Measurement of the top-quark Yukawa coupling from $t\overline{t}$ production in the lepton+jets final state using $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

TL;DR

This work presents a novel ATLAS measurement of the top-quark Yukawa coupling by exploiting electroweak virtual corrections in the ttbar invariant-mass spectrum near threshold, using $pp$ collisions at $\sqrt{s}=13$ TeV and $L=140~\mathrm{fb}^{-1}$. The analysis relies on NLO QCD ttbar signal modelling reweighted with $Y_t$-dependent electroweak corrections calculated via HATHOR, and a data-driven treatment of non-prompt/fake leptons, with a background model that includes a toponium-like resonance near threshold. A profile-likelihood fit to the $m_{t\bar{t}}$ distribution yields $Y_t^2=1.3\pm1.7$, corresponding to an observed (expected) 95% CL upper limit of $Y_t<2.1$ in the physical region, consistent with the Standard Model. The results provide a complementary, direct probe of the top Yukawa coupling in a regime dominated by virtual Higgs effects, and are compatible with CMS measurements in related channels. This approach demonstrates the sensitivity of differential ttbar observables near threshold to the Higgs-top sector and frameworks for incorporating EW corrections in precision top-quark studies.

Abstract

The top-quark Yukawa coupling is extracted from the distribution of the top-quark pair ($t \overline{t}$) invariant mass in proton-proton collisions using 140 ${fb}^{-1}$ of data at $\sqrt{s} = 13$ TeV collected in 2015-2018 by the ATLAS experiment at the Large Hadron Collider. In the region near the production threshold, the $t \overline{t}$ invariant mass spectrum is sensitive to electroweak virtual corrections, including contributions from Higgs boson exchange, thereby providing sensitivity to the top-quark Yukawa coupling. This is the first measurement in ATLAS that aims to obtain this coupling exploiting this approach. The $t\overline{t}$ system is reconstructed in the single-lepton final state, requiring exactly one isolated electron or muon and at least four jets with at least two identified as originating from $b$-quarks. The measured Yukawa coupling is found to be in good agreement with the Standard Model prediction. An upper limit on the top-quark Yukawa coupling strength of $Y_t < 2.1$ relative to the Standard Model prediction is observed at 95% confidence level, consistent with the expected sensitivity.

Figures (7)

• Figure 1: Example Feynman diagrams for (a) $gg$ and (b) production with virtual Higgs boson exchange.
• Figure 2: Ratio of the electroweak corrections ($\delta \sigma_\text{EW} = \sigma_\text{QCD+EW} - \sigma_\text{QCD}$) over the leading-order QCD cross-section ($\sigma_\text{QCD}$) at parton level, as a function of $m_{\Pqt{}\Paqt}\xspace^\textrm{gen}$, the mass calculated directly from the top quark and top antiquark four-momenta at generator level, for different $Y_{t}$ values from (a) quark--antiquark annihilation and (b) gluon--gluon fusion processes, calculated with HATHOR 2.1-b3 Hathor-21-b3.
• Figure 3: Reconstructed $m_{\Pqt{}\Paqt}$ resolution and the correlation of the reconstructed $m_{\Pqt{}\Paqt}$ with the generator level $m_{\Pqt{}\Paqt}$ distribution. (a) The $m_{\Pqt{}\Paqt}$ resolution distributions after the $m_t$ and $m_{W}^\textrm{had}$ requirements, compared with the case without these selections. (b) The reconstructed $m_{\Pqt{}\Paqt}$ after applying the selection of $m_{W}^\textrm{had} \in [40,120]~$$\text{Ge V}$ and $m_t \in [100,250]~$$\text{Ge V}$, plotted against the generated $m_{\Pqt{}\Paqt}$.
• Figure 4: Reconstructed $m_{\Pqt{}\Paqt}$ distributions corresponding to the various $Y_{t}^{2}$ templates used in the fit for (a) the electron and (b) the muon channel. The lower panel displays the ratio of the yields for each $Y_{t}^{2}$ template relative to the $Y_{t}^{2}\xspace=1$ template. The negative $Y_{t}^{2}$ templates are unphysical and are used only to improve the fit stability.
• Figure 5: The $m_{\Pqt{}\Paqt}$ distributions (a), (c) before the fit and (b), (d) after the combined fit, for $e$+jets and $\mu$+jets SRs, respectively. The signal in the plots corresponds to $Y_{t}\xspace=1$. The shaded bands represent the contribution of statistical and systematic uncertainties. The bottom panel in the figures show the ratio of the data over the total prediction.