A bstract A search for the exclusive decays of the Higgs and Z bosons to a ϕ or ρ meson and a photon is performed with a pp collision data sample corresponding to an integrated luminosity of up to 35 . 6 fb −1 collected at $$ \sqrt{s}=13 $$ s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. These decays have been suggested as a probe of the Higgs boson couplings to light quarks. No significant excess of events is observed above the background, as expected from the Standard Model. Upper limits at 95% confidence level were obtained on the branching fractions of the Higgs boson decays to ϕ γ and ρ γ of 4 . 8 × 10 −4 and 8 . 8 × 10 −4 , respectively. The corresponding 95% confidence level upper limits for the Z boson decays are 0 . 9 × 10 −6 and 25 × 10 −6 for ϕ γ and ρ γ, respectively.

The production yield of Z bosons is measured in the electron and muon decay channels in Pb+Pb collisions at sNN=5.02 TeV with the ATLAS detector. Data from the 2015 LHC run corresponding to an integrated luminosity of 0.49 nb−1 are used for the analysis. The Z boson yield, normalised by the total number of minimum-bias events and the mean nuclear thickness function, is measured as a function of dilepton rapidity and event centrality. The measurements in Pb+Pb collisions are compared with similar measurements made in proton–proton collisions at the same centre-of-mass energy. The nuclear modification factor is found to be consistent with unity for all centrality intervals. The results are compared with theoretical predictions obtained at next-to-leading order using nucleon and nuclear parton distribution functions. The normalised Z boson yields in Pb+Pb collisions lie 1–3σ above the predictions. The nuclear modification factor measured as a function of rapidity agrees with unity and is consistent with a next-to-leading-order QCD calculation including the isospin

http://sro.sussex.ac.uk Abraham, N L, Allbrooke, B M M, Asquith, L, Cerri, A, Jones, S D, De Santo, A, Salvatore, F, Shaw, K, Stevenson, T J, Suruliz, K, Sutton, M R, Tresoldi, F, Trovato, F, Vivarelli, I, Winkels, E et al. (2019) Correlated long-range mixed-harmonic fluctuations measured in pp, p+Pb and lowmultiplicity Pb+Pb collisions with the ATLAS detector. Physics Letters B, 789. pp. 444-471. ISSN 0370-2693

The production cross-sections for W +/- and Z bosons are measured using ATLAS data corresponding to an integrated luminosity of 4.0 pb-1 collected at a centre-ofmass energy v s = 2.76 TeV. The deca ...

http://sro.sussex.ac.uk Allbrooke, B M M, Asquith, L, Cerri, A, Chavez Barajas, C A, De Santo, A, Salvatore, F, Santoyo Castillo, I, Suruliz, K, Sutton, M R, Vivarelli, I and The ATLAS collaboration, (2016) Transverse momentum, rapidity, and centrality dependence of inclusive charged-particle production in sqrt (SNN) = 5.01 TeV p+Pb collisions measured by the ATLAS experiment. Physics Letters B, 763. pp. 313-336. ISSN 0370-2693

This change does not have any impact on the measured helicity fractions, but it changes the obtained limits on the anomalous couplings.

This paper reports searches for heavy resonances decaying into ZZ or ZW using data from proton-proton collisions at a centre-of-mass energy of √(s)=13 TeV. The data, corresponding to an integrated luminosity of 36.1 fb −1 , were recorded with the ATLAS detector in 2015 and 2016 at the Large Hadron Collider. The searches are performed in final states in which one Z boson decays into either a pair of light charged leptons (electrons and muons) or a pair of neutrinos, and the associated W boson or the other Z boson decays hadronically. No evidence of the production of heavy resonances is observed. Upper bounds on the production cross sections of heavy resonances times their decay branching ratios to ZZ or ZW are derived in the mass range 300-5000GeV within the context of Standard Model extensions with additional Higgs bosons, a heavy vector triplet or warped extra dimensions. Production through gluon-gluon fusion, Drell-Yan or vector-boson fusion are considered, depending on the assumed model.

Studies of the spin and parity quantum numbers of the Higgs boson in the W W ∗ → e νμν final state are pre-sented,basedonproton–protoncollisiondatacollectedbythe ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb − 1 at a centre-of-mass energy of √ s = 8 TeV. The Standard Model spin-parity J CP = 0 ++ hypothesis is compared with alternative hypotheses for bothspinandCP. Thecasewheretheobserved resonance is a mixture of the Standard-Model-like Higgs boson and CP-even ( J CP = 0 ++ ) or CP-odd ( J CP = 0 +− ) Higgs boson in scenarios beyond the Standard Model is also studied. The data are found to be consistent with the Standard Model prediction and limits are placed on alternative spin and CP hypotheses, including CP mixing in different scenarios.

The software for the ATLAS experiment on the Large Hadron Collider at CERN has evolved over many years to meet the demands of Monte Carlo simulation, particle detector reconstruction and data analysis. At present over 3.8 million lines of C++ code (and close to 6 million total lines of code) are maintained by an active worldwide developer community. To run the experiment software efficiently it is essential to maintain a high level of software quality standards. The methods proposed to improve software quality practices by incorporating checks into the new ATLAS software build infrastructure will be discussed.

Abstract A search for supersymmetry involving the pair production of gluinos decaying via third-generation squarks into the lightest neutralino $$ \left({\tilde{\chi}}_1^0\right) $$ χ ˜ 1 0 is reported. It uses LHC proton-proton collision data at a centre-of-mass energy $$ \sqrt{s}=13 $$ s = 13 TeV with an integrated luminosity of 36.1 fb−1 collected with the ATLAS detector in 2015 and 2016. The search is performed in events containing large missing transverse momentum and several energetic jets, at least three of which must be identified as originating from b-quarks. To increase the sensitivity, the sample is divided into subsamples based on the presence or absence of electrons or muons. No excess is found above the predicted background. For $$ {\tilde{\chi}}_1^0 $$ χ ˜ 1 0 masses below approximately 300 GeV, gluino masses of less than 1.97 (1.92) TeV are excluded at 95% confidence level in simplified models involving the pair production of gluinos that decay via top (bottom) squarks. An interpretation of the limits in terms of the branching ratios of the gluinos into third-generation squarks is also provided. These results improve upon the exclusion limits obtained with the 3.2 fb−1 of data collected in 2015.

This paper presents a search for direct electroweak gaugino or gluino pair production with a chargino nearly mass-degenerate with a stable neutralino. It is based on an integrated luminosity of 36.1 $\mathrm{fb}^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV collected by the ATLAS experiment at the LHC. The final state of interest is a disappearing track accompanied by at least one jet with high transverse momentum from initial-state radiation or by four jets from the gluino decay chain. The use of short track segments reconstructed from the innermost tracking layers significantly improves the sensitivity to short chargino lifetimes. The results are found to be consistent with Standard Model predictions. Exclusion limits are set at 95% confidence level on the mass of charginos and gluinos for different chargino lifetimes. For a pure wino with a lifetime of about 0.2 ns, chargino masses up to 460 GeV are excluded. For the strong production channel, gluino masses up to 1.65 TeV are excluded assuming a chargino mass of 460 GeV and lifetime of 0.2 ns.

A search for new charged massive gauge bosons, W′, is performed with the ATLAS detector at the LHC. Data were collected in proton–proton collisions at a center-of-mass energy of s=13 TeV and correspond to an integrated luminosity of 36.1 fb−1. This analysis searches for W′ bosons in the W′→tb¯ decay channel in final states with an electron or muon plus jets. The search covers resonance masses between 0.5 and 5.0 TeV and considers right-handed W′ bosons. No significant deviation from the Standard Model (SM) expectation is observed and upper limits are set on the W′→tb¯ cross section times branching ratio and the W′ boson effective couplings as a function of the W′ boson mass. For right-handed W′ bosons with coupling to the SM particles equal to the SM weak coupling constant, masses below 3.15 TeV are excluded at the 95% confidence level. This search is also combined with a previously published ATLAS result for W′→tb¯ in the fully hadronic final state. Using the combined searches, right-handed W′ bosons with masses below 3.25 TeV are excluded at the 95% confidence level. © 2018 The Author(s)

Searches for the exclusive decays of the Higgs and Z bosons into a J/ψ, ψ(2S), or ϒ(nS) (n=1,2,3) meson and a photon are performed with a pp collision data sample corresponding to an integrated luminosity of 36.1fb−1 collected at s=13TeV with the ATLAS detector at the CERN Large Hadron Collider. No significant excess of events is observed above the expected backgrounds, and 95% confidence-level upper limits on the branching fractions of the Higgs boson decays to J/ψγ, ψ(2S)γ, and ϒ(nS)γ of 3.5×10−4, 2.0×10−3, and (4.9,5.9,5.7)×10−4, respectively, are obtained assuming Standard Model production. The corresponding 95% confidence-level upper limits for the branching fractions of the Z boson decays are 2.3×10−6, 4.5×10−6 and (2.8,1.7,4.8)×10−6, respectively.

Abstract 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 production cross-sections are presented as a function of the dijet mass, covering the range from 300 GeV to 9 TeV, and the half absolute rapidity separation between the two leading jets within |y| < 3, y ∗, up to y ∗ = 3. Next-to-leading-order, and next-to-next-to-leading-order for the inclusive jet measurement, perturbative QCD calculations corrected for non-perturbative and electroweak effects are compared to the measured cross-sections.

AbstractAn angular analysis of the decayBd0 → K∗μ+μ−is presented, based on proton-proton collision data recorded by the ATLAS experiment at the LHC. The study is using 20.3 fb−1of integrated luminosity collected during 2012 at centre-of-mass energy of$$ \sqrt{s}=8 $$s=8TeV. Measurements of theK*longitudinal polarisation fraction and a set of angular parameters obtained for this decay are presented. The results are compatible with the Standard Model predictions.

A search for the decay of the Standard Model Higgs boson into a b ¯ b pair when produced in association with a W or Z boson is performed with the ATLAS detector. The data, corresponding to an integrated luminosity of 79 . 8 fb − 1 were collected in proton–proton collisions during Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.9 (4.3) standard deviations. A combination with the results from other searches in Run 1 and in Run 2 for the Higgs boson in the b ¯ b decay mode is performed, which yields an observed (expected) significance of 5.4 (5.5) standard deviations, thus providing direct observation of the Higgs boson decay into b -quarks. The ratio of the measured event yield for a Higgs boson decaying into b ¯ b to the Standard Model expectation is 1 . 01 ± 0 . 12 ( stat .) + 0 . 16 − 0 . 15 ( syst .) . Additionally, a combination of Run 2 results searching for the Higgs boson produced in association with a vector boson yields an observed (expected) significance of 5.3 (4.8) standard deviations.