Measurement of the top-quark pole mass in dileptonic $t\bar{t}+ 1\text{-jet}$ events at $\sqrt{s}=13$ TeV with the ATLAS experiment

TL;DR

This study measures the top-quark pole mass $m_t^{\text{pole}}$ using dileptonic $t\bar{t}+1$-jet events at $\sqrt{s}=13$ TeV with ATLAS Run 2 data ($L=140~\text{fb}^{-1}$). The normalized differential cross-section $\mathcal{R}(\rho_s; m_t^{\text{pole}})$ is unfolded to parton level via Iterative Bayesian Unfolding and compared to fixed-order NLO QCD predictions for two parton-level definitions: $2\to3$ (stable tops) and $2\to7$ (top decays with off-shell effects). A chi-squared fit yields $m_t^{\text{pole}} = 170.73 \pm 0.33{\text{(stat.)}} \pm 1.36{\text{(syst.)}} {}^{+0.34}_{-0.28}{\text{(scale)}} \pm 0.24{\text{(PDF}⊕\alpha_S)}~\text{GeV}$ for the $2\to3$ case, with a cross-check from the $2\to7$ calculation yielding $m_t^{\text{pole}} = 171.69 \pm 0.41{\text{(stat.)}} \pm 1.68{\text{(syst.)}} {}^{+0.66}_{-1.34}{\text{(scale)}} {}^{+0.49}_{-0.46}{\text{(PDF}⊕\alpha_S)}~\text{GeV}$. The results are consistent within uncertainties and align with other precise top-quark mass determinations, supporting the pole-mass interpretation of direct measurements. The analysis demonstrates the utility of the $\rho_s$ observable and differential FO predictions in probing the top-quark mass parameter and provides data for future theory improvements.

Abstract

A measurement of the top-quark pole mass $m_{t}^\text{pole}$ is presented in $t\bar{t}$ events with an additional jet, $t\bar{t}+1\text{-jet}$, produced in $pp$ collisions at $\sqrt{s}=13$ TeV. The data sample, recorded with the ATLAS experiment during Run 2 of the LHC, corresponds to an integrated luminosity of $140~\text{fb}^{-1}$. Events with one electron and one muon of opposite electric charge in the final state are selected to measure the $t\bar{t}+1\text{-jet}$ differential cross-section as a function of the inverse of the invariant mass of the $t\bar{t}+1\text{-jet}$ system. Iterative Bayesian Unfolding is used to correct the data to enable comparison with fixed-order calculations at next-to-leading-order accuracy in the strong coupling. The process $pp \to t\bar{t}j$ ($2 \rightarrow 3$), where top quarks are taken as stable particles, and the process $pp \to b\bar{b}l^+νl^- \barν j$ ($2 \to 7$), which includes top-quark decays to the dilepton final state and off-shell effects, are considered. The top-quark mass is extracted using a $χ^2$ fit of the unfolded normalized differential cross-section distribution. The results obtained with the $2 \to 3$ and $2 \to 7$ calculations are compatible within theoretical uncertainties, providing an important consistency check. The more precise determination is obtained for the $2 \to 3 $ measurement: $m_{t}^\text{pole}=170.7\pm0.3~(\text{stat.})\pm1.4~(\text{syst.})~\pm 0.3~(\text{scale})~\pm 0.2~(\text{PDF}\oplusα_\text{S})~\text{GeV},$ which is in good agreement with other top-quark mass results.

Figures (6)

• Figure 1: Resolution of the $\rho_{\text{s}}\xspace$ observable for events reconstructed with the combined loose kinematic and $\phi$-weighting reconstruction methods, in the $e^\pm\mu^\mp$ decay channel of the $t\bar{t} + 1 \textnormal{-jet}$ system, simulated using + 8. The resolution obtained from each individual reconstruction method is also shown together with the best possible scenario, where the neutrino momenta are known from truth information in the MC (blue). Only events satisfying the respective reconstruction method requirements are considered.
• Figure 2: Detector level distributions of the $\rho_{\text{s}}\xspace$ variable in the ${t\bar{t} + 1 \textnormal{-jet}\xspace}$ system after the final selection in the $e^\pm\mu^\mp$ decay channel, for a cut on the extra jet corresponding to (a) 50 $\text{Ge V}$ and (b) 60 $\text{Ge V}$ . Data (filled markers) are compared with the SM expectation with $m_t^\text{MC} = 172.5~\text{Ge V}\xspace$ (histogram). The uncertainty band represents the total uncertainty on the MC prediction, including all statistical and systematic components.
• Figure 3: The (a) migration matrix and (b) the acceptance and (c) efficiency factors for events considered in the unfolding to the $2\rightarrow3$ parton level. The matrix and correction factors are built from the nominal simulation of $t\bar{t} + 1 \textnormal{-jet}$ events using the + 8 generator. Vertical bars represent MC statistical uncertainties.
• Figure 4: The measured normalized differential cross-section $\mathcal{R}(\rho_{\text{s}}\xspace; m^{\text{pole}}_t)$ unfolded to (a) the $2\rightarrow3$ parton level and (b) the $2\rightarrow7$ parton level. The error bars on the marker indicate the statistical uncertainty, the gray band the total experimental uncertainty. Theoretical predictions at fixed-order NLO QCD for $m_t^{\text{pole}}=169~\text{Ge V}\xspace$ (dotted line) and $m_t^{\text{pole}}=171~\text{Ge V}\xspace$ (dashed line) are also shown, without their associated uncertainties.
• Figure 5: Difference between the extracted $m_t^\text{fit}$ value in the $\chi^2$ fit and the mass value $m_t^\text{MC}$ in the + 8 simulations, (a) for the $2\rightarrow3$ and (b) $2\rightarrow7$ measurements. Each point corresponds to a mass value extracted using the parameterized mass dependence of the parton-level distributions obtained from the simulated MC samples, with the inner uncertainty bars representing the statistical uncertainties due to the limited size of the simulated samples and the outer bars representing the total experimental uncertainty associated with that measurement. A linear fit is performed across the various mass points, considering only their fully uncorrelated statistical uncertainties. Another uncorrelated effect, which originates from the interplay of statistical and systematic effects in the covariance matrices, further scatters the fitted mass values and is not considered in the linear fit. The 68% confidence level (CL) associated with the fit is shown with a filled area. The 172.5 $\text{Ge V}$ mass point is not included among the fitted points.