilab from 1995 till 2011 have tried to search for the elusive particle, although without success. In their place a much stronger machine is built; the Large Hadron Collider (LHC) at CERN. LHC is designed to accelerate protons at centre-of-mass energy of 14 TeV and lead ions at total centre-of-mass energy of 1150 TeV. Collisions take place at four interaction points where four detectors have been installed.

The design energy has not yet been reached, due to technical problems. Instead the machine operated at 7 TeV for protons for two years (2010-2011) with great success. A fact that provided with technical knowledge and confidence to upgrade the run to 8 TeV for 2012. At the end of 2012, LHC will shut down for one year to enable an upgrade of the accelerator to the design energy and improvements on detector parts for the experiments. Further improvements of the LHC have already been planned for 2016 and 2020 and involve luminosity upgrades and, in many cases, complete sub-detector replacements. In this chapter the LHC and the setup of the experiment ATLAS, whose data are used for this thesis, are described.

3.1 Large Hadron Collider

The LHC is a ring accelerator of 26.7-km circumference. The tunnel is on average 100 m below the surface and extends from the Jura mountains till the L´eman lake. The accelerator is designed for circulating proton-proton (pp) beams in two independent rings [13]. Additionally, lead-ion beams (Pb⁺²) have been integrated in the physics program, giving the opportunity for studies in the QCD transition region and quark-gluon plasma physics. The trajectories of the protons or Pb ions is steered with the help of super-conducting magnets that operate at 1.9 K.

The two beams meet at four interaction points underground, where detectors have been placed. The four detectors, i.e. four experiments, are ALICE, ATLAS, CMS and LHCb. ATLAS and CMS are general purpose detectors and their physics interests vary from Higgs searches to studies of rare B meson decays or forward physics. ALICE and LHCb, on the other hand, are specialised experiments in relativistic heavy ion physics the former andB meson physics and CP-violation the latter. Two more detectors are placed in the LHC ring, the LHCf and TOTEM.

LHCf, situated on either side of the ATLAS detector, measures neutral pions in the forward direction accumulating data that will help to get closer to understanding the ultrahigh-energy

Figure 3.1: LHC acceleration chain. In red are shown the injection point and the pre-acceleration rings for protons and in green for lead ions.

cosmic ray events. TOTEM shares the cavern with CMS and, also, detects forward escaping particles from the collision point to study the structure of the proton and to measure the LHC luminosity.

In order to achieve the high energies necessary, the proton (Pb-ion) beams are going through a series of pre-accelerators. From the source of protons (Pb-ion) the particles are sent to the Linac2 (Linac3) for the first acceleration, then they continue to the Proton Synchrotron Booster (PSB) (Low Energy Ion Ring (LEIR)), to the Proton Synchrotron (PS), then to the Super Proton Synchrotron (SPS) to end up in LHC. A schematic of the acceleration chain is shown in Fig. 3.1. The source of protons is hydrogen atoms which are stripped of their electrons. The protons are accelerated in the Linac2 up to 50 MeV before they are injected into the PSB. The Booster is composed of four, superimposed rings and gets the protons to energies up to 1.4 GeV¹. Then, the PS takes over and pushes the energy of the protons to 26 GeV (5.9 GeV/nucleon).

SPS follows with a ring of 6.9 km in circumference and maximum energy 450 GeV for protons (177 GeV/nucleon). In the LHC ring the proton beams are accelerated to their maximum energy; 3.5 TeV in 2010, 2011 runs and 4.0 TeV in 2012. The design energy per beam is 7 TeV (1.38 TeV/nucleon) and it is planned to be achieved after a machine upgrade in 2014.