Precise measurement of the $t\bar{t}$ production cross-section and lepton differential distributions in $eμ$ dilepton events from $\sqrt{s}=13$ TeV $pp$ collisions with the ATLAS detector

Abstract

The inclusive top quark pair ($t\bar{t}$) cross-section $σ_{t\bar{t}}$ has been measured in $\sqrt{s}=13$ TeV proton-proton collisions, using 140 fb$^{-1}$ of data collected by the ATLAS experiment at the Large Hadron Collider. Using events with an opposite-charge $eμ$ pair and $b$-tagged jets, the cross-section is measured to be: $\begin{equation}\nonumber σ_{t\bar{t}} = 829.3 \pm 1.3\,\mathrm{(stat)}\ \pm 8.0\,\mathrm{(syst)}\ \pm 7.3\,\mathrm{(lumi)}\ \pm 1.9\,\mathrm{(beam)}\,\mathrm{pb}, \end{equation}$ where the uncertainties reflect the limited size of the data sample, experimental and theoretical systematic effects, the integrated luminosity, and the proton beam energy, giving a total uncertainty of 1.3%. The result is used to determine the top quark pole mass via the dependence of the predicted cross-section on $m_t^\mathrm{pole}$, giving $m_t^\mathrm{pole}=172.8^{+1.5}_{-1.7}$ GeV. The same event sample is used to measure absolute and normalised differential cross-sections for the $t\bar{t}\rightarrow eμν\barνb\bar{b}$ process as a function of single-lepton and dilepton kinematic variables. Complementary measurements of $eμb\bar{b}$ production, treating both $t\bar{t}$ and $Wt$ events as signal, are also provided. Both sets of differential cross-sections are compared to the predictions of various Monte Carlo event generators, demonstrating that the state-of-the-art generators Powheg MiNNLO and Powheg $bb4l$ describe the data better than Powheg hvq. The sensitivity of some of the measured differential distributions to quasi-bound-state formation near the $t\bar{t}$ threshold is investigated in an addendum.

Figures (22)

• Figure 1: Distributions of (a) the number of $b$-tagged jets in selected opposite-charge $e\mu$ events; and (b) the $p_{\text{T}}$ of $b$-tagged jets, (c) the $p_{\text{T}}$ of the electron, (d) the $|\eta|$ of the electron, (e) the $p_{\text{T}}$ of the muon and (f) the $|\eta|$ of the muon, in events with an opposite-charge $e\mu$ pair and at least one $b$-tagged jet. The reconstruction-level data are compared with the expectation from simulation, broken down into contributions from (modelled with Powheg MiNNLO + Pythia8), $Wt$, $Z$+jets, dibosons, and events with misidentified electrons or muons. The simulation prediction is normalised to the same integrated luminosity as the data in (a) and to the same number of entries as the data in (b--f). The lower panels show the ratios of simulation to data, using various simulation samples and with the cyan shaded band indicating the statistical uncertainty. The last bin includes the overflows in panels (b), (c) and (e).
• Figure 2: Distributions of (a) the dilepton $p_\mathrm{T}^{e\mu}$, (b) invariant mass $m^{e\mu}$, (c) rapidity $|y^{e\mu}|$, (d) azimuthal angle difference $\Delta\phi^{e\mu}$, (e) lepton $p_{\text{T}}$ sum $p_{\mathrm T}^{e}+p_{\mathrm T}^{\mu}$ and (f) lepton energy sum $E^{e}+E^{\mu}$, in events with an opposite-charge $e\mu$ pair and at least one $b$-tagged jet. The reconstruction-level data are compared with the expectation from simulation, broken down into contributions from (modelled with Powheg MiNNLO + Pythia8), $Wt$, $Z$+jets, dibosons, and events with misidentified electrons or muons, normalised to the same number of entries as the data. The lower panels show the ratios of simulation to data, using various signal samples and with the cyan shaded band indicating the statistical uncertainty. The last bin includes the overflow in panels (a), (b), (e) and (f).
• Figure 3: Distributions of (a) the number of $b$-tagged jets in selected opposite-sign $e\mu$ events; and (b) the $p_{\text{T}}$ of $b$-tagged jets, (c) the $p_{\text{T}}$ of the electron, (d) the $|\eta|$ of the electron, (e) the $p_{\text{T}}$ of the muon and (f) the $|\eta|$ of the muon, in events with an opposite-sign $e\mu$ pair and exactly two $b$-tagged jets with the 77% efficiency working point. The reconstruction-level data are compared with the expectation from simulation, broken down into contributions from (modelled with Powheg MiNNLO + Pythia8), $Wt$, $Z$+jets, dibosons, and events with misidentified electrons or muons. The simulation prediction is normalised to the same integrated luminosity as the data in (a) and to the same number of entries as the data in (b--f). The lower panels show the ratios of simulation to data, using various simulation samples and with the cyan shaded band indicating the statistical uncertainty. The last bin includes the overflows in panels (b), (c) and (e).
• Figure 4: Distributions of (a) the electron $p_{\text{T}}$, (b) the electron $|\eta|$, (c) the muon $p_{\text{T}}$ and (d) the muon $|\eta|$, in events with a same-charge $e\mu$ pair and at least one $b$-tagged jet, also requiring the electron to pass the charge misidentification BDT. The simulation prediction is normalised to the same integrated luminosity as the data, and broken down into contributions where both leptons are prompt and reconstructed with correct charge signs (RS), both leptons are prompt but one has a misreconstructed charge sign (WS), or one is a misidentified lepton from a photon conversion, a heavy-flavour (HF) hadron decay to an electron, a heavy-flavour hadron decay to a muon, or other sources of misidentified electrons (such as misidentified hadrons) and muons (such as decays in flight of pions and kaons). In the $p_{\text{T}}$ distributions, the last bin includes the overflows.
• Figure 5: Results of pseudo-experiment studies on simulated events for the extraction of the normalised $\Pqt{}\Paqt\rightarrow e\mu$ differential cross-section distributions. The upper two plots show (a) $p_\mathrm{T}^{\ell}$ and (b) $p_\mathrm{T}^{e\mu}$ using Powheg + Pythia8 events with $\hbox{$m_{t}$}=172.5$$\text{Ge V}$ for the reference sample and events with $\hbox{$m_{t}$}=176$$\text{Ge V}$ for the alternative sample. The lower two plots show (c) $|\eta^{\ell}|$ and (d) $|y^{e\mu}|$ using events reweighted to the CT14 PDF set instead of NNPDF3.0 for the alternative sample. The black filled points show the mean deviations from the reference values of the results from pseudo-data samples generated with the reference simulation sample, with error bars indicating the uncertainties due to the limited number of simulated events. The cyan shaded bands indicate the expected statistical uncertainties for a single sample corresponding to the data integrated luminosity. The open red points show the mean deviations from the reference values obtained from pseudo-experiments generated from the alternative simulation sample. The red error bars represent the uncertainty due to the limited size of these alternative samples, and the red dotted lines show the true deviations from the reference in the alternative samples.