A search is presented for hadronic signatures of beyond the Standard Model (BSM) physics, with an emphasis on signatures of a strongly-coupled hidden dark sector accessed via resonant production of a $Z'$ mediator. The ATLAS experiment dataset collected at the Large Hadron Collider from 2015 to 2018 is used, consisting of proton-proton collisions at $\sqrt{s}$ = 13 TeV and corresponding to an integrated luminosity of 140 fb$^{-1}$. The $Z'$ mediator is considered to decay to two dark quarks, which each hadronize and decay to showers containing both dark and Standard Model particles, producing a topology of interacting and non-interacting particles within a jet known as ``semi-visible". Machine learning methods are used to select these dark showers and reject the dominant background of mismeasured multijet events, including an anomaly detection approach to preserve broad sensitivity to a variety of BSM topologies. A resonance search is performed by fitting the transverse mass spectrum based on a functional form background estimation. No significant excess over the expected background is observed. Results are presented as limits on the production cross section of semi-visible jet signals, parameterized by the fraction of invisible particles in the decay and the $Z'$ mass, and by quantifying the significance of any generic Gaussian-shaped mass peak in the anomaly region.
Acceptance times efficiency weighted yields across the signal grid.
The 95% CL limits on the cross-section $\sigma(pp \rightarrow Z' \rightarrow \chi \chi$) times branching ratio B in fb with all statistical and systematic uncertainties, for the $R_{\text{inv}}=$0.2 signal points.
The 95% CL limits on the cross-section $\sigma(pp \rightarrow Z' \rightarrow \chi \chi$) times branching ratio B in fb with all statistical and systematic uncertainties, for the $R_{\text{inv}}=$0.4 signal points.
A measurement of the $B^{0}$ meson lifetime and related properties using $B^0 \to J/ψK^{*0}$ decays in data from 13 TeV proton-proton collisions with an integrated luminosity of 140 fb$^{-1}$ recorded by the ATLAS detector at the LHC is presented. The measured effective lifetime is $$ τ= 1.5053 \pm 0.0012 ~\mathrm{(stat.)} \pm 0.0035 ~\mathrm{(syst.)~ps}. $$ The average decay width extracted from the effective lifetime, using parameters from external sources, is $$ Γ_d = 0.6639 \pm 0.0005 ~\mathrm{(stat.)} \pm 0.0016 ~\mathrm{(syst.)}\pm 0.0038 ~\textrm{(ext.)} \textrm{ ps}^{-1}, $$ where the uncertainties are statistical, systematic and from external sources. The earlier ATLAS measurement of $Γ_s$ in the $B^0_s \to J/ψϕ$ decay was used to derive a value for the ratio of the average decay widths $Γ_d$ and $Γ_s$ for $B^{0}$ and $B_s^{0}$ mesons respectively, of $$ \frac{Γ_d }{Γ_s } = 0.9905 \pm 0.0022 ~\textrm{(stat.)} \pm 0.0036 ~\textrm{(syst.)} \pm 0.0057 ~\textrm{(ext.)}. $$ The measured lifetime, average decay width and decay width ratio are in agreement with theoretical predictions and with measurements by other experiments. This measurement provides the most precise result of the effective lifetime of the $B^{0}$ meson to date.
The measured effective lifetime for the $B^0 \rightarrow J/\psi\,K^{*0}$ decay.
The measured average decay width $\Gamma_{d}\,$ extracted from the average lifetime.
The measured ratio $\Gamma_{d} / \Gamma_{s}\,$ of the average decay widths.
A search is presented for pair production of a new heavy quark ($Q$) that decays into a $W$ boson and a light quark ($q$) in the final state where one $W$ boson decays leptonically (to an electron or muon plus a neutrino) and the other $W$ boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 8$ TeV collected by the ATLAS detector at the LHC. No evidence of $Q\bar{Q}$ production is observed. New chiral quarks with masses below 690 GeV are excluded at 95% confidence level, assuming BR$(Q\to Wq)=1$. Results are also interpreted in the context of vectorlike quark models, resulting in the limits on the mass of a vectorlike quark in the two-dimensional plane of BR$(Q\to Wq)$ versus BR$(Q\to Hq)$.
The expected and observed 95% CL upper limits on the cross section as a function of $m_Q$ when setting BR$(Q\to Wq) = 1$, which would be the case for a new chiral quark. Also shown are the $\pm 1\sigma$ and $\pm 2\sigma$ intervals on the distribution of expected results for the chiral model if no signal exists.
The upper and lower bounds on the range of heavy quark masses expected and observed to be excluded at 95% CL, as a function of the branching ratio of the heavy quark to $Wq$ versus $Hq$, with the branching ratio to $Zq$ fixed by the requirement BR$(Q\!\to\!Zq) = 1 - \text{BR}(Q\!\to\!Wq) - \text{BR}(Q\!\to\!Hq)$.
The results of a search for charged Higgs bosons decaying to a $\tau$ lepton and a neutrino, $H^{\pm} \rightarrow \tau^{\pm} \nu$, are presented. The analysis is based on 19.5 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV collected by the ATLAS experiment at the Large Hadron Collider. Charged Higgs bosons are searched for in events consistent with top-quark pair production or in associated production with a top quark. The final state is characterised by the presence of a hadronic $\tau$ decay, missing transverse momentum, $b$-tagged jets, a hadronically decaying $W$ boson, and the absence of any isolated electrons or muons with high transverse momenta. The data are consistent with the expected background from Standard Model processes. A statistical analysis leads to 95\% confidence-level upper limits on the product of branching ratios $ {\cal B}(t\rightarrow bH^\pm) \times {\cal B} (H^\pm\rightarrow \tau^{\pm}\nu) $, between 0.23% and 1.3% for charged Higgs boson masses in the range 80-160 GeV. It also leads to 95% confidence-level upper limits on the production cross section times branching ratio, $\sigma(pp \rightarrow tH^{\pm} + X) \times {\cal B}(H^{\pm} \rightarrow \tau^{\pm} \nu)$, between 0.76 pb and 4.5 fb, for charged Higgs boson masses ranging from 180 GeV to 1000 GeV. In the context of different scenarios of the Minimal Supersymmetric Standard Model, these results exclude nearly all values of $\tan\beta$ above one for charged Higgs boson masses between 80 GeV and 160 GeV, and exclude a region of parameter space with high $\tan\beta$ for $H^{\pm}$ masses between 200 GeV and 250 GeV.
The measured B(t->H+)xB(H+->tau+ nu) limit.
The measured P P --> t H+ --> t tau+nu limit.
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