Analysis of W^± bosons with ALICE: Effect of alignment on W^± bosons analysis

Abstract

The ALICE (A Large Ion Collider Experiment) detector at the CERN Large Hadron Collider (LHC) is dedicated to studying the deconfined medium called the quark-gluon plasma (QGP), which is formed at extreme energy densities in heavy-ion collisions. ALICE can study hadrons, photons, electrons and muons up to the highest multiplicities expected at the LHC and down to very low transverse momentum (p_T ~ 30 MeV/c) by employing excellent particle identification (PID) and tracking over a broad momentum range (p ~ 100 MeV/c – 100 GeV/c). It consists of the central barrel which covers mid-rapidity (|y|< 0.9) and the Muon Spectrometer covering the forward rapidity region (2.5<y<4). The Muon Spectrometer detects dimuons decaying from heavy quarkonia (e.g. J/Ψ) which are hard, penetrating probes as well as high-p_T single muons from W^± bosons which are initial-state observables. These probes are essential tools for determining medium induced effects and studying the initial conditions of the interaction. The W^± boson has a high mass of M_W = 80.385 ± 0.015 GeV and is therefore formed in the early stages of the collision. It decays to single muons (μ^±←W^±) which are detected in the high-p_T region (30 – 80 GeV/c). The high centre-of-mass energies (√s) obtained during proton-proton (pp) and lead-lead (Pb-Pb) collisions at the LHC are sufficient for the formation of the W^± boson. Due to the increase in luminosity for the LHC in 2011 it is now thought possible to perform a data analysis of the W^± boson in ALICE. The results can then be compared to previous performance studies and to results from other LHC experiments (ATLAS, CMS and LHCb). As a first requirement of the analysis, the effect of the alignment of the Muon Spectrometer has to be determined. Misalignment of the Muon Spectrometer could result in a systematic uncertainty in the measurement of the muon track, thereby influencing the efficiency of the detector. By analysing simulations of W^± boson signals generated with PYTHIA in pp collisions at √s = 8 TeV with ideal and residual misalignment configurations of the detector, these alignment effects on the p_T and pseudorapidity (η) distributions, as well as the ratio (μ^+←W^+)/(μ^-←W^- ) (charge asymmetry) were studied using the AliROOT framework. It was found that the misalignment does cause a systematic uncertainty in the p_T distributions and charge asymmetry, especially in the region p_T > 40 GeV/c where the systematic uncertainty grows above 50 %. Analyses of Pb-Pb collisions conducted in 2011 at √(s_NN ) = 2.76 TeV were then performed on data reconstructed with original alignment information and data refitted with improved alignment information. They were compared to establish the effect of alignment on the single muon distributions. The improved alignment has a limited effect in the high-p_T region and therefore also on possible W^± boson studies. Due to lack of statistics at high-p_T the W^± boson signal and the nuclear modification factor (R_AA) could not be extracted, but it is foreseen that the extraction will later be possible with the use of 2012 pp and Pb-Pb data.