A measurement of the $B^0_s \to J/\psi\phi$ decay parameters using 80.5 $\mathrm{fb}^{-1}$ of integrated luminosity collected with the ATLAS detector from 13 TeV proton-proton collisions at the LHC is presented. The measured parameters include the $CP$-violating phase $\phi_s$, the width difference $\Delta\Gamma_{s}$ between the $B^0_s$ meson mass eigenstates and the average decay width $\Gamma_{s}$. The values measured for the physical parameters are combined with those from 19.2 $\mathrm{fb}^{-1}$ of 7 TeV and 8 TeV data, leading to the following: \begin{eqnarray*} \phi_s & = & -0.087\phantom{0} \pm 0.036\phantom{0} ~\mathrm{(stat.)} \pm 0.021\phantom{0} ~\mathrm{(syst.)~rad} \\ \Delta\Gamma_{s} & = & \phantom{-}0.0657 \pm 0.0043 ~\mathrm{(stat.)} \pm 0.0037 ~\mathrm{(syst.)~ps}^{-1} \\ \Gamma_{s} & = & \phantom{-}0.6703 \pm 0.0014 ~\mathrm{(stat.)} \pm 0.0018 ~\mathrm{(syst.)~ps}^{-1} \\ \end{eqnarray*} Results for $\phi_s$ and $\Delta\Gamma_{s}$ are also presented as 68% confidence level contours in the $\phi_s$-$\Delta\Gamma_{s}$ plane. Furthermore, the transversity amplitudes and corresponding strong phases are measured. $\phi_s$ and $\Delta\Gamma_{s}$ measurements are in agreement with the Standard Model predictions.
The results of a search for new heavy $W^\prime$ bosons decaying to 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 dataset was collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 36.1 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 5.1 TeV are excluded at the 95% confidence level.
The fragmentation of high-energy gluons at small opening angles is largely unconstrained by present measurements. Gluon splitting to $b$-quark pairs is a unique probe into the properties of gluon fragmentation because identified $b$-tagged jets provide a proxy for the quark daughters of the initial gluon. In this study, key differential distributions related to the $g\rightarrow b\bar{b}$ process are measured using 33 fb$^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collision data recorded by the ATLAS experiment at the LHC in 2016. Jets constructed from charged-particle tracks, clustered with the anti-$k_t$ jet algorithm with radius parameter $R = 0.2$, are used to probe angular scales below the $R=0.4$ jet radius. The observables are unfolded to particle level in order to facilitate direct comparisons with predictions from present and future simulations. Multiple significant differences are observed between the data and parton shower Monte Carlo predictions, providing input to improve these predictions of the main source of background events in analyses involving boosted Higgs bosons decaying into $b$-quarks.
A search for the direct production of charginos and neutralinos in final states with at least two hadronically decaying tau leptons is presented. The analysis uses a dataset of $pp$ collisions corresponding to an integrated luminosity of 36.1 fb$^{-1}$, recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. No significant deviation from the Standard Model background expectation is observed. Limits are derived in scenarios of $\tilde{\chi}_{1}^{+}\tilde{\chi}_{1}^{-}$ pair production and of $\tilde{\chi}_{1}^{\pm}\tilde{\chi}_{2}^{0}$ and $\tilde{\chi}_{1}^{+}\tilde{\chi}_{1}^{-}$ production in simplified models where the neutralinos and charginos decay solely via intermediate left-handed staus and tau sneutrinos, and the mass of the $\tilde{\tau}_{\mathrm L}$ state is set to be halfway between the masses of the $\tilde{\chi}_{1}^{\pm}$ and the $\tilde{\chi}_{1}^{0}$. Chargino masses up to 630 GeV are excluded at 95% confidence level in the scenario of direct production of $\tilde{\chi}_{1}^{+}\tilde{\chi}_{1}^{-}$ for a massless $\tilde{\chi}_{1}^{0}$. Common $\tilde{\chi}_{1}^{\pm}$, $\tilde{\chi}_{2}^{0}$ masses up to 760 GeV are excluded in the case of production of $\tilde{\chi}_{1}^{\pm}\tilde{\chi}_{2}^{0}$ and $\tilde{\chi}_{1}^{+}\tilde{\chi}_{1}^{-}$ assuming a massless $\tilde{\chi}_{1}^{0}$. Exclusion limits for additional benchmark scenarios with large and small mass-splitting between the $\tilde{\chi}_{1}^{\pm}$ and the $\tilde{\chi}_{1}^{0}$ are also studied by varying the $\tilde{\tau}_{\mathrm L}$ mass between the masses of the $\tilde{\chi}_{1}^{\pm}$ and the $\tilde{\chi}_{1}^{0}$.
A measurement of the Standard Model Higgs boson produced in association with a $W$ or $Z$ boson and decaying into a pair of $\tau$-leptons is presented. This search is based on proton-proton collision data collected at $\sqrt{s}=13$ TeV by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 140 fb$^{-1}$. For the Higgs boson candidate, only final states with at least one $\tau$-lepton decaying hadronically ($\tau\rightarrow \mathrm{hadrons} + \nu_\tau$) are considered. For the vector bosons, only leptonic decay channels are considered: $Z \rightarrow \ell\ell$ and $W\rightarrow \ell\nu_\ell$, with $\ell=e,\mu$. An excess of events over the expected background is found with an observed (expected) significance of 4.2 (3.6) standard deviations, providing evidence of the Higgs boson produced in association with a vector boson and decaying into a pair of $\tau$-leptons. The ratio of the measured cross-section to the Standard Model prediction is $\mu_{\text{VH}}^{\tau\tau} = 1.28\ ^{+0.30}_{-0.29}\ (\mathrm{stat.})\ ^{+0.25}_{-0.21}\ (\mathrm{syst.})$. This result represents the most accurate measurement of the \vh process achieved to date.
Several extensions of the Standard Model predict associated production of dark-matter particles with a Higgs boson. Such processes are searched for in final states with missing transverse momentum and a Higgs boson decaying to a $b\bar b$ pair with the ATLAS detector using 36.1 fb$^{-1}$ of $pp$ collisions at a center-of-mass energy of 13 TeV at the LHC. The observed data are in agreement with the Standard Model predictions and limits are placed on the associated production of dark-matter particles and a Higgs boson.
Measurements of jet activity in top-quark pair events produced in proton--proton collisions are presented, using 3.2 fb$^{-1}$ of $pp$ collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Events are chosen by requiring an opposite-charge $e\mu$ pair and two $b$-tagged jets in the final state. The normalised differential cross-sections of top-quark pair production are presented as functions of additional-jet multiplicity and transverse momentum, $p_{\mathrm T}$. The fraction of signal events that do not contain additional jet activity in a given rapidity region, the gap fraction, is measured as a function of the $p_{\mathrm T}$ threshold for additional jets, and is also presented for different invariant mass regions of the $e\mu b\bar{b}$ system. All measurements are corrected for detector effects and presented as particle-level distributions compared to predictions with different theoretical approaches for QCD radiation. While the kinematics of the jets from top-quark decays are described well, the generators show differing levels of agreement with the measurements of observables that depend on the production of additional jets.
This paper presents a measurement of the polarisation of $\tau$ leptons produced in $Z/\gamma^{*}\rightarrow\tau\tau$ decays which is performed with a dataset of proton-proton collisions at $\sqrt{s}=8$ TeV, corresponding to an integrated luminosity of 20.2 fb$^{-1}$ recorded with the ATLAS detector at the LHC in 2012. The $Z/\gamma^{*}\rightarrow\tau\tau$ decays are reconstructed from a hadronically decaying $\tau$ lepton with a single charged particle in the final state, accompanied by a $\tau$ lepton that decays leptonically. The $\tau$ polarisation is inferred from the relative fraction of energy carried by charged and neutral hadrons in the hadronic $\tau$ decays. The polarisation is measured in a fiducial region that corresponds to the kinematic region accessible to this analysis. The $\tau$ polarisation extracted over the full phase space within the $Z/\gamma^{*}$ mass range of 66$ < m_{Z/\gamma^{*}} < $ 116 GeV is found to be $P_{\tau} =-0.14 \pm 0.02 (\text{stat}) \pm 0.04 (\text{syst})$. It is in agreement with the Standard Model prediction of $P_{\tau} =-0.1517 \pm 0.0019$, which is obtained from the ALPGEN event generator interfaced with the PYTHIA 6 parton shower modelling and the TAUOLA $\tau$ decay library.
Multi-particle azimuthal cumulants are measured as a function of centrality and transverse momentum using 470 $\mu$b$^{-1}$ of Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ATLAS detector at the LHC. These cumulants provide information on the event-by-event fluctuations of harmonic flow coefficients $v_n$ and correlated fluctuations between two harmonics $v_n$ and $v_m$. For the first time, a non-zero four-particle cumulant is observed for dipolar flow, $v_1$. The four-particle cumulants for elliptic flow, $v_2$, and triangular flow, $v_3$, exhibit a strong centrality dependence and change sign in ultra-central collisions. This sign change is consistent with significant non-Gaussian fluctuations in $v_2$ and $v_3$. The four-particle cumulant for quadrangular flow, $v_4$, is found to change sign in mid-central collisions. Correlations between two harmonics are studied with three- and four-particle mixed-harmonic cumulants, which indicate an anti-correlation between $v_2$ and $v_3$, and a positive correlation between $v_2$ and $v_4$. These correlations decrease in strength towards central collisions and either approach zero or change sign in ultra-central collisions. To investigate the possible flow fluctuations arising from intrinsic centrality or volume fluctuations, the results are compared between two different event classes used for centrality definitions. In peripheral and mid-central collisions where the cumulant signals are large, only small differences are observed. In ultra-central collisions, the differences are much larger and transverse momentum dependent. These results provide new information to disentangle flow fluctuations from the initial and final states, as well as new insights on the influence of centrality fluctuations.
A search is conducted for new resonances decaying into a $WW$ or $WZ$ boson pair, where one $W$ boson decays leptonically and the other $W$ or $Z$ boson decays hadronically. It is based on proton-proton collision data with an integrated luminosity of 36.1 fb$^{-1}$ collected with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV in 2015 and 2016. The search is sensitive to diboson resonance production via vector-boson fusion as well as quark-antiquark annihilation and gluon-gluon fusion mechanisms. No significant excess of events is observed with respect to the Standard Model backgrounds. Several benchmark models are used to interpret the results. Limits on the production cross section are set for a new narrow scalar resonance, a new heavy vector-boson and a spin-2 Kaluza-Klein graviton.
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