A measurement of the charge asymmetry in top-quark pair ($t\bar{t}$) production in association with a photon is presented. The measurement is performed in the single-lepton $t\bar{t}$ decay channel using proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN at a centre-of-mass-energy of 13 TeV during the years 2015-2018, corresponding to an integrated luminosity of 139 fb$^{-1}$. The charge asymmetry is obtained from the distribution of the difference of the absolute rapidities of the top quark and antiquark using a profile likelihood unfolding approach. It is measured to be $A_\text{C}=-0.003 \pm 0.029$ in agreement with the Standard Model expectation.
The measured asymmetry of top quark pairs in $t\bar{t}\gamma$ production in a fiducial region at particle level.
Normalised differential cross section as a function of $|y(t)| - |y(\bar{t})|$. The observed data is compared with the SM expectation using aMC@NLO+Pythia8 at NLO QCD precision. The value of the charge asymmetry corresponds to the difference between the two bins. Underflow and overflow events are included in corresponding bins of the distribution.
Definition of the fiducial phase space at particle level. where, $\gamma$: photon $\ell$: lepton j: jet
A measurement of $\textit{W}$ boson production in lead-lead collisions at $\sqrt{s_{\mathrm{NN}}}=$2.76 TeV is presented. It is based on the analysis of data collected with the ATLAS detector at the LHC in 2011 corresponding to an integrated luminosity of 0.14 $\mathrm{nb}^{-1}$ and 0.15 $\mathrm{nb}^{-1}$ in the muon and electron decay channels, respectively. The differential production yields and lepton charge asymmetry are each measured as a function of the average number of participating nucleons $< N_{\mathrm{part}} >$ and absolute pseudorapidity of the charged lepton. The results are compared to predictions based on next-to-leading-order QCD calculations. These measurements are, in principle, sensitive to possible nuclear modifications to the parton distribution functions and also provide information on scaling of $\textit{W}$ boson production in multi-nucleon systems.
Ratio of W+ and W- candidates in $W\rightarrow \ell \nu_{\ell}$ as a function of the mean number of participants $N_{part}$.
$W^\pm$ boson production yield per binary collision as a function of the mean number of participants $N_{part}$.
Differential production yield per binary collision for $W^{+}$ bosons as a function of $|\eta_\ell|$.
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 $\ell\ell q q$ contact interaction scale $\Lambda$ between 15.4 TeV and 26.3 TeV, at the 95% credibility level. For large extra spatial dimensions, lower limits are set on the string scale $M_{S}$ between 3.2 TeV to 5.0 TeV.
Reconstructed dielectron mass distributions for data and the SM background estimate.
Reconstructed dimuon mass distributions for data and the SM background estimate.
Reconstructed $\cos\theta^*$ distributions for data and the SM background estimate in the dielectron channel.
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/gamma cross sections in the e and mu channels are combined, and compared with perturbative QCD calculations, based on a number of different parton distribution sets available at NNLO.
Cross sections for Z0 production from the combined electron and muon data sets in the defined fiducial regions. The first (sys) error is the uncorrelated systematic error and the second is the correlated systematic error.
Correlated Systematic Uncertainties for Z0 production.
Cross sections for W- production from the combined electron and muon data sets in the defined fiducial regions. The first (sys) error is the uncorrelated systematic error and the second is the correlated systematic error.
Using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally-segmented electromagnetic and hadronic calorimeters. The underlying event is measured and subtracted event-by-event, giving estimates of jet transverse energy above the ambient background. The transverse energies of dijets in opposite hemispheres is observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, and which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.
Asymmetry in the different centrality regions for 2.76 TeV/Nucleon PB-PB collisions.
Asymmetry in 7 TeV P-P collisions.
DeltaPhi distribution in the different centrality regions for 2.76 TeV/Nucleon PB-PB collisions.
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