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 NLO calculations with Next-to-Leading-Log high pt resummation.
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.
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.
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.
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 …
Latest results on inclusive top-quark pair production cross-sections are presented using collision data collected by ATLAS and CMS experiments at the LHC. Inclusive and differential measurements of top-quark pair production cross-sections from ATLAS and CMS are presented in the resolved and boosted kinematic regions. The cross-sections are measured as a function of various kinematic observables of the top quarks, the jets and leptons of the event final state.
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…
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.
This Thesis presents the measurement of the inclusive jet cross section using the full 2010 data set, corresponding to 37 pb^{-1} collected by ATLAS. Jets with p_{T} > 20 GeV and |y| < 4.4 are considered in this analysis. The measurement of the jet shapes using the first 3 pb^{-1} is also presented, for jets with p_{T} > 30 GeV and |y| < 2.8. Both measurements are unfolded back to the particle level. Results are compared to theoretical predictions, showing the potential of the measurements to test QCD, and its importance in the search for new physics.
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…
A search for flavour-changing neutral current (FCNC) processes in top-quark decays is presented. Data collected from proton-proton collisions at the LHC at a centre-of-mass energy of $\sqrt{s}=13$ TeV corresponding to an integrated luminosity of 36 fb$^{-1}$, are analysed. A search is performed for top-quark pair-production events, with one top-quark decaying through the $t\to qZ$ ($q=u,c$) FCNC channel, and the other through the dominant Standard Model mode $t\to bW$. Only the decays of the $Z$ boson to charged leptons and leptonic $W$ boson decays are considered as signal. No evidence for a …
Several electroweak precision measurements are performed by the ATLAS and CMS collaborations at the LHC. The main ones are carried out using Drell-Yan production of single W and Z boson. They regard the measurement of the production cross sections of W and Z bosons, the mass of the W boson, and $\sin^2θ_W$. The results of the $\sin^2θ_W$ measurements have an accuracy of approximately twice that reached at LEP and SLD. Other measurements reported are about the Drell-Yan differential production cross section.
A direct search for charged lepton-flavour violation in top-quark decays is presented. The data analysed correspond to $79.8\ \text{fb}^{-1}$ of proton--proton collisions at a centre-of-mass energy of $\sqrt{s}=13\ \text{TeV}$ recorded by the ATLAS experiment at the LHC. The process studied is the production of top-quark pairs, where one top quark decays into a pair of opposite-sign different-flavour charged leptons and an up-type quark, while the other decays semileptonically according to the Standard Model. The signature of the signal is thus characterised by the presence of three charged le…
Results are presented of a search for supersymmetric particles decaying into final states with significant missing transverse momentum and exactly two identical flavour leptons (e or mu) of opposite charge in sqrt{s}=7 TeV collisions at the Large Hadron Collider. This channel is particularly sensitive to supersymmetric particle cascade decays producing flavour correlated lepton pairs. Flavour uncorrelated backgrounds are subtracted using a sample of opposite flavour lepton pair events. Observation of an excess beyond Standard Model expectations following this subtraction procedure would offer …
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 s…
Leptons with essentially the same properties apart from their mass are grouped into three families (or flavours). The number of leptons of each flavour is conserved in interactions, but this is not imposed by fundamental principles. Since the formulation of the standard model of particle physics, the observation of flavour oscillations among neutrinos has shown that lepton flavour is not conserved in neutrino weak interactions. So far, there has been no experimental evidence that this also occurs in interactions between charged leptons. Such an observation would be a sign for undiscovered part…
The results of the most recent measurements of the inclusive $\mathrm{t\bar{t}}$ cross section performed by the ATLAS and CMS experiments at the CERN LHC are summarized. These include results obtained in proton-proton collisions at different centre-of-mass energies, and the first observation of $\mathrm{t\bar{t}}$ production in proton-lead collisions. A new result at $\sqrt{s} = 13~\mathrm{TeV}$ by the CMS Collaboration is presented for the first time at this conference. In this analysis, the total cross section is determined both for a fixed top quark mass value in the simulation of $m_{\math…
The measurement of the muon inclusive differential cross section dsigma/dpT in pp collisions at sqrt(s)=7 TeV with the ATLAS detector is presented. The analysis is performed in the pseudorapidity interval |eta|< 2.5 for muon of transverse momentum 4 < pT < 100 GeV and with an integrated luminosity of 1.4 pb-1. The result is compared with the next-to-leading order with next-to-leading log high pT resummation prediction for the heavy avour production and with MC@NLO prediction for W-Z bosons production. The measurement is sensitive for the first time to the next-to-leading log contribution to th…
This paper describes a measurement of the flavour composition of dijet events produced in pp collisions at sqrt{s}=7 TeV using the ATLAS detector. The measurement uses the full 2010 data sample, corresponding to an integrated luminosity of 39 pb^-1. Six possible combinations of light, charm and bottom jets are identified in the dijet events, where the jet flavour is defined by the presence of bottom, charm or solely light flavour hadrons in the jet. Kinematic variables, based on the properties of displaced decay vertices and optimised for jet flavour identification, are used in a multidimensio…
The inclusive jet cross-section has been measured in proton-proton collisions at sqrt(s)=2.76 TeV in a dataset corresponding to an integrated luminosity of 0.20pb-1 collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-kt algorithm with two radius parameters of 0.4 and 0.6. The inclusive jet double-differential cross-section is presented as a function of the jet transverse momentum pT and jet rapidity y, covering a range of 20 <= pT < 430 GeV and |y| < 4.4. The ratio of the cross-section to the inclusive jet cross-section measurement at sqrt…
Inclusive isolated-photon production in $pp$ collisions at a centre-of-mass energy of 13 TeV is studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 3.2 fb$^{-1}$. The cross section is measured as a function of the photon transverse energy above 125 GeV in different regions of photon pseudorapidity. Next-to-leading-order perturbative QCD and Monte Carlo event-generator predictions are compared to the cross-section measurements and provide an adequate description of the data.
This paper presents a search for the t-channel exchange of an R-parity violating scalar top quark (t) in the emu continuum using 2.1/fb of data collected by the ATLAS detector in sqrt(s) = 7 TeV pp collisions at the Large Hadron Collider. Data are found to be consistent with the expectation from the Standard Model backgrounds. Limits on R-parity-violating couplings at 95% C.L. are calculated as a function of the scalar top mass (m_{t}). The upper limits on the production cross section for pp->emuX, through the t-channel exchange of a scalar top quark, ranges from 170 fb for m_{t}=95 GeV to 30 …
The sensitivity of the ATLAS experiment to the top-quark rare decay via flavor-changing neutral currents $t \rightarrow Zq$ ($q$ represents $c$ and $u$ quarks) has been studied at $\sqrt{s}$=14 TeV in two decay modes: 1.The pure leptonic decay of gauge bosons: $t\bar{t} \rightarrow ZqWb \rightarrow l^{+} l^{-} j l^{\pm} νb$ , (l=e, $μ$). 2.The leptonic decay of Z bosons and hadronic decay of W bosons: $t\bar{t} \rightarrow ZqWb \rightarrow l^{+} l^{-} jjjb$ , (l=e, $μ$). The dominant backgrounds $Z+jets$, $WZ$ and $t\bar{t}$ have been analysed. The signal and backgrounds were generated via PYT…
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 the flavour-changing neutral-current decay $t\to qZ$ is presented. Data collected by the ATLAS detector during 2012 from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=8$ TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, are analysed. Top-quark pair-production events with one top quark decaying through the $t\to qZ$ ($q=u,c$) channel and the other through the dominant Standard Model mode $t\to bW$ are considered as signal. Only the decays of the $Z$ boson to charged leptons and leptonic $W$ boson decays are used. No ev…
A search for flavour changing neutral current (FCNC) processes in top-quark decays by the ATLAS Collaboration is presented. Data collected from pp collisions at the LHC at a centre-of-mass energy of sqrt(s) = 7 TeV during 2011, corresponding to an integrated luminosity of 2.1 fb^-1, were used. A search was performed for top-quark pair-production events, with one top quark decaying through the t->Zq FCNC (q=u,c) channel, and the other through the Standard Model dominant mode t->Wb. Only the decays of the Z boson to charged leptons and leptonic W-boson decays were considered as signal. Consequen…
The inclusive $t\bar{t}$ production cross-section is measured in the lepton+jets channel using 20.2 fb$^{-1}$ of proton--proton collision data at a centre-of-mass energy of 8 TeV recorded with the ATLAS detector at the LHC. Major systematic uncertainties due to the modelling of the jet energy scale and $b$-tagging efficiency are constrained by separating selected events into three disjoint regions. In order to reduce systematic uncertainties in the most important background, the W+jets process is modelled using Z+jets events in a data-based approach. The inclusive $t\bar{t}$ cross-section is m…
Inclusive jet and dijet double-differential cross sections have been measured in proton-proton collisions at a centre-of-mass energy of 7 TeV using the ATLAS detector at the LHC. The cross sections were measured using jets clustered with the anti-kt algorithm. The measurements are performed in the jet rapidity range |y| < 4.4, covering jet transverse momenta from 20 GeV to 1.5 TeV and dijet invariant masses from 70 GeV to 5 TeV. The data are compared to expectations based on next-to-leading order QCD calculations corrected for non-perturbative effects, as well as to next-to-leading order Monte…
The inclusive as well as differential cross section of the associated production of top-antitop quark pairs and a $Z$ boson ($t\bar{t}Z$) is measured in final states with exactly three or four isolated leptons (electrons or muons). For this purpose, the full LHC Run 2 dataset of proton-proton collisions recorded by the ATLAS detector from $2015$ to $2018$, which corresponds to an integrated luminosity of $139\,\text{fb}^{-1}$, is used. The inclusive production cross section is measured to be $σ_{t\bar{t}Z}=1.05\pm0.05\,(\text{stat.})\,\pm0.09\,(\text{syst.})\,\text{pb}$, which is in agreement …
A measurement of the cross section for the inclusive production of isolated prompt photons in pp collisions at a centre-of-mass energy sqrt(s) = 7TeV is presented. The measurement covers the pseudorapidity ranges |eta|<1.37 and 1.52<|eta|<1.81 in the transverse energy range 15 < E_T <100 GeV. The results are based on an integrated luminosity of 880 nb-1, collected with the ATLAS detector at the Large Hadron Collider. Photon candidates are identified by combining information from the calorimeters and from the inner tracker. Residual background in the selected sample is estimated from data based…
We present a measurement of two-particle angular correlations in proton-proton collisions at sqrt(s) = 900 GeV and 7 TeV. The collision events were collected during 2009 and 2010 with the ATLAS detector at the Large Hadron Collider using a single-arm minimum bias trigger. Correlations are measured for charged particles produced in the kinematic range of transverse momentum pT > 100 MeV and pseudorapidity |eta| < 2.5. A complex structure in pseudorapidity and azimuth is observed at both collision energies. Results are compared to Pythia 8 and Herwig++ as well as to the AMBT2B, DW and Perugia 20…
A first measurement of the ttgamma cross section in pp collisions at the LHC using 1.04 fb^-1 of data taken with the ATLAS detector is presented. A total of 122 candidate events were identified in the single electron and single muon channels. The contributions from background processes with prompt photons, and with electrons or hadrons misidentified as photons were estimated reducing the dependence on simulations by the use of data-driven techniques. The resulting cross section times branching ratio into the single lepton and dilepton decay channels for photons with pt > 8 GeV reads σ_ttgamma …
The production cross sections of the inclusive Drell-Yan processes W to l nu and Z/gamma to ll (l=e,mu) are measured in proton-proton collisions at sqrt(s) = 7 TeV with the ATLAS detector. The cross sections are reported integrated over a fiducial kinematic range, extrapolated to the full range and also evaluated differentially as a function of the W decay lepton pseudorapidity and the Z boson rapidity, respectively. Based on an integrated luminosity of about 35 pb^-1 collected in 2010, the precision of these measurements reaches a few per cent. The integrated and the differential W+- and Z/ga…
A measurement of the differential cross-section for the inclusive production of isolated prompt photons in pp collisions at a center-of-mass energy sqrt(s) = 7 TeV is presented. The measurement covers the pseudorapidity ranges |eta|<1.37 and 1.52<=|eta|<2.37 in the transverse energy range 45<=E_T<400GeV. The results are based on an integrated luminosity of 35 pb-1, collected with the ATLAS detector at the LHC. The yields of the signal photons are measured using a data-driven technique, based on the observed distribution of the hadronic energy in a narrow cone around the photon candidate and th…
A search is presented for particles that decay producing a large jet multiplicity and invisible particles. The event selection applies a veto on the presence of isolated electrons or muons and additional requirements on the number of b-tagged jets and the scalar sum of masses of large-radius jets. Having explored the full ATLAS 2015-2016 dataset of LHC proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$, which corresponds to 36.1 fb$^{-1}$ of integrated luminosity, no evidence is found for physics beyond the Standard Model. The results are interpreted in the context of simplified models ins…
Calibration of the light-flavour jet mistagging efficiency of the $b$-tagging algorithms with $Z$+jets events using 139 $\mathrm{fb}^{-1}$ of ATLAS proton-proton collision data at $\sqrt{s} = 13$ TeV
Identifying the flavour of neutral $B$ mesons production is one of the most important components needed in the study of time-dependent $CP$ violation. The harsh environment of the Large Hadron Collider makes it particularly hard to succeed in this task. We present an inclusive flavour-tagging algorithm as an upgrade of the algorithms currently used by the LHCb experiment. Specifically, a probabilistic model which efficiently combines information from reconstructed vertices and tracks using machine learning is proposed. The algorithm does not use information about underlying physics process. It…
Inclusive jet and dijet cross-sections are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV. The measurement uses a dataset with an integrated luminosity of 3.2 fb$^{-1}$ recorded in 2015 with the ATLAS detector at the Large Hadron Collider. Jets are identified using the anti-${k_t}$ algorithm with a radius parameter value of $R=0.4$. The inclusive jet cross-sections are measured double-differentially as a function of the jet transverse momentum, covering the range from 100 GeV to 3.5 TeV, and the absolute jet rapidity up to $|y|=3$. The double-differential dijet produ…
Despite the absence of experimental evidence, weak scale supersymmetry remains one of the best motivated and studied Standard Model extensions. This report summarises recent ATLAS results on inclusive searches for supersymmetric squarks and gluinos, including third generation squarks produced in the decay of gluinos. Results are presented for both R-parity conserving and R-parity violating scenarios, with final states containing jets with and without missing transverse momentum, light leptons, taus or photons.
This Letter presents the measurement of differential cross sections of isolated prompt photons produced in association with a b-jet or a c-jet. These final states provide sensitivity to the heavy-flavour content of the proton and aspects related to the modelling of heavy-flavour quarks in perturbative QCD. The measurement uses proton-proton collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of up to 20.2 fb$^{-1}$. The differential cross sections are measured for each jet flavour with respect to the tra…
A measurement of charged-particle distributions sensitive to the properties of the underlying event is presented for an inclusive sample of events containing a Z-boson , decaying to an electron or muon pair. The measurement is based on data collected using the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 7 TeV with an integrated luminosity of $4.6$ fb$^{-1}$. Distributions of the charged particle multiplicity and of the charged particle transverse momentum are measured in regions of azimuthal angle defined with respect to the Z-boson direction. The measur…
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…
Jet cross sections have been measured for the first time in proton-proton collisions at a centre-of-mass energy of 7 TeV using the ATLAS detector. The measurement uses an integrated luminosity of 17 nb-1 recorded at the Large Hadron Collider. The anti-kt algorithm is used to identify jets, with two jet resolution parameters, R = 0.4 and 0.6. The dominant uncertainty comes from the jet energy scale, which is determined to within 7% for central jets above 60 GeV transverse momentum. Inclusive single-jet differential cross sections are presented as functions of jet transverse momentum and rapidit…
The inclusive production of four isolated charged leptons in $pp$ collisions is analysed for the presence of hard double-parton scattering, using 20.2 fb$^{-1}$ of data recorded in the ATLAS detector at the LHC at centre-of-mass energy $\sqrt{s}$ = 8 TeV. In the four-lepton invariant-mass range of $80<m_{4\ell}<1000$ GeV, an artificial neural network is used to enhance the separation between single- and double-parton scattering based on the kinematics of the four leptons in the final state. An upper limit on the fraction of events originating from double-parton scattering is determined at 95% …
