Summary of ATLAS 13 TeV results using the full 2015 − 2017 data-set

Abstract:A search for dark matter produced in association with a Higgs boson decaying to $b$-quarks is performed using $pp$ collisions at a centre-of-mass energy of $\sqrt{s}= 13\,$TeV. The dataset has an integrated luminosity of 79.8$\,$fb$^{-1}$ and was recorded with the ATLAS detector at the Large Hadron Collider. Selected collision events comprise large missing transverse momentum and either two $b$-tagged small radius jets or a single large radius jet containing two $b$-tagged subjets. The reconstruction of these subjets is based on a jet algorithm that uses inner detector tracks as inputs and a radius parameter that decreases as the subjet transverse momentum increases. The observed data are in agreement with the Standard Model predictions. The results are interpreted in the context of a simplified model ($Z^{\prime}$-2HDM) which describes the interaction of dark matter and Standard Model particles via new heavy mediator particles. Masses of the $Z^{\prime}$ are excluded up to $2.8\,$TeV depending on the choices for other model parameters.

Keywords:13 TeV,BSM search,Dark matter,Higgs,Boosted,MET,b-jets,2 jets,1 jet

Abstract:A search is presented for single production of a vector-like $B$ quark that decays into a Higgs boson and a $b$ quark. The search uses a dataset corresponding to 79.8 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=13$ TeV collected during 2015-2017 with the ATLAS detector at the Large Hadron Collider. The analysis focuses on the presence of a Higgs boson that decays into two photons, a high-momentum $b$-jet and a forward jet, characteristic of $t$-channel production of the signal process. The main background process is non-resonant diphoton production, with additional small contributions from Standard Model processes containing $H\rightarrow\gamma\gamma$ decays. No significant excess above the background expectation is observed, and upper limits on the signal production cross section are set as a function of the $B$-quark mass. Assuming a generalized coupling $\kappa_B = 0.5$ and doublet branching ratios of 50% for $B\rightarrow Hb$ and $B\rightarrow Zb$, $B$ quarks with masses less than 1210 GeV are excluded at the 95% confidence level.

Keywords:Photons,13 TeV,Vector-like quarks,BSM resonance,BSM search,Higgs,Di-photon,b-hadron,b-jets

Abstract:Narrow resonances decaying into $WW$, $WZ$ or $ZZ$ boson pairs are searched for in 79.8 fb$^{-1}$ of proton-proton data at a centre-of-mass energy of $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the Large Hadron Collider from 2015 to 2017. The diboson system is reconstructed using pairs of high transverse momentum, large-radius jets build from a combination of calorimeter- and tracker-inputs tagged as compatible with the hadronic decay of a boosted $W$ or $Z$ boson, using jet mass and substructure properties. The search covers diboson resonances with masses in the range 1.2 TeV - 5.0 TeV. No significant deviations from the background expectations are seen. Exclusion limits at the $95 confidence level are set on the production cross section times branching ratio to dibosons for a range of theories beyond the Standard Model, with the highest lower limit on the mass of a new gauge boson at 4.15 TeV in the context of mass-degenerate resonances that couple predominantly to bosons.

Keywords:13 TeV,BSM resonance,BSM search,ZZ,WZ,WW,Boosted,All hadronic,2 jets

Abstract:A search for the pair production of heavy leptons ($N^0$, $L^\pm$) as predicted by the type-III seesaw mechanism is presented. The search uses proton-proton collision data at a centre-of-mass energy of 13 TeV corresponding to 79.8 fb$^{-1}$ of integrated luminosity recorded in 2015, 2016 and 2017 by the ATLAS detector at the Large Hadron Collider. The analysis focuses on the $N^0$ and $L^\pm$ decays with intermediate $W$ bosons and yielding two final-state light leptons (electron or muon) of different flavor and charge combinations, with at least two jets and large missing transverse momentum in the final state. The search is optimized in six channels distinguished by the flavor combination and charge product of the final state lepton pair, where same charge or opposite charge final states are considered. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits on heavy lepton $m(N^0, L^\pm)$ masses, where $N^0$ and $L^\pm$ are considered mass-degenerate. The observed lower 95 % confidence level limit on the mass of the type-III seesaw heavy leptons, where the branching fractions to all lepton flavors are assumed to be equal, is 560 GeV.

Keywords:13 TeV,Heavy neutrinos,BSM search,MET,2 jets,2 leptons (same charge),2 leptons

Abstract:A search for dijet resonances in events with identified leptons is performed using $79.8~{\text{fb}}^{-1}$ of $\sqrt{s}=13$ TeV proton-proton collision data recorded by the ATLAS detector. Dijet invariant mass distributions ranging from $0.22-6.3$ TeV in events with at least one isolated lepton ($e$ or $\mu$) are searched for excesses above background expectations. Selecting events based on the presence of a lepton can probe lower $m_{jj}$ than traditional inclusive dijet searches and targets a large range of new physics models with a final-state lepton. Signatures of such processes include associated production of a $W$ or $Z$ boson. No statistically significant deviation from the background hypothesis is found. In the absence of signals indicating the presence of new physics, limits are calculated based on generic Gaussian signals with a width ranging from that determined by the detector resolution to $15\%$ of the resonance mass. Contributions from a signal with effective cross-section ranging from $50$ fb to $0.1$ fb are excluded in the mass range between 0.25 TeV and 6 TeV.

Keywords:13 TeV,BSM resonance,BSM search,MET,2 jets,1 lepton

Abstract:The results of a search for a new heavy, charged $W^\prime$ boson decaying into an electron or muon and a neutrino using proton-proton collision data at a centre-of-mass energy of $\sqrt{s } = 13$ TeV are presented. The data were collected in 2015, 2016 and 2017 by the ATLAS experiment at the Large Hadron Collider and correspond to an integrated luminosity of 79.8 fb$^{-1}$. As no excess of events above the Standard Model prediction is observed, the results are used to set upper limits on the $W^\prime$ boson cross-section times branching ratio to an electron or muon and a neutrino as a function of the $W^\prime$ mass. Assuming a $W^\prime$ boson with the same couplings as the Standard Model $W$ boson, $W^\prime$ masses below 5.6 TeV are excluded at the 95\% confidence level.

Keywords:13 TeV,BSM resonance,BSM search,MET,1 lepton

Abstract:A search for the decay of the Standard Model Higgs boson into a $b\bar{b}$ pair when produced in association with a $W$ or $Z$ boson is performed with the ATLAS detector. The data, corresponding to an integrated luminosity of 79.8 $\mathrm{fb}^{-1}$ were collected in proton-proton collisions during Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.9 (4.3) standard deviations. A combination with the results from other searches in Run 1 and in Run 2 for the Higgs boson in the $b\bar{b}$ decay mode is performed, which yields an observed (expected) significance of 5.4 (5.5) standard deviations, thus providing direct observation of the Higgs boson decay into $b$-quarks. The ratio of the measured event yield for a Higgs boson decaying into $b\bar{b}$ to the Standard Model expectation is $1.01 \pm 0.12 \mathrm{(stat.)} ^{+0.16}_{-0.15} \mathrm{(syst.)}$. Additionally, a combination of Run 2 results searching for the Higgs boson produced in association with a vector boson yields an observed (expected) significance of 5.3 (4.8) standard deviations.

Keywords:13 TeV,Coupling measurements,H(125) measurements,VH production,Higgs to bb,Higgs,VH,b-jets,2 leptons,1 lepton,0 lepton

Abstract:Combined measurements of Higgs boson production cross-sections, branching fractions and couplings are presented in the Higgs boson decay channels into $\gamma\gamma$, $ZZ^*$, $WW^*$, $\tau\tau$, $\mu\mu$ and $b\bar{b}$, using up to $79.8$ fb$^{-1}$ of proton--proton collision data at $\sqrt{s}= $ 13 TeV collected with the ATLAS experiment. Results are presented for the gluon-gluon fusion and vector-boson fusion processes, and associated production with vector bosons or top quarks. The combined measurement yields an observed (expected) significance for the vector-boson fusion production process of $6.5\sigma$ ($5.3\sigma$). This single-experiment observation follows an earlier observation in the combination of ATLAS and CMS measurements. The results are interpreted in terms of modifiers applied to the Standard Model couplings of the Higgs boson to other particles, and are used to set exclusion limits on new phenomena. No significant deviations from Standard Model predictions are observed.

Keywords:13 TeV,Coupling measurements,kappa,H(125) measurements,Statistical combination

Abstract:This note reports measurements of Higgs boson properties in the two-photon final state using 79.8 fb$^{-1}$ of data recorded at $\sqrt{s}=13$ TeV by the ATLAS experiment at the Large Hadron Collider. The cross sections of Higgs boson production through gluon--gluon fusion, vector-boson fusion, and in association with a vector boson or a top-quark pair are measured. Measurements of the Higgs boson production divided further into kinematic regions, called simplified template cross sections, are also reported. Additionally, the cross section for the production of the Higgs boson decaying to two isolated photons is measured in a fiducial phase space designed to closely match the ATLAS detector acceptance, and is found to be 60.4 $\pm$ 6.1 (stat.) $\pm$ 6.0 (exp.) $\pm$ 0.3 (theo.) fb, in agreement with the Standard Model prediction of 63.5 $\pm$ 3.3 fb. Finally, the fiducial cross section is measured differentially in bins of several kinematic observables with sensitivity to properties of the Higgs boson. Among these, the number of $b$-jets produced in association with the Higgs boson is measured to probe Higgs production in association with heavy flavor hadrons. No significant deviations between the observed data and the Standard Model prediction are observed.

Keywords:13 TeV,Simplified template cross-section,Coupling measurements,H(125) measurements,ttH production,VH production,VBF production,Gluon fusion production,Higgs to 2 photons,VBF,Gluon fusion,Differential measurement,Cross-section measurement,Higgs,Di-photon,Photons

Abstract:A search for low-mass diphoton resonances is performed using 80 fb$^{-1}$ of $pp$ collision data collected with the ATLAS detector at the Large Hadron Collider. Pairs of isolated photon candidates with high transverse momentum are selected, probing the diphoton invariant mass spectrum in the range 65 to 110 GeV. No significant excess with respect to the Standard Model expectation is found, and a limit at the 95% confidence level is set on narrow resonance fiducial cross-section times branching ratio ranging from 30 to 101 fb.

Keywords:13 TeV,BSM Higgs searches,Higgs to 2 photons,BSM search,Di-photon,Photon,Photons

Abstract:A search for the dimuon decay of the Standard Model Higgs boson was performed using data corresponding to an integrated luminosity of $79.8$ fb$^{-1}$ collected so far in Run 2 with the ATLAS detector in $pp$ collisions at $\sqrt{s}=13$ TeV at the Large Hadron Collider. No significant excess is observed above the expected background. The observed upper limit on the cross section times branching ratio is $2.1$ times the Standard Model prediction at $95$\% confidence level for a Higgs boson mass of $125$ GeV, while the expected limit in the absence of a $H \to \mu\mu$ signal was $2.0$. The measured signal strength, defined as the ratio of the observed signal yield to the one expected in the Standard Model, is $\mu = 0.1 ^{+1.0}_{-1.1}$.

Keywords:13 TeV,Higgs to 2 muons,Higgs,2 leptons

Abstract:The observation of Higgs boson production in association with a top quark pair ($t\bar{t}H$), based on the analysis of proton--proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider, is presented. Using data corresponding to integrated luminosities of up to 79.8 fb$^{-1}$, and considering Higgs boson decays into $b\bar{b}$, $WW^*$, $\tau\tau$, $\gamma\gamma$, and $ZZ^*$, the observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the $t\bar{t}H$ searches using a dataset corresponding to integrated luminosities of 4.5 fb$^{-1}$ at 7 TeV and 20.3 fb$^{-1}$ at 8 TeV, the observed (expected) significance is 6.3 (5.1) standard deviations. Assuming Standard Model branching fractions, the total $t\bar{t}H$ production cross section at 13 TeV is measured to be 670 $\pm$ 90 (stat.) $^{+110}_{-100}$ (syst.) fb, in agreement with the Standard Model prediction.

Keywords:13 TeV,Coupling measurements,H(125) measurements,ttH production,Higgs to 4 lepton,Higgs to 2 photons,Statistical combination,Cross-section measurement,Higgs,ZZ,Photon,Photons,>=3 leptons

Abstract:Inclusive, differential and production mode cross sections of the Higgs boson production in proton-proton collisions are measured in the $H \to ZZ^{*} \to 4\ell$ decay channel. The proton$-$proton collision data were produced at the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector from 2015 to 2017, corresponding to an integrated luminosity of 79.8 fb$^{-1}$. The inclusive fiducial cross section for the process $H \to ZZ^{*} \to 4\ell$ is measured to be $\sigma_\mathrm{fid } = 4.04 \pm 0.47$ fb, while the Standard Model prediction is $\sigma_\mathrm{fid, SM } = 3.35 \pm 0.15 $ fb. The cross-section times $H \to ZZ^{*}$ branching ratio for gluon fusion and vector-boson fusion production are measured to be $1.21 \pm 0.18$ pb and $0.24 \pm 0.09$ pb, respectively. All measurements are in agreement with the Standard Model prediction.

Keywords:13 TeV,Simplified template cross-section,Coupling measurements,H(125) measurements,ttH production,VH production,VBF production,Gluon fusion production,Higgs to ZZ,Higgs to 4 lepton,VBF,Gluon fusion,Differential measurement,Cross-section measurement,Higgs,ZZ,>=3 leptons

Abstract:The results of a search for the direct pair production of charginos decaying via W-bosons are reported using 80.5 fb-1 of integrated luminosity from LHC $pp$ collisions at 13 TeV collected by the ATLAS detector in the 2015, 2016 and 2017 data taking campaigns. Charginos are searched for in final states with two isolated leptons (electron or muon), missing transverse momentum and at most one light jet in the final state. No excess above Standard Model expectations is observed. Exclusion limits are derived on the chargino mass assuming a simplified model in which it decays with a 100% branching ratio to an on-shell W-boson and the lightest neutralino: $\tilde{\chi}_1^\pm \rightarrow W^\pm \tilde{\chi}_1^{0}$. For a massless neutralino, chargino masses up to 410~GeV are excluded at 95% confidence level.

Keywords:13 TeV,Chargino/neutralino pair production,BSM search,MET,0 jets,2 leptons

Abstract:A search for supersymmetry involving the pair production of gluinos decaying via third-generation squarks into the lightest neutralino ($\displaystyle\tilde\chi^0_1$) is reported. It uses LHC proton-proton collision data at a centre-of-mass energy $\sqrt{s } = 13$ TeV with an integrated luminosity of 79.9 fb$^{-1}$ collected with the ATLAS detector from 2015 to 2017. The search is performed in events containing large missing transverse momentum and several energetic jets, at least three of which must be identified as originating from $b$-quarks. No excess is found above the predicted background. For $\displaystyle\tilde\chi^0_1$ masses below approximately 800 GeV, gluino masses of less than 2.23 TeV (2.19) TeV are excluded at 95 % confidence level in simplified models involving the pair production of gluinos that decay via top (bottom) squarks. An interpretation of the limits in terms of the branching ratios of the gluinos into third-generation squarks is also provided.

Keywords:13 TeV,Gluino pair production,BSM search,MET,b-jets,>=3 jets,1 lepton,0 lepton

Abstract:The result of a search for the production of the supersymmetric partner of the bottom-quark ($\tilde{b}_{1}$) using $79.8\,\mathrm{fb}^{-1}$ of data collected at $\sqrt{s } = 13\,\mathrm{TeV}$ by the ATLAS detector in Run 2 is reported. In the supersymmetric scenarios considered the bottom-squarks decay to a $b$-quark and the second lightest neutralino, $\tilde{b}_{1} \rightarrow b + \tilde{\chi}^{0}_{2}$. The $\tilde{\chi}^{0}_{2}$ is assumed to subsequently decay with $100\%$ branching ratio to the Standard-Model-like Higgs boson ($h$) and the lightest neutralino: $\tilde{\chi}^{0}_{2} \rightarrow h + \tilde{\chi}^{0}_{1}$. The $\tilde{\chi}^{0}_{1}$ is assumed to be the lightest supersymmetric particle (LSP) and is stable. Two signal mass configurations are targeted: the first has a constant LSP mass of $60\,\mathrm{GeV}$; and the second has a constant mass difference between the $\tilde{\chi}^{0}_{2}$ and $\tilde{\chi}^{0}_{1}$ of $130\,\mathrm{GeV}$. The final states considered contain zero leptons, three or more $b$-jets, and large missing transverse momentum. No significant excess of events over the Standard Model background expectation is observed in any of the signal regions considered. Limits at the $95\%$ confidence level are placed in the simplified models considered, and bottom-squarks with mass up to $1400\,\mathrm{GeV}$ are excluded.

Keywords:13 TeV,Sbottom pair production,BSM search,MET,b-jets

Abstract:A search for Standard Model Higgs bosons decaying into at least one photon and missing transverse momentum is performed using 79.8 fb$^{-1}$ of proton–proton collisions collected at $\sqrt{s}=13$ TeV with the ATLAS detector, where the photon can arise from a neutralino decaying into a gravitino. Higgs bosons produced in association with a $Z$ boson are considered in order to reduce the number of background events. No excess with respect to the Standard Model prediction is observed. Assuming a Standard Model $Zh$ production cross-section, the branching fraction of a Higgs boson to neutralinos or neutralino/gravitino is constrained to be less than 5–11% at the 95% confidence level for near massless gravitinos.

Keywords:13 TeV,Higgs to SUSY,GGM/GMSB,VH production,BSM search,Photon,Z,MET,Photons,2 leptons

Abstract:A direct search for charged lepton-flavour violation in top-quark decays is presented. The data analysed correspond to 79.8 $\text{fb}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}= 13$ TeV delivered by the LHC. The process studied is the production of top-quark pairs, where one top quark decays into a pair of opposite-sign different-flavour charged leptons and an up-type quark, while the other decays semileptonically according to the Standard Model. The signature of the signal is thus characterised by the presence of three charged leptons, a light jet and a $b$-jet. A multivariate discriminant is deployed and its distribution used as input to extract the signal strength. In the absence of a signal, an upper limit on the branching ratio of ${\cal B}(t \to \ell \ell q) < 1.86 \times 10^{-5}$ is set at 95% confidence level.

Keywords:>=3 leptons,Top properties measurements,ttbar rare decays,LFV