Measurement of the top quark charge in pp collisions at $ \sqrt{s}=7 $ TeV with the ATLAS detector

Abstract

A measurement of the top quark electric charge is carried out in the ATLAS experiment at the Large Hadron Collider using 2.05 fb −1 of data at a centre-of-mass energy of 7 TeV. In units of the elementary electric charge, the top quark charge is determined to be 0.64 ± 0.02 (stat.) ± 0.08 (syst.) from the charges of the top quark decay products in single lepton tt candidate events. This excludes models that propose a heavy quark of electric charge -4/3, instead of the Standard Model top quark, with a significance of more than 8σ. The ATLAS collaboration 26 IntroductionIt is generally accepted that the particle discovered at Fermilab in 1995 [1,2] is the Standard Model (SM) top quark. However, a few years after the discovery a theoretical model appeared proposing an "exotic" quark of charge -4/3 and mass ≈ 170 GeV as an alternative to the SM top quark at this mass value [3]. Though this model has already been experimentally excluded a precise measurement of the top quark charge is important as it is one of the basic top quark properties. A strong preference for the SM top quark with electric charge of +2/3 (in units of the electron charge magnitude) was reported by the D0 and CDF collaborations [4,5] but without the ultimate 5σ exclusion of a possible exotic quark with charge of -4/3. The CDF and D0 exclusion limits are 95% 1 and 92%, 1 The CDF collaboration has recently submitted an update of their analysis for publication, which results in a limit of 99% [6].-1 - JHEP11(2013)031respectively. Therefore, it is still important to carry out a more precise measurement to definitively resolve this question with more than 5σ confidence level. Due to the excellent ATLAS detector performance, the analysis presented here not only demonstrates that the particle presently denoted by "top quark" is really the SM top quark decaying into a bquark and a W + boson, but also allows for a direct measurement of its electric charge with a significantly improved precision. Moreover, from an experimental point of view it is interesting to demonstrate the high flavour tagging performance of the ATLAS experiment, i.e. its capability to distinguish between jets initiated by quarks and anti-quarks used in this study to find the correct W b pairing in the W + W − bb system of the assumed tt final state.The dominant decay channel of the top quark is to a b-quark through the charged weak current: t → W + b (t → W −b ). The measurement of the top quark charge requires the charges of both the W boson and the b-quark to be determined. While the former can be determined through W's leptonic decay, the b-quark charge is not directly measurable due to quark confinement in hadrons. However, it is possible to establish a correlation between the charge of the b-quark and the charges of the collimated hadrons from the bquark hadronization that form a b-jet. Within this approach, the charge can be determined using the lepton + jets (tt → ℓ ± νjjbb) or the dilepton (tt → ℓ + νℓ −ν bb) channel. This paper presents the results of a top quark cha...

Off-shell vs on-shell modelling of top quarks in photon associated production

“…The latter is known to be Q t = +2/3, i.e. consistent with the SM, albeit has only been measured indirectly in tt production [8,9]. More exotic physics scenarios, that propose a heavy quark of electric charge Q t = −4/3, instead of the SM top quark, have been excluded with a significance of more than 8σ.…”

Off-shell single-top production at NLO matched to parton showers

“…Single-top production at hadron colliders has continued to be an active field of research, challenging both experimental and theoretical communities, since its observation at the Tevatron [1,2]. Measurements of the dominant t-channel subprocess have been presented by CDF and D0 [3][4][5] at the Tevatron, as well as by the ATLAS [6][7][8] and CMS [9][10][11] collaborations, at both the 7 TeV and 8 TeV LHC, with preliminary results [12,13] available at 13 TeV as well. This channel has also been exploited by the ATLAS collaboration in the first top-mass extraction from single-top events in ref.…”

HYDRO + JETS (HYDJET+ +) event generator for Pb+Pb collisions at LHC

“…Event-by-Event (EbyE) fluctuations. The EbyE distributions of harmonics of anisotropic flow in lead-lead collisions at LHC were studied, e.g., by ATLAS Collaboration in [26]. The results were obtained after the application of the so-called unfolding procedure [27] in order to extract the "true" value of the flow vector and get rid of the nonflow effects caused by the finite event multiplicities, jet fragmentation and decays of resonances.…”

“…system. The difference between the EbyE flow vectors (to exclude the collective flow) of the two subevents is fitted to the Gaussian with the width δ 2SE = 2δ, which enters the response function [26]…”

“…Dashed and solid histograms present the results for simulated HYDJET++ events before and after the unfolding procedure, respectively. The full circles are the ATLAS data from [26]. Figure 6 displays the probability density distributions of elliptic and triangular EbyE flows obtained in three centrality intervals: σ/σ geo = 5−10%, 20−25%, and 35−40%.…”