Measurements of the production cross-sections of the Standard Model (SM) Higgs boson ($H$) decaying into a pair of $\tau$-leptons are presented. The measurements use data collected with the ATLAS detector from $pp$ collisions produced at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13\,\text{TeV}$, corresponding to an integrated luminosity of $139\,\text{fb}^{-1}$. Leptonic ($\tau\to\ell\nu_{\ell}\nu_{\tau}$) and hadronic ($\tau\to\text{hadrons}~\nu_{\tau}$) decays of the $\tau$-lepton are considered. All measurements account for the branching ratio of $H\to\tau\tau$ and are performed with a requirement $|y_H|<2.5$, where $y_H$ is the true Higgs boson rapidity. The cross-section of the $pp\to H\to\tau\tau$ process is measured to be $2.94 \pm 0.21 \text{(stat)} ^{+\,0.37}_{-\,0.32} \text{(syst)}$ pb, in agreement with the SM prediction of $3.17\pm0.09~ \mbox{pb}$. Inclusive cross-sections are determined separately for the four dominant production modes: $2.65 \pm 0.41 \text{(stat)} ^{+\,0.91}_{-\,0.67} \text{(syst)}$ pb for gluon$-$gluon fusion, $0.197 \pm 0.028 \text{(stat)} ^{+\,0.032}_{-\,0.026} \text{(syst)}$ pb for vector-boson fusion, $0.115 \pm 0.058 \text{(stat)} ^{+\,0.042}_{-\,0.040} \text{(syst)}$ pb for vector-boson associated production, and $0.033 \pm 0.031 \text{(stat)} ^{+\,0.022}_{-\,0.017} \text{(syst)}$ pb for top-quark pair associated production. Measurements in exclusive regions of the phase space, using the simplified template cross-section framework, are also performed. All results are in agreement with the SM predictions.
Observed yields in the boost_0_1J signal region category of the hh channel.
Observed yields in the boost_0_ge2J signal region category of the hh channel.
Observed yields in the boost_1_1J signal region category of the hh channel.
A search for new-physics resonances decaying into a lepton and a jet performed by the ATLAS experiment is presented. Scalar leptoquarks pair-produced in $pp$ collisions at $\sqrt{s}=13$ TeV at the Large Hadron Collider are considered using an integrated luminosity of 139 fb$^{-1}$, corresponding to the full Run 2 dataset. They are searched for in events with two electrons or two muons and two or more jets, including jets identified as arising from the fragmentation of $c$- or $b$-quarks. The observed yield in each channel is consistent with the Standard Model background expectation. Leptoquarks with masses below 1.8 TeV and 1.7 TeV are excluded in the electron and muon channels, respectively, assuming a branching ratio into a charged lepton and a quark of 100%, with minimal dependence on the quark flavour. Upper limits on the aforementioned branching ratio are also given as a function of the leptoquark mass.
Distribution of the resonance mass in the pretag Signal Region of the $ qe$ channel for the post-fit background, the observed data, and the expected signal with $m_{LQ} = 1$ TeV.
Distribution of the resonance mass in the pretag Signal Region of the $ q\mu$ channel for the post-fit background, the observed data, and the expected signal with $m_{LQ} = 1$ TeV.
Distribution of the resonance mass in the untagged Signal Region of the $ ce$ channel for the post-fit background, the observed data, and the expected signal with $m_{LQ} = 1$ TeV.
A search for Higgs boson decays into a $Z$ boson and a light resonance in two-lepton plus jet events is performed, using a $pp$ collision dataset with an integrated luminosity of 139 fb$^{-1}$ collected at $\sqrt{s}=13$ TeV by the ATLAS experiment at the CERN LHC. The resonance considered is a light boson with a mass below 4 GeV from a possible extended scalar sector, or a charmonium state. Multivariate discriminants are used for the event selection and for evaluating the mass of the light resonance. No excess of events above the expected background is found. Observed (expected) 95$\% $ confidence-level upper limits are set on the Higgs boson production cross section times branching fraction to a $Z$ boson and the signal resonance, with values in the range 17 pb to 340 pb ($16^{+6}_{-5}$ pb to $320^{+130}_{-90}$ pb) for the different light spin-0 boson mass and branching fraction hypotheses, and with values of 110 pb and 100 pb ($100^{+40}_{-30}$ pb and $100^{+40}_{-30}$ pb) for the $\eta_c$ and $J/\psi$ hypotheses, respectively.
Observed number of data events and expected number of background events in the signal region.
Efficiencies of the MLP selection, complete selection and total expected signal yields for each signal sample, assuming B$(H\to Z(Q/a))=100\%$ and $\sigma(pp\to H) = \sigma_\text{SM}(pp\to H)$. Pythia 8 branching fractions of $a$ are assumed using a $\tan\beta$ value of 1. The MLP efficiencies, total efficiencies, and expected yields are determined using MC samples, with uncertainties due to MC sample statistics, except for the expected background yield. The expected background yield and its uncertainty is calculated as described in the main text of the paper.
Expected and observed 95% CL upper limits on $\sigma(pp\to H)B(H\to Za)/$pb. These results are quoted for $B(a\to gg)=100\%$ and $B(a\to s\bar{s})=100\%$ for each signal sample. The smaller (larger) quoted ranges around the expected limits represent $\pm 1\sigma$ ($\pm 2\sigma$) fluctuations.