A search for high-mass dielectron and dimuon resonances in the mass range of 250 GeV to 6 TeV is presented. The data were recorded by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=$13 TeV during Run 2 of the Large Hadron Collider and correspond to an integrated luminosity of 139 fb$^{-1}$. A functional form is fitted to the dilepton invariant-mass distribution to model the contribution from background processes, and a generic signal shape is used to determine the significance of observed deviations from this background estimate. No significant deviation is observed and upper limits are placed at the 95% confidence level on the fiducial cross-section times branching ratio for various resonance width hypotheses. The derived limits are shown to be applicable to spin-0, spin-1 and spin-2 signal hypotheses. For a set of benchmark models, the limits are converted into lower limits on the resonance mass and reach 4.5 TeV for the E6-motivated $Z^\prime_\psi$ boson. Also presented are limits on Heavy Vector Triplet model couplings.
Distribution of the dielectron invariant mass for events passing the full selection.
Distribution of the dielectron invariant mass for events passing the full selection.
Distribution of the dielectron invariant mass for events passing the full selection.
A search for new physics with non-resonant signals in dielectron and dimuon final states in the mass range above 2 TeV is presented. This is the first search for non-resonant signals in dilepton final states at the LHC to use a background estimate from the data. The data, corresponding to an integrated luminosity of 139 fb$^{-1}$, were recorded by the ATLAS experiment in proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV during Run 2 of the Large Hadron Collider. The benchmark signal signature is a two-quark and two-lepton contact interaction, which would enhance the dilepton event rate at the TeV mass scale. To model the contribution from background processes a functional form is fit to the dilepton invariant-mass spectra in data in a mass region below the region of interest. It is then extrapolated to a high-mass signal region to obtain the expected background there. No significant deviation from the expected background is observed in the data. Upper limits at 95 % CL on the number of events and the visible cross-section times branching fraction for processes involving new physics are provided. Observed (expected) 95 % CL lower limits on the contact interaction energy scale reach 35.8 (37.6) TeV.
Expected and observed event yields in each signal bin.
Model-independent upper limits at 95% CL on the number of signal events in the (constructive/destructive interference) SRs used in the analysis for dielectrons and dimuons.
Lower limits at 95$\%$ CL on $\Lambda$ for the dielectron channel for different signal chiralities in the (constructive/destructive interference) SRs of the analysis.
A search for the pair production of heavy leptons 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 139 fb$^{-1}$ of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on the final state with two light leptons (electrons or muons) of different flavour and charge combinations, with at least two jets and large missing transverse momentum. No significant excess over the Standard Model expectation is observed. The results are translated into exclusion limits on heavy-lepton masses, and the observed lower limit on the mass of the type-III seesaw heavy leptons is 790 GeV at 95% confidence level.
Cross-sections of the type-III seesaw process for mass points used in the analysis. Branching ratios into at least two leptons are presented with the corresponding effective cross-section.
Expected and observed 95 % CLs exclusion limits for the type-III seesaw process with the corresponding one- and two-standard-deviation bands, showing the 95 % CL upper limit on the cross-section.
Selection efficiencies in percentage relative to the events with at least two leptons for signal mass points used in the analysis. The efficiency is defined as the ratio of expected signal events in a signal region compared with the number of expected events produced, for integrated luminosity 139 fb$^{-1}$.
A search for pair production of doubly charged Higgs bosons ($H^{\pm \pm}$), each decaying into a pair of prompt, isolated, highly energetic leptons with the same electric charge, is presented. The search uses a proton-proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb$^{-1}$ recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. This analysis focuses on same-charge leptonic decays, $H^{\pm \pm} \rightarrow \ell^{\pm} \ell^{\prime \pm}$ where $\ell, \ell^\prime=e, \mu, \tau$, in two-, three-, and four-lepton channels, but only considers final states which include electrons or muons. No evidence of a signal is observed. Corresponding limits on the production cross-section and consequently a lower limit on $m(H^{\pm \pm})$ are derived at 95% confidence level. Assuming that the branching ratios to each of the possible leptonic final states are equal, $\mathcal{B}(H^{\pm \pm} \rightarrow e^\pm e^\pm) = \mathcal{B}(H^{\pm \pm} \rightarrow e^\pm \mu^\pm) = \mathcal{B}(H^{\pm \pm} \rightarrow \mu^\pm \mu^\pm) = \mathcal{B}(H^{\pm \pm} \rightarrow e^\pm \tau^\pm) = \mathcal{B}(H^{\pm \pm} \rightarrow \mu^\pm \tau^\pm) = \mathcal{B}(H^{\pm \pm} \rightarrow \tau^\pm \tau^\pm) = 1/6$, the observed lower limit on the mass of a doubly charged Higgs boson is 1080 GeV within the left-right symmetric type-II seesaw model, which is an improvement over previous limits. Additionally, a lower limit of $m(H^{\pm \pm})$ = 900 GeV is obtained in the context of the Zee-Babu neutrino mass model.
LO, NLO cross-sections and K-factors for the pair-production of doubly charged Higgs bosons in pp collisions at $\sqrt{s}$ = 13 TeV. The K-factors (K=$\sigma_{NLO}/\sigma_{LO}$) are identical for $H^{\pm\pm}_L$, $H^{\pm\pm}_R$, and $k^{\pm\pm}$. The values are calculated using the NNPDF3.1NLO and NNPDF2.3LO PDF sets.
Observed (solid line) and expected (dashed line) 95% CL upper limits on the $H^{\pm\pm}$ pair production cross-section as a function of $m(H^{\pm\pm})$ resulting from the combination of all analysis channels, assuming $\sum_{\ell \ell^\prime} \mathcal{B}(H^{\pm\pm} \rightarrow \ell^{\pm} \ell^{\prime \pm})=100%$, where $\ell, \ell^\prime = e, \mu, \tau$.
Distribution of $m(e^{\pm},e^{\pm})_{\mathrm{lead}}$ in the electron-electron signal region after the background-only fit.
Forum