Measurement of the Higgs boson production in association with top quarks in multilepton final states in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

TL;DR

The ATLAS analysis measures ttH production in multilepton final states using 140 fb$^{-1}$ of 13 TeV data, combining six channels to extract the signal via a global likelihood fit. It presents both inclusive and differential ttH cross-sections (via STXS) and performs a CP-violation interpretation of the top–Higgs Yukawa coupling, including a CP-mixing angle scan. The results yield $\sigma_{t\bar{t}H} = 321^{+102}_{-99}$ fb and a cross-section ratio $\sigma_{t\bar{t}H}/\sigma^{\text{SM}} = 0.63^{+0.20}_{-0.19}$, with an observed $3.3\sigma$ significance. CP-structure constraints exclude large CP-odd admixtures, demonstrating consistency with the SM and placing bounds on new sources of CP violation in the top–Higgs sector.

Abstract

A measurement of the associated production of a top-quark pair with the Higgs boson ($t\bar{t}H$) in multilepton final states is presented. The analysis is based on a data sample of proton-proton collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 140 fb$^{-1}$. Six final states defined by the number and flavour of reconstructed charged leptons are combined in a simultaneous likelihood fit to extract the $t\bar{t}H$ signal and constrain the most relevant backgrounds. The measured $t\bar{t}H$ cross-section normalised to Standard Model (SM) prediction is $σ_{t\bar tH}/σ^{\text{SM}}=0.63^{+0.20}_{-0.19}$. This result corresponds to an observed (expected) significance of 3.3$σ$ (5.3$σ$). Additionally, two other fits are used to measure the $t\bar{t}H$ cross-section differentially in bins of the Higgs boson transverse momentum in the simplified template cross-section framework, and to extract the associated production cross-section of a single top-quark with the Higgs boson ($tH$) together with the $t\bar{t}H$ one. The $CP$ structure of the top quark-Higgs boson Yukawa coupling is probed through analysis of $t\bar{t}H$ and $tH$ events. The results are compatible with the SM hypothesis, and values of the mixing angle between $CP$-even and $CP$-odd top-Higgs Yukawa couplings of $| α| > 62^\circ$ are excluded at 68$\% $ confidence level.

Figures (17)

• Figure 1: (a) Analysis channel definition in terms of light lepton and $\Pgt_{\text{had-vis}}$ multiplicities. (b) Fraction of the expected $t\bar{t}H\xspace$ signal arising from different Higgs boson decay modes in each signal channel.
• Figure 2: Comparison between data and the signal-plus-background prediction for the distribution of jet multiplicity in the (a) $2\ell SS0\Pgt_{\text{had}}\xspace$, (b) $3\ell0\Pgt_{\text{had}}\xspace$, (c) $4\ell$, (d) $2\ell SS1\Pgt_{\text{had}}\xspace$, (e) $1\ell2\Pgt_{\text{had}}\xspace$, and (f) $2\ell2\Pgt_{\text{had}}\xspace$ channels. The signal and background normalisation factors and the nuisance parameters are set to their best-fit values as described in Section \ref{['sec:result']}. The shaded band indicates the total uncertainty, including correlations.
• Figure 3: STXS migration matrices evaluated in the $t\bar{t}H$ MC sample for the analysis channels featuring a $p_\text{T}^{H}$ estimation. Colours denote the overall fraction of events in each STXS bin, while numbers display the predicted yield fractions normalised to each truth STXS bin.
• Figure 4: Observed and expected event yields in all the categories in the $2\ell$SS0$\Pgt_{\text{had}}$ channel. The signal and background normalisation factors and the nuisance parameters are set to their best-fit values as described in Section \ref{['sec:result']}. The categories denoted as 'STXS $n$' correspond to the $n$-th bin in the STXS formalism described in Section \ref{['sec:evtselection']}. When several STXS bins are merged together, all numbers are quoted (e.g. 'STXS 56' corresponds to STXS bins 5 and 6). The notation '++' or '$- -$' correspond to the total charge of the leptons in the category. The shaded band indicates the total uncertainty, including correlations. The vertical line indicates the bins in which the event yields have been scaled down by a factor 10.
• Figure 5: Observed and expected event yields in all the categories in the $3\ell$0$\Pgt_{\text{had}}$ channel. The signal and background normalisation factors and the nuisance parameters are set to their best-fit values as described in Section \ref{['sec:result']}. The categories denoted as 'STXS $n$' correspond to the $n$-th bin in the STXS formalism described in Section \ref{['sec:evtselection']}. When several STXS bins are merged together, all numbers are quoted (e.g. 'STXS 56' corresponds to STXS bins 5 and 6). The shaded band indicates the total uncertainty, including correlations.