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Measurement of the top-quark mass using decays with a $J/ψ$ meson at $\sqrt{s}=$13 TeV with the ATLAS detector

ATLAS Collaboration

TL;DR

This ATLAS study measures the top-quark mass in decays producing a $J/\psi$ from a $b$-hadron and an isolated lepton, exploiting the purely leptonic invariant mass $m(\ell\,\mu^+\mu^-)$ and an unbinned likelihood to extract $m_{top}$. Using 140 fb$^{-1}$ of 13 TeV data, the result $m_{top}=172.17^{+0.80}_{-0.80}$ (stat) $^{+0.81}_{-0.81}$ (syst) $^{+1.07}_{-1.07}$ (recoil) GeV achieves a total uncertainty of 1.56 GeV, with the third component arising from the dipole gluon-recoil scheme in top-quark decays. The analysis demonstrates a robust, jet-energy-scale–insensitive approach to top-mass measurements and identifies the dominant systematics from parton-shower/hadronisation modelling, ISR/FSR, and $b$-hadron fragmentation, while jet calibrations remain comparatively small. This channel provides a valuable cross-check of jet-based top-mass determinations and informs future precision studies at ATLAS and the LHC.

Abstract

The top-quark mass is measured using top-quark decays producing an isolated lepton and $J/ψ$ meson reconstructed in its $μ^+μ^-$ decay mode. The data sample was recorded with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb$^{-1}$. The measurement is based on the invariant mass $m(\ell μ^+μ^-)$ of the system made of the isolated lepton $\ell$ from the $W$ boson decay and the non-isolated $μ^+μ^-$ pair from a $J/ψ$ decay of a $b$-hadron, exploiting its sensitivity to the top-quark mass. An unbinned maximum-likelihood fit to the $m(\ell μ^+μ^-)$ distribution is performed to extract the top-quark mass. The top-quark mass is measured to be $m_{top} = 172.17 \pm 0.80 (stat) \pm 0.81 (syst) \pm 1.07 (recoil)$ GeV, with a total uncertainty of 1.56 GeV. The third uncertainty arises from changing the dipole parton shower gluon-recoil scheme used in top-quark decays.

Measurement of the top-quark mass using decays with a $J/ψ$ meson at $\sqrt{s}=$13 TeV with the ATLAS detector

TL;DR

This ATLAS study measures the top-quark mass in decays producing a from a -hadron and an isolated lepton, exploiting the purely leptonic invariant mass and an unbinned likelihood to extract . Using 140 fb of 13 TeV data, the result (stat) (syst) (recoil) GeV achieves a total uncertainty of 1.56 GeV, with the third component arising from the dipole gluon-recoil scheme in top-quark decays. The analysis demonstrates a robust, jet-energy-scale–insensitive approach to top-mass measurements and identifies the dominant systematics from parton-shower/hadronisation modelling, ISR/FSR, and -hadron fragmentation, while jet calibrations remain comparatively small. This channel provides a valuable cross-check of jet-based top-mass determinations and informs future precision studies at ATLAS and the LHC.

Abstract

The top-quark mass is measured using top-quark decays producing an isolated lepton and meson reconstructed in its decay mode. The data sample was recorded with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of TeV during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb. The measurement is based on the invariant mass of the system made of the isolated lepton from the boson decay and the non-isolated pair from a decay of a -hadron, exploiting its sensitivity to the top-quark mass. An unbinned maximum-likelihood fit to the distribution is performed to extract the top-quark mass. The top-quark mass is measured to be GeV, with a total uncertainty of 1.56 GeV. The third uncertainty arises from changing the dipole parton shower gluon-recoil scheme used in top-quark decays.

Paper Structure

This paper contains 12 sections, 5 equations, 5 figures, 3 tables.

Table of Contents

  1. Introduction
  2. ATLAS detector
  3. Data and simulated event samples
  4. Object definition
  5. Event selection
  6. Systematic uncertainties
  7. Data and Monte Carlo samples
  8. Modelling of and single-top-quark processes
  9. Modelling of background processes
  10. Modelling of the detector response
  11. Template method and result
  12. Conclusion

Figures (5)

  • Figure 1: Distributions of the invariant mass $m$($\mu^+\mu^-$) of candidates in (a) the 2--3.6 $\text{Ge V}$ and (b) the 2.9--3.3 $\text{Ge V}$ mass range, (c) the invariant mass $m$($\ell\mu^+\mu^-$) and (d) the transverse momentum $\pt$($\ell\mu^+\mu^-$) of the system made of the selected isolated lepton and the two muons from the candidate. The data is shown compared with the expectation from simulation, broken down into contributions from and single-top-quark with and without $b\rightarrow\PJgy \rightarrow \Pgmp{}\Pgmm\xspace$ decay, $\Pqt{}\Paqt V+\Pqt{}\Paqt H$, $\PW\text{+\,jets}$, $W+\PJgy$, $\PZ\text{\,+\,jets}$, dibosons and events with non-prompt and fake leptons (referred to as 'NP & Fake Lep.'). The data is represented as closed circles with statistical uncertainties. The predictions are shown as solid coloured histograms and are normalised to the same integrated luminosity as the data. The shaded area represents the combination of statistical and systematic uncertainties. The lower panels show the ratios of the data to the predictions.
  • Figure 2: Distribution of $x_B$, the fraction of energy taken by the $b$-hadron in the top-quark rest frame, as obtained from theoretical prediction at NLO+NLL Cacciari:2002Corcella:2005, accounting for soft-gluon resummation with the hadronisation according to the Kartvelishvili model Kartvelishvili:1977pi, and obtained at particle level using samples. The [8] A14--$r_b$ MC samples are based on different recoil schemes using either the ('recoil-to-colour=ON') or the top quark ('recoil-to-top') as the recoil particle. The lower panel shows the ratios of the theoretical and 'recoil-to-top' predictions to the 'recoil-to-colour=ON' one.
  • Figure 3: Template fit functions for (a) the $m_{\textrm{top}}$-dependent sample shown for $m_{\textrm{top}}^{\textrm{gen}}$ values of 169, 172.5 and 176 $\text{Ge V}$ and (b) the $m_{\textrm{top}}$-independent samples compared with the histograms used in the parameterisation. The lower panel of the $m_{\textrm{top}}$-dependent template plot shows the ratios of the fit functions obtained with other $m_{\textrm{top}}^{\textrm{gen}}$ values to the one obtained with $m_{\textrm{top}}^{\textrm{gen}}\xspace=172.5$$\text{Ge V}$. The lower panel of the $m_{\textrm{top}}$-independent template plot shows the ratios of the histogram to the fit function.
  • Figure 4: (a) Calibration curve obtained showing the reconstructed $m_{\textrm{top}}$ as a function of the generated one $m_{\textrm{top}}^{\textrm{gen}}$. The fitted values are represented as closed circles with statistical uncertainties. A linear function is fitted to the mass points. (b) The $m$($\ell\mu^+\mu^-$) distribution in data compared with the predicted distribution. The shaded uncertainty band is constructed by varying the template fit function within the statistical and systematic uncertainties of the measurement. The lower panel shows the ratio of data to the template fit function.
  • Figure 5: This measurement of the top-quark mass $m_{\textrm{top}}$ compared with earlier ATLAS measurements, in particular at 13 $\text{Te V}$TOPQ-2017-17TOPQ-2022-24, the ATLAS and CMS Run 1 combination TOPQ-2019-13, and a CMS measurement in the same lepton+ channel CMS-TOP-15-014. The dashed line and shaded band represent the central value and uncertainty of the ATLAS+CMS Run 1 combination.