A search is performed for a heavy particle decaying into different flavour dilepton pairs ($eμ$, $eτ$ or $μτ$), using 3.2 $fb^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV collected in 2015 by the ATLAS detector at the Large Hadron Collider. No excess over the Standard Model prediction is observed. Limits at the 95% credibility level are set on the mass of a $Z^{\prime}$ boson with lepton-flavour-violating couplings at 3.0, 2.7 and 2.6 TeV, and on the mass of a supersymmetric $τ$ sneutrino with $R$-parity-violating couplings at 2.3, 2.2 and 1.9 TeV, for $eμ$, $eτ$ and $μτ$ final states, respectively. The results are also interpreted as limits on the threshold mass for quantum black hole production.
We report a measurement of the charge asymmetry $A_C$ in top quark pair production with the ATLAS experiment. The measurement focuses on dilepton channels ($ee$, $eμ$, $μμ$). The data are unfolded to parton level at full phase space using a fully Bayesian unfolding method. Inclusive and differential measurements of the charge asymmetry are performed.
The ATLAS detector has been used to search for high mass ee or mumu resonances, such as new heavy neutral gauge bosons. Over 1 fb^{-1} of proton-proton collisions at s = 7 TeV recorded by the ATLAS experiment are used to search for a high mass state decaying to dilepton pairs. No excess over Standard Model expectations is observed, and limits are placed on benchmark models predicting spin-1 and spin-2 resonances.
Models with extra gauge symmetry were well motivated extensions of the Standard Model. In this paper, we search for an excess of events in search of a heavy neutral gauge boson Z which is fermiophobic. We are looking at llbb final states where the dilepton channel provides the most stringent constraint on the fermiophobic (FB) model. Since Z is fermiophobic, the production of such particles can occur via vector boson fusion, with subsequent decays into WW or Zh. We observe no significant excess of events over the background-only hypothesis and are able to put the constraint on efficiency times…
Dilepton-jet final states are used to study physical phenomena not predicted by the standard model. The ATLAS discovery potential for leptoquarks and Majorana Neutrinos is presented using a full simulation of the ATLAS detector at the Large Hadron Collider. The study is motivated by the role of the leptoquark in the Grand Unification of fundamental forces and the see-saw mechanism that could explain the masses of the observed neutrinos. The analysis algorithms are presented, background sources are discussed and estimates of sensitivity and the discovery potential for these processes are report…
This Letter reports on a search for narrow high-mass resonances decaying into dilepton final states. The data were recorded by the ATLAS experiment in pp collisions at sqrt(s) = 7 TeV at the LHC and correspond to a total integrated luminosity of 1.08/fb (1.21/fb) in the ee (mumu) channel. No statistically significant excess above the Standard Model expectation is observed and upper limits are set at the 95% C.L. on the cross section times branching ratio of Z' resonances and Randall-Sundrum gravitons decaying into dileptons as a function of the resonance mass. A lower mass limit of 1.83 TeV on…
A measurement of the top--antitop ($t\bar{t}$) charge asymmetry is presented using 2011 LHC data collected by the ATLAS detector corresponding to an integrated luminosity of 4.6 fb$^{-1}$ at a centre-of-mass energy of 7 TeV. The analysis is performed in the dilepton channel, and two different observables are studied: $A^{\ell\ell}_{\mathrm{C}}$, based on the identified charged leptons, and $A^{t\bar{t}}_{\mathrm{C}}$, based on the reconstructed $t\bar{t}$ final state. The asymmetries, measured to be $A^{\ell\ell}_{\mathrm{C}} = 0.024 \pm 0.015 \mathrm{(stat.)} \pm 0.009 \mathrm{(syst.)}$ and $…
A measurement of the production cross section of top quark pairs (ttbar) in proton-proton collisions at a center-of-mass energy of 7 TeV recorded with the ATLAS detector at the Large Hadron Collider is reported. Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets. Using a data sample corresponding to an integrated luminosity of 35 pb^-1, a ttbar production cross section of 171 +/- 20(stat.) +/- 14(syst.) +8-6(lum.) pb is measured for an assumed top quark mass of 172.5 GeV. A second measurement requiring at least one jet identified a…
This article presents a search for high mass e e or mu mu resonances in pp collisions at sqrt{s} = 7 TeV at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of ~40/pb. No statistically significant excess above the Standard Model expectation is observed in the search region of dilepton invariant mass above 110 GeV. Upper limits at the 95% confidence level are set on the cross section times branching ratio of Z' resonances decaying to dielectrons and dimuons as a function of the resonance mass. A lower mass limit of 1.048 TeV on …
The top quark mass is measured in the $t\bar{t}\to$ dilepton channel (lepton $= e, μ$) using ATLAS data recorded in the year 2012 at the LHC. The data were taken at a proton--proton centre-of-mass energy of $\sqrt{s}=$ 8 TeV and correspond to an integrated luminosity of about 20.2 fb$^{-1}$. Exploiting the template method, and using the distribution of invariant masses of lepton--b-jet pairs, the top quark mass is measured to be $m_{top} =$ 172.99 $\pm$ 0.41 (stat) $\pm$ 0.74 (syst) GeV, with a total uncertainty of 0.84 GeV. Finally, a combination with previous ATLAS $m_{top}$ measurements fro…
We present a search for high-mass l+l- resonances in pp collisions at a centre-of-mass energy of 7 TeV recorded by the ATLAS experiment in 2011. No statistically significant excess above the Standard Model expectation is observed in a dataset corresponding to an integrated luminosity of approximately 1/fb. Consequently, upper limits are set on the cross-section times branching ratio of resonances decaying to muon pairs as a function of the resonance mass. In particular, a Sequential Standard Model Z' is excluded for masses below 1.83 TeV, and a Randall-Sundrum Kaluza-Klein graviton with coupli…
A search is conducted for non-resonant new phenomena in dielectron and dimuon final states, originating from either contact interactions or large extra spatial dimensions. The LHC 2012 proton-proton collision dataset recorded by the ATLAS detector is used, corresponding to 20 fb$^{-1}$ at $\sqrt{s}$ = 8 TeV. The dilepton invariant mass spectrum is a discriminating variable in both searches, with the contact interaction search additionally utilizing the dilepton forward-backward asymmetry. No significant deviations from the Standard Model expectation are observed. Lower limits are set on the $\…
A search for a high mass ttbar resonance in pp collisions at sqrt(s) = 7 TeV at the LHC is presented. We search for such a resonance using the final state where the $W$ bosons from the top quark decay into either an electron or a muon (dilepton final state). The data were recorded by the ATLAS experiment during 2011 and correspond to a total integrated luminosity of L = 1.04 fb^-1. No statistically significant excess above the Standard Model expectation is observed. Upper limits at the 95% Confidence Level (C.L.) are set on the cross section times branching ratio of the resonance decaying to t…
A search is presented for exotic processes that result in final states containing jets including at least one b jet, sizable missing transverse momentum, and a pair of leptons with the same electric charge. Using a sample of 14.3 fb^-1 of pp collisions at sqrt(s) =8 TeV recorded by the ATLAS detector at the Large Hadron Collider, no significant excess of events over the background expectation is observed. This observation is interpreted as constraining several signal hypotheses beyond the Standard Model and limits are set at 95% confidence level on relevant parameters of the signal hypotheses.
Measurements of the top--antitop quark pair production charge asymmetry in the dilepton channel, characterized by two high-${p}_{\rm{T}}$ leptons (electrons or muons), are presented using data corresponding to an integrated luminosity of 20.3 $\textrm{fb}^{-1}$ from $pp$ collisions at a center-of-mass energy $\sqrt{s} = 8$~TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. Inclusive and differential measurements as a function of the invariant mass, transverse momentum, and longitudinal boost of the $t\bar{t}$ system are performed both in the full phase space and in a f…
An inclusive search is presented for new physics in events with two isolated leptons (electron or muon) having the same electric charge. The data are selected from events collected from pp collisions at sqrt(s) = 7 TeV by the ATLAS detector and correspond to an integrated luminosity of 34 inverse picobarns. The spectra in dilepton invariant mass, missing transverse momentum and jet multiplicity are presented and compared to Standard Model predictions. In this event sample, no evidence is found for contributions beyond those of the Standard Model. Limits are set on the cross-section in a fiduci…
Two searches for new phenomena in final states containing a same-flavour opposite-lepton (electron or muon) pair, jets, and large missing transverse momentum are presented. These searches make use of proton--proton collision data, collected during 2015 and 2016 at a centre-of-mass energy $\sqrt{s}=13$ TeV by the ATLAS detector at the Large Hadron Collider, which correspond to an integrated luminosity of 14.7 fb$^{-1}$. Both searches target the pair production of supersymmetric particles, squarks or gluinos, which decay to final states containing a same-flavour opposite-sign lepton pair via one…
A search is conducted for new resonant and non-resonant high-mass phenomena in dielectron and dimuon final states. The search uses 36.1 fb$^{-1}$ of proton-proton collision data, collected at $\sqrt{s}$ = 13 TeV by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the Standard Model prediction is observed. Upper limits at 95% credibility level are set on the cross-section times branching ratio for resonances decaying into dileptons, which are converted to lower limits on the resonance mass, up to 4.1 for the E$_{6}$-motivated Z'$_χ$. Lower limits on the $qq \ell\e…
A search is conducted for both resonant and non-resonant high-mass new phenomena in dielectron and dimuon final states. The search uses 3.2 fb$^{-1}$ of proton-proton collision data, collected at $\sqrt{s} = 13$ TeV by the ATLAS experiment at the LHC in 2015. The dilepton invariant mass is used as the discriminating variable. No significant deviation from the Standard Model prediction is observed; therefore limits are set on the signal model parameters of interest at 95% credibility level. Upper limits are set on the cross-section times branching ratio for resonances decaying to dileptons, and…
Prospects for heavy flavour studies with the CMS and ATLAS detectors are presented. Many studies are aimed for early LHC data, taking advantage of the large $b$ production cross-section. Rare decay studies as the $B_s \to μ^+μ^-$ decay have also been performed.
An inclusive search for the non-Standard Model production of two isolated leptons of the same electrical charge is presented. The search was performed with 2010 collision data in the ATLAS detector at the Large Hadron Collider for ee, eμ, and μμ channels. With 34 pb-1 of integrated luminosity, no disagreement with the Standard Model prediction was found, and limits on the fiducial cross section for generic same-sign production as well as four model-dependent limits for models of new physics were set. An updated same sign analysis with 1.6 fb-1 is underway and nearly complete for the μμ channel…
A search for resonances in the dielectron and dimuon mass spectra from 250 GeV to 6 TeV is presented. The data were recorded during Run 2 of the LHC by the ATLAS experiment using proton-proton ($pp$) collisions with a center-of-mass energy of $\sqrt{s} = 13$ TeV. The integrated luminosity of the data corresponds to 139 fb$^{-1}$. The background models are a functional form fit to the data, and generic signal shapes are used to represent various models with different resonant widths and masses. No significant deviation from the expected background is observed and 95\% confidence level upper lim…
Measurements of normalized differential cross-sections of top quark pair ($t\bar t$) production are presented as a function of the mass, the transverse momentum and the rapidity of the $t\bar t$ system in proton-proton collisions at center-of-mass energies of $\sqrt{s}$ = 7 TeV and 8 TeV. The dataset corresponds to an integrated luminosity of 4.6 fb$^{-1}$ at 7 TeV and 20.2 fb$^{-1}$ at 8 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Events with top quark pair signatures are selected in the dilepton final state, requiring exactly two charged leptons and at least two jets …
A selection of heavy-flavour physics results from the ATLAS experiment is presented, based on data collected in proton-proton collisions at the LHC during 2010. Differential cross-sections for the production of heavy flavours, charmonium and bottomonium states and D-mesons are presented and compared to various theoretical models. Results of B-hadron lifetime measurements are also reported.
A search for four-top-quark production, $t\bar{t}t\bar{t}$, is presented. It is based on proton-proton collision data with a center-of-mass energy $\sqrt{s}$ = 13 TeV collected by the ATLAS detector at the Large Hadron Collider during the year 2015 and 2016, corresponding to an integrated luminosity of $36.1$ fb$^{-1}$. Data are analyzed in both the single-lepton and opposite-sign dilepton channels, characterized by the presence of one or two isolated electrons or muons with high-transverse momentum and multiple jets. A data-driven method is used to estimate the dominant background from top-qu…
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 di…
The sensitivity to $B_s$ meson mixing phenomena can be significantly improved by distinguishing the initial meson flavour. The flavour tagging techniques used in the ATLAS measurement of the angular amplitudes contributing to $B_s \rightarrow J/ψϕ$ decays are presented using 4.9 fb$^{-1}$ of $pp$ data at $\sqrt{s}$ = 7 TeV from 2011. The methods, their performance and calibration on self-tagged $B^\pm \rightarrow J/ψK^\pm$ data are shown.
Two searches for supersymmetric particles in final states containing a same-flavour opposite-sign lepton pair, jets and large missing transverse momentum are presented. The proton-proton collision data used in these searches were collected at a centre-of-mass energy $\sqrt{s}=8$ TeV by the ATLAS detector at the Large Hadron Collider and corresponds to an integrated luminosity of 20.3 fb$^{-1}$. Two leptonic production mechanisms are considered: decays of squarks and gluinos with $Z$ bosons in the final state, resulting in a peak in the dilepton invariant mass distribution around the $Z$-boson …
We investigate the impact of flavour-conserving, non-universal quark-lepton contact interactions on the dilepton invariant mass distribution in $p~p \to \ell^+ \ell^-$ processes at the LHC. After recasting the recent ATLAS search performed at 13 TeV with 36.1 fb$^{-1}$ of data, we derive the best up-to-date limits on the full set of 36 relevant four-fermion operators, as well as estimate the sensitivity achievable at the HL-LHC. We discuss how these high-$p_T$ measurements can provide complementary information to the low-$p_T$ rare meson decays. In particular, we find that the recent hints on …
Measurements of top quark spin observables in $t\bar{t}$ events are presented based on 20.2 fb$^{-1}$ of $\sqrt{s} = 8$ TeV proton-proton collisions recorded with the ATLAS detector at the LHC. The analysis is performed in the dilepton final state, characterised by the presence of two isolated leptons (electrons or muons). There are 15 observables, each sensitive to a different coefficient of the spin density matrix of $t\bar{t}$ production, which are measured independently. Ten of these observables are measured for the first time. All of them are corrected for detector resolution and acceptan…
This Letter presents a search for contact interactions in the dielectron and dimuon channels using data from proton-proton collisions produced by the LHC at sqrt(s) = 7 TeV and recorded by the ATLAS detector. The data sample, collected in 2011, corresponds to an integrated luminosity of 1.08 and 1.21 fb^-1 in the e+ e- and mu+ mu- channels, respectively. No significant deviations from the standard model are observed. Using a Bayesian approach with a prior flat in 1/Lambda^2, the following 95% CL lower limits are placed on the energy scale of llqq contact interactions: Lambda^- > 10.1 TeV (Lamb…
Precision determinations of the flavour sector allow the search for indirect new physics signatures. At the forefront of these studies are the determinations of interference of new physics with known $\upDelta F=1$ and $\upDelta F=2$ processes. The ATLAS collaboration explores this area with competitive results measuring the CP violating phase $ϕ_s$ from $B^0_s\to J/ψϕ$ decays and investigating rare $B$ decays with dileptons in the final state with data collected at the Large Hadron Collider. In this paper, the latest ATLAS results relevant for new physics searches in the heavy flavour sector …
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 ar…
Measurements of normalized differential cross-sections of top quark pair ($t\bar t$) production are presented as a function of the mass, the transverse momentum and the rapidity of the $t\bar t$ system in proton-proton collisions at center-of-mass energies of $\sqrt{s}$ = 7 TeV and 8 TeV. The dataset corresponds to an integrated luminosity of 4.6 fb$^{-1}$ at 7 TeV and 20.2 fb$^{-1}$ at 8 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Events with top quark pair signatures are selected in the dilepton final state, requiring exactly two charged leptons and at least two jets …
Search for lepton-flavour violation in high-mass dilepton final states using 139 $\mathrm{fb}^{-1}$ of $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector
Measurements of $K_S^0$ and $Λ^0$ production in $t\bar{t}$ final states have been performed. They are based on a data sample with integrated luminosity of 4.6 $\mathrm{fb}^{-1}$ from proton-proton collisions at a centre-of-mass energy of 7 TeV, collected in 2011 with the ATLAS detector at the Large Hadron Collider. Neutral strange particles are separated into three classes, depending on whether they are contained in a jet, with or without a $b$-tag, or not associated with a selected jet. The aim is to look for differences in their main kinematic distributions. A comparison of data with several…
The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to an electron-positron pair or a muon-antimuon pair. The search is sensitive to heavy neutral Z' gauge bosons, Randall-Sundrum gravitons, Z* bosons, techni-mesons, Kaluza-Klein Z/gamma bosons, and bosons predicted by Torsion models. Results are presented based on an analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.9/fb in the dielectron channel and 5.0/fb in the dimuon channel. A Z' boson with Standard Model-like couplings is excluded …
A search for doubly-charged Higgs bosons decaying to pairs of electrons and/or muons is presented. The search is performed using a data sample corresponding to an integrated luminosity of 4.7 fb-1 of pp collisions at sqrt(s) = 7 TeV collected by the ATLAS detector at the LHC. Pairs of prompt, isolated, high-pT leptons with the same electric charge (ee, emu, mumu) are selected, and their invariant mass distribution is searched for a narrow resonance. No significant excess over Standard Model background expectations is observed, and limits are placed on the cross section times branching ratio fo…
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 deviati…
A search for new phenomena in final states containing an $e^+e^-$ or $μ^+μ^-$ pair, jets, and large missing transverse momentum is presented. This analysis makes use of proton--proton collision data with an integrated luminosity of $36.1 \; \mathrm{fb}^{-1}$, collected during 2015 and 2016 at a centre-of-mass energy $\sqrt{s}$ = 13 TeV with the ATLAS detector at the Large Hadron Collider. The search targets the pair production of supersymmetric coloured particles (squarks or gluinos) and their decays into final states containing an $e^+e^-$ or $μ^+μ^-$ pair and the lightest neutralino ($\tilde…
A measurement of the top-antitop ($t\bar{t}$) charge asymmetry is presented using data corresponding to an integrated luminosity of 4.6 fb$^{-1}$ of LHC $pp$ collisions at a centre-of-mass energy of 7 TeV collected by the ATLAS detector. Events with two charged leptons, at least two jets and large missing transverse momentum are selected. Two observables are studied: $A^{\ell\ell}_{\textrm{C}}$ based on the identified charged leptons, and $A^{t\bar{t}}_{\textrm{C}}$, based on the reconstructed $t\bar{t}$ final state. The asymmetries are measured to be \begin{eqnarray*} A^{\ell\ell}_{\textrm{C}…
The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton (pp) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb-1 in the dielectron channel and 20.5 fb-1 in the dimuon channel. A narrow resonance with Standard Model Z couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limit…
The top quark mass was measured in the channels $t\bar{t} \to \mathrm{lepton+jets}$ and $t\bar{t} \to \mathrm{dilepton}$ (lepton=$e, μ$) based on ATLAS data recorded in 2011. The data were taken at the LHC with a proton--proton centre-of-mass energy of $\sqrt{s}=7$ TeV and correspond to an integrated luminosity of 4.6fb$^{-1}$. The $t\bar{t} \to \mathrm{lepton+jets}$ analysis uses a three-dimensional template technique which determines the top quark mass together with a global jet energy scale factor (JSF), and a relative $b$-to-light-jet energy scale factor (bJSF), where the terms $b$-jets an…
A search for the Higgs boson decaying into a photon and a pair of electrons or muons with an invariant mass $m_{\ell\ell} < 30$ GeV is presented. The analysis is performed using 139 fb$^{-1}$ of proton-proton collision data, produced by the LHC at a centre-of-mass energy of 13 TeV and collected by the ATLAS experiment. Evidence for the $H \rightarrow \ell \ell γ$ process is found with a significance of 3.2$σ$ over the background-only hypothesis, compared to an expected significance of 2.1$σ$. The best-fit value of the signal strength parameter, defined as the ratio of the observed signal yield…
The most recent results of the top-quark mass measurements with the ATLAS detector using data collected from proton-proton collisions at the Large Hadron Collider are presented. Although several decay modes of the top-quark pairs have been used in ATLAS for top-quark mass measurements, only the latest results are presented (single lepton and dilepton channels). The top-quark pole mass from the $t\overline{t}$ cross-section measurement in the dilepton channel and the top-antitop mass difference measurement in the single-lepton channel are also shown. The systematic uncertainties associated to t…
Measurements of the top--antitop quark pair production charge asymmetry in the dilepton channel, characterized by two high-${p}_{\rm{T}}$ leptons (electrons or muons), are presented using data corresponding to an integrated luminosity of $20.3$ $\textrm{fb}^{-1}$ from $pp$ collisions at a center-of-mass energy of $\sqrt{s} = 8$ TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. Inclusive and differential measurements as a function of the invariant mass, transverse momentum, and longitudinal boost of the $t\bar{t}$ system are performed both in the full phase space and i…
Measurement of the inclusive and differential top-quark charge asymmetry is carried out using full Run 2 data from proton-proton collisions at a center-of-mass energy of $13$ TeV collected by the ATLAS detector. The single-lepton and dilepton $t\bar{t}$ decay channels are combined. Distorting detector effects are removed using fully Bayesian unfolding. In the dilepton decay channel, the leptonic charge asymmetry is determined. Results from the measurements are compared with the Standard Model prediction. The excess of charge asymmetry from zero charge-asymmetry hypothesis at the level of $4.7$…
Direct searches for lepton flavour violation in decays of the Higgs and $Z$ bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: $H\to eτ$, $H\toμτ$, and $Z\toμτ$. The searches are based on the data sample of proton--proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 fb$^{-1}$ at a centre-of-mass energy of $\sqrt{s} = 8$ TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95% confidence level: Br$(H\to eτ) <$ 1.04%, Br$(H\toμτ) <$ …
This article presents the sensitivity of the ATLAS experiment to the lepton-flavour-violating decays of $τ\to 3μ$. A method utilising the production of $τ$ leptons via $W\toτν$ decays is used. This method is applied to the sample of 20.3 fb$^{-1}$ of $pp$ collision data at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment at the LHC in 2012. No event is observed passing the selection criteria, and the observed (expected) upper limit on the $τ$ lepton branching fraction into three muons, ${\rm Br}(τ\to 3μ)$, is $3.76\times 10^{-7}$ ($3.94\times 10^{-7}$) at 90% confidence level…
A search for flavour-changing neutral currents (FCNC) processes in proton-proton ($pp$) collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the CERN Large Hadron Collider (LHC) is presented. The analysed data collected during the years of 2015 and 2016 corresponds to an integrated luminosity of 36.1 fb$^{-1}$. A search considering top-quark pair-production events is performed, with one top-quark decaying through the dominant Standard Model (SM) mode $t$ $\to$ $Wb$, and the other through the $t$ $\to$ $qZ$ ($q$=$u$,$c$) FCNC channel. The data are consistent with the SM ex…
An inclusive search for anomalous production of two prompt, isolated leptons with the same electric charge is presented. The search is performed in a data sample corresponding to 4.7 fb-1 of integrated luminosity collected in 2011 at sqrt(s) = 7 TeV with the ATLAS detector at the LHC. Pairs of leptons (ee, emu, and mumu) with large transverse momentum are selected, and the dilepton invariant mass distribution is examined for any deviation from the Standard Model expectation. No excess is found, and upper limits on the production cross section of like-sign lepton pairs from physics processes be…
A direct search for lepton-flavour-violating (LFV) $H\toμτ$ decays of the recently discovered Higgs boson with the ATLAS detector at the LHC is presented. The analysis is performed in the $H\toμτ_{\mathrm{had}}$ channel, where $τ_{\mathrm{had}}$ is a hadronically decaying $τ$-lepton. The search is based on the data sample of proton--proton collisions collected by the ATLAS experiment corresponding to an integrated luminosity of 20.3 fb$^{-1}$ at a centre-of-mass energy of $\sqrt{s}=8$ TeV. No statistically significant excess of data over the predicted background is observed. The observed (expe…
This Letter presents direct searches for lepton flavour violation in Higgs boson decays, $H\rightarrow eτ$ and $H\rightarrowμτ$, performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of $36.1\,\mathrm{fb}^{-1}$. No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95 % confidence-level upper limits on the lepton-flavour-violating branching ratios are $0.47\%$ ($0.34^{+0.13}_…
Leptoquarks (LQ) are predicted by many new physics theories to describe the similarities between the lepton and quark sectors of the Standard Model and offer an attractive potential explanation for the lepton flavour anomalies observed at LHCb and flavour factories. The ATLAS experiment has a broad program of direct searches for Leptoquarks, coupling to the first-, second- or third-generation particles. The most recent 13 TeV results on the searches for Leptoquarks and contact interactions with the ATLAS detector are reviewed, covering flavour-diagonal and cross-generational final states.
We present the measurement of the inclusive electron spectrum in proton-proton collisions at a centre-of-mass energy of 7 TeV, using 1.3 pb-1 of data collected by the ATLAS experiment at the Large Hadron Collider. Signal electrons in the transverse momentum range 7 < pt < 26 GeV and within |eta| <2.0, excluding 1.37<|eta|<1.52, are extracted from the dominant hadron and conversion backgrounds. After subtraction of the small W/Z/gamma* contribution, the cross-section as a function of pt is found to be in good agreement with theoretical predictions for heavy flavour production from Fixed Order N…
Given the hints of lepton-flavour non-universality in semi-leptonic $B$ decays, leptoquark (LQ) models with sizeable couplings to heavy-quark flavours are enjoying a renaissance. While such models are subject to stringent constraints from low-energy experiments also bounds from non-resonant dilepton searches at the Large Hadron Collider (LHC) turn out to be phenomenologically relevant. Based on the latest LHC dilepton analyses corresponding to an integrated luminosity of around $140 \, {\rm fb}^{-1}$ of proton-proton collisions at $\sqrt{s} = 13 \, {\rm TeV}$, we present improved limits on the…
The sensitivity of the ATLAS experiment to the top-quark rare decay via flavour-changing neutral currents t --> Hq (q represents c and u quarks) has been studied at sqrt{s} = 14 TeV in the decay mode of ttbar --> HqWb --> WW*q,Wb -->l νl νj, l^{\pm} νb, (l=e, μ). The Standard Model backgrounds ttbar, ttbar H, WZ and WH have been analysed. The signal and backgrounds were generated via PYTHIA 5.7 and simulated and analysed using ATLFAST 2.51. A branching ratio for t --> Hq -->WW*q as low as 2.0x10^{-3} for m_H=150 GeV and 2.1x10^{-3} for m_H=160 GeV could be discovered at the 5 σlevel with an in…
