Measurement of the top quark mass in the and channels using ATLAS data

The mass of the top quark (mtop) is an important parameter of the Standard Model (SM) of particle physics. Precise measurements of mtop provide critical inputs to fits of global electroweak parameters [1–3] that help assess the internal consistency of the SM. In addition, the value of mtop affects the stability of the SM Higgs potential, which has cosmological implications [4–6]. Many measurements of mtop were performed by the CDF and D0 collaborations based on Tevatron proton–antiproton collision data corresponding to integrated luminosities of up to 9.7  fb-1. A selection of these measurements was used in the recent Tevatron mtop combination resulting in mtop = 174.34 ± 0.37(stat) ± 0.52(syst) GeV = 174.34 ± 0.64 GeV [7]. Since 2010, measurements of mtop from the LHC by the ATLAS and CMS collaborations have become available. They are based on proton–proton (pp) collisions at a centre-of-mass energy of s=7TeV, recorded during 2010 and 2011 for integrated luminosities of up to 4.9 fb-1 [8–13]. The corresponding LHC combination, based on s=7TeV data and including preliminary results, yields mtop = 173.29 ± 0.23(stat) ± 0.92(syst) GeV = 173.29 ± 0.95 GeV [14]. Using the same LHC input measurements and a selection of the mtop results from the Tevatron experiments, the first Tevatron + LHC mtop combination results in mtop = 173.34 ± 0.27(stat) ± 0.71(syst) GeV, with a total uncertainty of 0.76 GeV [15]. Recently, improved individual measurements with a total uncertainty compatible with that achieved in the Tevatron + LHC mtop combination have become available; the most precise single measurement is obtained by the D0 Collaboration using tt¯→lepton+jets events and yields mtop = 174.98 ± 0.76 GeV [16]. This article presents a measurement of mtop using events with one or two isolated charged leptons (electrons or muons) in the final state (the tt¯→lepton+jets and tt¯→dilepton decay channels), in 4.6  fb -1 of pp collision data collected by the ATLAS detector at a centre-of-mass energy of s=7 TeV during 2011. It supersedes Ref. [8], where, using a two-dimensional fit to reconstructed observables in the tt¯→lepton+jets channel, mtop was determined together with a global jet energy scale factor. The use of this scale factor allows the uncertainty on mtop stemming from imperfect knowledge of the jet energy scale (JES) to be considerably reduced, albeit at the cost of an additional statistical uncertainty component. The single largest systematic uncertainty on mtop in Ref. [8] was due to the relative b-to-light-jet energy scale (bJES) uncertainty, where the terms b-jets and light-jets refer to jets originating from b-quarks and u, d, c, s-quarks or gluons, respectively. To reduce this uncertainty in the present analysis, a three-dimensional template fit is used for the first time in the tt¯→lepton+jets channel, again replacing the corresponding uncertainty by a statistical uncertainty and a reduced systematic uncertainty. This concept will be even more advantageous with increasing data luminosity. In addition, for the combination of the measurements of mtop in the two decay channels an in-depth investigation of the correlation of the two estimators for all components of the sources of systematic uncertainty is made. This leads to a much smaller total correlation of the two measurements than what is typically assigned, such that their combination yields a very significant improvement in the total uncertainty on mtop. To retain this low correlation, the jet energy scale factors measured in the tt¯→lepton+jets channel have not been propagated to the tt¯→dilepton channel. In the tt¯→lepton+jets channel, one W boson from the top or antitop quark decays directly or via an intermediate τ decay into an electron or muon and at least one neutrino, while the other W boson decays into a quark–antiquark pair. The tt¯ decay channels with electrons and muons are combined and referred to as the lepton + jets (or as a shorthand l+jets ) final state. The tt¯→dilepton channel corresponds to the case where both W bosons from the top and antitop quarks decay leptonically, directly or via an intermediate τ decay, into an electron or muon and at least one neutrino. The tt¯ decay channels ee, eμ, μμ are combined and referred to as the dilepton final state. For both the l+jets and dilepton final states, the measurements are based on the template method [17]. In this technique, Monte Carlo (MC) simulated distributions are constructed for a chosen quantity sensitive to the physics parameter under study, using a number of discrete values of that parameter. These templates are fitted to analytical functions that interpolate between different input values of the physics parameter, fixing all other parameters of the functions. In the final step a likelihood fit to the observed distribution in data is used to obtain the value for the physics parameter that best describes the data. In this procedure the top quark mass determined from data corresponds to the mass definition used in the MC simulation. It is expected that the difference between this mass definition and the pole mass is of order 1 GeV [18–21]. In the l+jets channel, events are reconstructed using a kinematic fit that assumes a tt¯ topology. A three-dimensional template method is used, where mtop is determined simultaneously with a light-jet energy scale factor ( JSF ), exploiting the information from the hadronic W decays, and a separate b-to-light-jet energy scale factor ( bJSF ). The JSF and bJSF account for residual differences of data and simulation in the light-jet and in the relative b-to-light-jet energy scale, respectively, thereby mitigating the corresponding systematic uncertainties on mtop. The analysis in the dilepton channel is based on a one-dimensional template method, where the templates are constructed for the mlb observable, defined as the per-event average invariant mass of the two lepton - b-jet systems from the decay of the top quarks. Due to the underconstrained kinematics associated with the dilepton final state, no in situ constraint of the jet energy scales is performed. This article is organised as follows: after a short description of the ATLAS detector in Sect. 2, the data and MC simulation samples are discussed in Sect. 3. Details of the event selection and reconstruction are given in Sect. 4. The template fits are explained in Sect. 5. The measurement of mtop in the two final states is given in Sect. 6, and the evaluation of the associated systematic uncertainties are discussed in Sect. 7. The results of the combination of the mtop measurements from the individual analyses are reported in Sect. 8. Finally, the summary and conclusions are given in Sect. 9.