A search for the dimuon decay of the Standard Model (SM) Higgs boson is performed using data corresponding to an integrated luminosity of 139 fb$^{-1}$ collected with the ATLAS detector in Run2 $pp$ collisions at $\sqrt{s}=13$ TeV at the Large Hadron Collider. The observed (expected) significance over the background-only hypothesis for a Higgs boson with a mass of 125.09 GeV is $2.0\,σ$ ($1.7\,σ$). The observed upper limit on the cross section times branching ratio for $pp \to H \to μμ$ is 2.2 times the SM prediction at 95% confidence level, while the expected limit on a $H \to μμ$ signal assuming the absence (presence) of a SM signal is $1.1$ ($2.0$). The best-fit value of the signal strength parameter, defined as the ratio of the observed signal yield to the one expected in the SM, is $μ= 1.2 \pm 0.6$.
A search is performed for a long-lived particle decaying into a final state that includes a pair of muons of opposite-sign electric charge, using proton-proton collision data collected at $\sqrt{s}=$ 13 TeV by the ATLAS detector at the Large Hadron Collider corresponding to an integrated luminosity of 32.9 fb$^{-1}$. No significant excess over the Standard Model expectation is observed. Limits at 95% confidence level on the lifetime of the long-lived particle are presented in models of new phenomena including gauge-mediated supersymmetry or decay of the Higgs boson, $H$, to a pair of dark phot…
Exclusive dimuon production in ultraperipheral collisions (UPC), resulting from photon-photon interactions in the strong electromagnetic fields of colliding high-energy lead nuclei, $\mathrm{PbPb}(γγ) \rightarrow μ^+μ^- (\mathrm{Pb}^{(\star)}\mathrm{Pb}^{(\star)} )$, is studied using $\mathcal{L}_{\mathrm{int}} = 0.48$ nb$^{-1}$ of $\sqrt{s_\mathrm{NN}}=5.02$ TeV lead-lead collision data at the LHC with the ATLAS detector. Dimuon pairs are measured in the fiducial region $p_{\mathrm{T}μ} > 4$ GeV, $|η_μ| < 2.4$, invariant mass $m_{μμ} > 10$ GeV, and $p_{\mathrm{T,μμ}} < 2$ GeV. The primary bac…
We investigate the potential reach of a search for a long-lived/prompt dark vector boson $Z_D$, also called dark $Z$, and a prompt dark Higgs boson $h_D$ through exotic decays of the observed Higgs boson $h$ into either $Z_DZ_D$, $h_Dh_D$, or $ZZ_D$ with $Z$ being the hypercharge gauge boson. The $Z_D$ production through the Higgs portal is completed via one of two mechanisms, kinetic mixing of $Z_D$ with $Z$ and the mixing of $h_D$ with $h$. All production modes of $h$ are considered, while the branching fractions are calculated in Monte Carlo simulation using the {\textsc{MadGraph5}}\_aMC@NL…
This paper presents a search for decays of the Higgs boson with a mass of 125 GeV into a pair of new pseudoscalar particles, $H\rightarrow aa$, where one $a$-boson decays into a $b$-quark pair and the other into a muon pair. The search uses 139 fb$^{-1}$ of proton-proton collision data at a center-of-mass energy of $\sqrt{s}=13$ TeV recorded between 2015 and 2018 by the ATLAS experiment at the LHC. A narrow dimuon resonance is searched for in the invariant mass spectrum between 16 GeV and 62 GeV. The largest excess of events above the Standard Model backgrounds is observed at a dimuon invarian…
The ATLAS Higgs results are reviewed using Run-2 data taken at a center-of-mass energy of 13 TeV with up to an integrated luminosity of 80 fb$^{-1}$. So far, the data are consistent with the standard model expectations. ATLAS now has observed the Higgs Yukawa coupling to the third generation fermions with $H\rightarrow ττ$, ttH, and $H\rightarrow b b$ in the VH process. The Higgs boson will continue to provide an important probe for new physics and beyond.
Recent measurements of Higgs boson cross-section and properties are presented using up to $139~\text{fb}^{-1}$ of proton-proton collision data delivered by the Large Hadron Collider at $\sqrt{s}~=~13~\text{TeV}$ and recorded by the ATLAS and CMS detectors. Three measurements are discussed. The first is the measurement of Higgs boson production with sizeable transverse momentum decaying to a $b\bar{b}$ pair. The remaining two measurements exploit the $H \rightarrow WW^*$ decay channel in various production modes: gluon fusion, vector boson fusion, and production in association with a $W$ or $Z$…
ATLAS has developed a new approach for vertex selection in VBF invisible events at HL-LHC conditions, exploiting its new forward tracking capabilities, integrating calorimeter and tracking information to mitigate the impact of pileup vertex merging, and introducing a new way to apply pile-up jet suppression methods for the selection of VBF jets. The new algorithm is insensitive to pileup density and improves the average vertex selection efficiency from 86% to 95% under tight VBF event selection cuts.
