Measurement of the top-quark mass in t t-bar events with lepton+jets final states in pp collisions at sqrt(s) = 7 TeV

TL;DR

This CMS study measures the top-quark mass using tt̄ events in the lepton+jets channel at √s = 7 TeV, employing a constrained kinematic fit and a two-dimensional ideogram likelihood to extract m_t simultaneously with the jet energy scale (JES). The in situ JES calibration leverages the W → qq̄ mass, reducing a dominant systematic, while comprehensive calibrations and pseudo-experiments ensure unbiased results. The combined analysis of μ+jets and e+jets yields m_t = 173.49 ± 0.43 (stat.+JES) ± 0.98 (syst) GeV with JES = 0.994 ± 0.003 (stat) ± 0.008 (syst), and a cross-check cross-channel result supports consistency. Including dilepton measurements via BLUE yields m_t = 173.32 ± 0.27 (stat) ± 1.02 (syst) GeV, establishing a highly precise single-measurement top-quark mass value at the time. These results validate the in situ JES technique and demonstrate CMS’s capability for high-precision top-quark mass determinations at the LHC.

Abstract

The mass of the top quark is measured using a sample of t t-bar candidate events with one electron or muon and at least four jets in the final state, collected by CMS in pp collisions at sqrt(s) = 7 TeV at the LHC. A total of 5174 candidate events is selected from data corresponding to an integrated luminosity of 5.0 inverse femtobarns. For each event the mass is reconstructed from a kinematic fit of the decay products to a t t-bar hypothesis. The top-quark mass is determined simultaneously with the jet energy scale (JES), constrained by the known mass of the W boson in q anti-q decays, to be 173.49 +/- 0.43 (stat.+JES) +/- 0.98 (syst.) GeV.

Figures (7)

• Figure 1: Reconstructed masses of (a) the W bosons decaying to ${q}\overline{{q}}$ pairs and (b) the corresponding top quarks, prior to the kinematic fitting to the ${t}\overline{{t}}$ hypothesis. (c) and (d) show, respectively, the reconstructed W-boson masses and the fitted top-quark masses after the goodness-of-fit selection and the weighting by $P_\mathrm{gof}$. The distributions are normalized to the theoretical predictions described in Refs. Kidonakis:2010dkMelnikov:2006kvCampbell:2010ff. The uncertainty on the predicted ${t}\overline{{t}}$ cross section is indicated by the hatched area. The top-quark mass assumed in the simulation is $172.5$$\,\text{Ge\spaceV}$.
• Figure 2: Simulated $m_{t}\xspace^\text{fit}$ distributions of (a,d) correct, (b,e) wrong, and (c,f) unmatched ${t}\overline{{t}}$ permutations, for three generated masses $m_{{t},\text{gen}}$ with $\mathrm{JES} =1$ in (a), (b) and (c), and for three jet energy scales with $m_{{t},\text{gen}} = 172.5$$\,\text{Ge\spaceV}$ in (d), (e) and (f). The vertical dashed line corresponds to $m_{t}\xspace^\text{fit} = 172.5$$\,\text{Ge\spaceV}$. The $m_\mathrm{W} ^\text{reco}$ distributions are shown for (g) correct, (h) wrong, and (i) unmatched ${t}\overline{{t}}$ permutations for three jet energy scales with $m_{{t},\text{gen}} = 172.5$$\,\text{Ge\spaceV}$. The vertical dashed lines in (g), (h) and (i) indicate the accepted value of the W-boson mass of $80.4$$\,\text{Ge\spaceV}$. All distributions are shown for the muon+jets channel.
• Figure 3: Mean difference between the extracted $m_{{t},\text{extr}}$ and each generated $m_{{t},\text{gen}}$ and between $\mathrm{JES}_\text{extr}$ and $\mathrm{JES}_\text{gen}$ for the (a) muon channel and (b) electron channel, before the calibration, as a function of different generated $m_{{t},\text{gen}}$ and three values of JES. The colored dashed lines correspond to straight line fits, which are used to correct the final likelihoods. The black solid line corresponds to an assumption of a constant calibration for all mass and JES points in each channel.
• Figure 4: Width of the pull distribution for the calibrated measurement of $m_{t}\xspace$ and JES as a function of different generated $m_{\rm t,gen}$ and three values of JES. The muon channel is shown in (a) and the electron channel in (b). The black solid lines correspond to fits of constants to all calibration points, assuming no dependence on $m_{t}\xspace$ or JES.
• Figure 5: (a) Mean difference between the calibrated and generated values of $m_{t}\xspace$ and JES as a function of different generated $m_{{t},\text{gen}}$ and three values of JES for combined lepton+jets events; (b) width of the pull distributions for the combined channel after the single-channel calibration. The colored dashed lines correspond to straight line fits, the black solid line corresponds to a constant fit to all calibration points. The error bars in (a) indicate the statistical uncertainty on the mean difference for the uncalibrated likelihood.