Measurements of the variation of inclusive jet suppression as a function of relative azimuthal angle, Δϕ, with respect to the elliptic event plane provide insight into the path-length dependence of jet quenching. ATLAS has measured the Δϕ dependence of jet yields in 0.14  nb-1 of sNN=2.76  TeV Pb+Pb collisions at the LHC for jet transverse momenta pT>45  GeV in different collision centrality bins using an underlying event subtraction procedure that accounts for elliptic flow. The variation of the jet yield with Δϕ was characterized by the parameter, v2jet, and the ratio of out-of-plane (Δϕ∼π/2) to in-plane (Δϕ∼0) yields. Nonzero v2jet values were measured in all centrality bins for pT<160  GeV. The jet yields are observed to vary by as much as 20% between in-plane and out-of-plane directions.

Charged-particle spectra obtained in Pb+Pb interactions at $ \sqrt{s_{\mathrm{NN}}}=2.76 $ TeV and pp interactions at $ \sqrt{s_{\mathrm{NN}}}=2.76 $ TeV with the ATLAS detector at the LHC are presented, using data with integrated luminosities of 0.15 nb$^{−1}$ and 4.2 pb$^{−1}$, respectively, in a wide transverse momentum (0.5 < p$_{T}$ < 150 GeV) and pseudorapidity (|η| < 2) range. For Pb+Pb collisions, the spectra are presented as a function of collision centrality, which is determined by the response of the forward calorimeters located on both sides of the interaction point. The nuclear modification factors R$_{AA}$ and R$_{CP}$ are presented in detail as a function of centrality, p$_{T}$ and η. They show a distinct p$_{T}$-dependence with a pronounced minimum at about 7 GeV. Above 60 GeV, R$_{AA}$ is consistent with a plateau at a centrality-dependent value, within the uncertainties. The value is 0.55 ± 0.01(stat.) ± 0.04(syst.) in the most central collisions. The R$_{AA}$ distribution is consistent with flat |η| dependence over the whole transverse momentum range in all centrality classes.

<p>The transverse polarization of <inline-formula><mml:math display="inline"><mml:mi mathvariant="normal">Λ</mml:mi></mml:math></inline-formula> and <inline-formula><mml:math display="inline"><mml:mover accent="true"><mml:mi mathvariant="normal">Λ</mml:mi><mml:mo accent="true" stretchy="false">¯</mml:mo></mml:mover></mml:math></inline-formula> hyperons produced in proton-proton collisions at a center-of-mass energy of 7 TeV is measured. The analysis uses <inline-formula><mml:math display="inline"><mml:mrow><mml:mn>760</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>μ</mml:mi><mml:msup><mml:mrow><mml:mi mathvariant="normal">b</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math></inline-formula> of minimum bias data collected by the ATLAS detector at the LHC in the year 2010. The measured transverse polarization averaged over Feynman <inline-formula><mml:math display="inline"><mml:msub><mml:mi>x</mml:mi><mml:mi mathvariant="normal">F</mml:mi></mml:msub></mml:math></inline-formula> from <inline-formula><mml:math display="inline"><mml:mn>5</mml:mn><mml:mo>×</mml:mo><mml:mn>1</mml:mn><mml:msup><mml:mn>0</mml:mn><mml:mrow><mml:mo>-</mml:mo><mml:mn>5</mml:mn></mml:mrow></mml:msup></mml:math></inline-formula> to 0.01 and transverse momentum <inline-formula><mml:math display="inline"><mml:msub><mml:mi>p</mml:mi><mml:mi mathvariant="normal">T</mml:mi></mml:msub></mml:math></inline-formula> from 0.8 to 15 GeV is <inline-formula><mml:math display="inline"><mml:mrow><mml:mo>-</mml:mo><mml:mn>0.010</mml:mn><mml:mo>±</mml:mo><mml:mn>0.005</mml:mn><mml:mo stretchy="false">(</mml:mo><mml:mtext>stat</mml:mtext><mml:mo stretchy="false">)</mml:mo><mml:mo>±</mml:mo><mml:mn>0.004</mml:mn><mml:mo stretchy="false">(</mml:mo><mml:mtext>syst</mml:mtext><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:math></inline-formula> for <inline-formula><mml:math display="inline"><mml:mi mathvariant="normal">Λ</mml:mi></mml:math></inline-formula> and <inline-formula><mml:math display="inline"><mml:mrow><mml:mn>0.002</mml:mn><mml:mo>±</mml:mo><mml:mn>0.006</mml:mn><mml:mo stretchy="false">(</mml:mo><mml:mtext>stat</mml:mtext><mml:mo stretchy="false">)</mml:mo><mml:mo>±</mml:mo><mml:mn>0.004</mml:mn><mml:mo stretchy="false">(</mml:mo><mml:mtext>syst</mml:mtext><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:math></inline-formula> for <inline-formula><mml:math display="inline"><mml:mover accent="true"><mml:mi mathvariant="normal">Λ</mml:mi><mml:mo accent="true" stretchy="false">¯</mml:mo></mml:mover></mml:math></inline-formula>. It is also measured as a function of <inline-formula><mml:math display="inline"><mml:msub><mml:mi>x</mml:mi><mml:mi mathvariant="normal">F</mml:mi></mml:msub></mml:math></inline-formula> and <inline-formula><mml:math display="inline"><mml:msub><mml:mi>p</mml:mi><mml:mi mathvariant="normal">T</mml:mi></mml:msub></mml:math></inline-formula>, but no significant dependence on these variables is observed. Prior to this measurement, the polarization was measured at fixed-target experiments with center-of-mass energies up to about 40 GeV. The ATLAS results are compatible with the extrapolation of a fit from previous measurements to the <inline-formula><mml:math display="inline"><mml:msub><mml:mi>x</mml:mi><mml:mi mathvariant="normal">F</mml:mi></mml:msub></mml:math></inline-formula> range covered by this measurement.</p>

The semiconductor tracker is a silicon microstrip detector forming part of the inner tracking system of the ATLAS experiment at the LHC. The operation and performance of the semiconductor tracker during the first years of LHC running are described. More than 99% of the detector modules were operational during this period, with an average intrinsic hit efficiency of (99.74±0.04)%. The evolution of the noise occupancy is discussed, and measurements of the Lorentz angle, δ-ray production and energy loss presented. The alignment of the detector is found to be stable at the few-micron level over long periods of time. Radiation damage measurements, which include the evolution of detector leakage currents, are found to be consistent with predictions and are used in the verification of radiation background simulations.

The production of opposite-charge W -boson pairs in proton–proton collisions at s=13 TeV is measured using data corresponding to 3.16 fb −1 of integrated luminosity collected by the ATLAS detector at the CERN Large Hadron Collider in 2015. Candidate W -boson pairs are selected by identifying their leptonic decays into an electron, a muon and neutrinos. Events with reconstructed jets are not included in the candidate event sample. The cross-section measurement is performed in a fiducial phase space close to the experimental acceptance and is compared to theoretical predictions. Agreement is found between the measurement and the most accurate calculations available.

The ATLAS experiment at the CERN Large Hadron Collider has performed searches for new, heavy bosons decaying to WW , WZ and ZZ final states in multiple decay channels using 20.3 fb−1 of pp collision data at s=8 TeV . In the current study, the results of these searches are combined to provide a more stringent test of models predicting heavy resonances with couplings to vector bosons. Direct searches for a charged diboson resonance decaying to WZ in the ℓνℓ′ℓ′ ( ℓ=μ,e ), ℓℓqq¯ , ℓνqq¯ and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WZ bosons are compared with predictions of an extended gauge model with a heavy W′ boson. In addition, direct searches for a neutral diboson resonance decaying to WW and ZZ in the ℓℓqq¯ , ℓνqq¯ , and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WW and ZZ bosons are compared with predictions for a heavy, spin-2 graviton in an extended Randall–Sundrum model where the Standard Model fields are allowed to propagate in the bulk of the extra dimension.

The integrated elliptic flow of charged particles produced in Pb+Pb collisions at sqrt(s_NN)=2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v_2, was measured in the pseudorapidity range |eta| <= 2.5 with the event-plane method. In order to include tracks with very low transverse momentum pT, thus reducing the uncertainty in v_2 integrated over pT, a 1 mu b-1 data sample without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v_2 is compared to other measurements obtained with higher pT thresholds. A weak pseudorapidity dependence of the integrated elliptic flow is observed for central collisions, and a small decrease when moving away from mid-rapidity is observed only in peripheral collisions. The integrated v2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data.

A measurement is reported of the production cross section of top-quark pairs in proton-proton collisions at a center-of-mass energy of 7 TeV recorded with the ATLAS detector at the LHC. Candidate events have a signature consistent with containing two isolated leptons, large missing transverse momentum, and at least two jets. Using a data sample corresponding to an integrated luminosity of 0.70 fb^-1, a t-tbar production cross section sigmattbar=176 +/-5 (stat) +14 -11 (syst) +/- 8 (lum.) pb is measured for an assumed top-quark mass of m_t = 172.5 GeV.

Charged Higgs bosons heavier than the top quark and decaying via $H^{\pm} \rightarrow tb$ are searched for in proton--proton collisions measured with the ATLAS experiment at $\sqrt{s}=8$ TeV corresponding to an integrated luminosity of 20.3 fb$^{-1}$. The production of a charged Higgs boson in association with a top quark, $gb \rightarrow tH^{\pm}$, is explored in the mass range 200 to 600 GeV using multi-jet final states with one electron or muon. In order to separate the signal from the Standard Model background, analysis techniques combining several kinematic variables are employed. An excess of events above the background-only hypothesis is observed across a wide mass range, amounting to up to 2.4 standard deviations. Upper limits are set on the $gb\rightarrow tH^{\pm}$ production cross section times the branching fraction $\mathrm{BR}(H^{\pm} \rightarrow tb)$. Additionally, the complementary $s$-channel production, $qq' \rightarrow H^{\pm}$, is investigated through a reinterpretation of $W' \rightarrow tb$ searches in ATLAS. Final states with one electron or muon are relevant for $H^{\pm}$ masses from 0.4 to 2.0 TeV, whereas the all-hadronic final state covers the range 1.5 to 3.0 TeV. In these search channels, no significant excesses from the predictions of the Standard Model are observed, and upper limits are placed on the $qq' \rightarrow H^{\pm}$ production cross section times the branching fraction BR$(H^{\pm} \rightarrow tb)$.

Searches for high-mass resonances in the dijet invariant mass spectrum with one or two jets identified as b -jets are performed using an integrated luminosity of 3.2 fb−1 of proton–proton collisions with a centre-of-mass energy of s=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% credibility level, excited b⁎ quarks with masses from 1.1 TeV to 2.1 TeV and leptophobic Z′ bosons with masses from 1.1 TeV to 1.5 TeV. Contributions of a Gaussian signal shape with effective cross sections ranging from approximately 0.4 to 0.001 pb are also excluded in the mass range 1.5–5.0 TeV.