Bruno Amadio

© 2016, CERN for the benefit of the ATLAS collaboration.The number of charged particles inside jets is a widely used discriminant for identifying the quark or gluon nature of the initiating parton and is sensitive to both the perturbative and non-perturbative components of fragmentation. This paper presents a measurement of the average number of charged particles with >500 pT> 500 {\mathrm{MeV}} MeV inside high-momentum jets in dijet events using 20.3 fb-1 of data recorded with the ATLAS detector in pp collisions at √s = 8 TeV collisions at the LHC. The jets considered have transverse momenta from 50 GeV up to and beyond 1.5 TeV. The reconstructed charged-particle track multiplicity distribution is unfolded to remove distortions from detector effects and the resulting charged-particle multiplicity is compared to several models. Furthermore, quark and gluon jet fractions are used to extract the average charged-particle multiplicity for quark and gluon jets separately.

© 2016, CERN for the benefit of the ATLAS collaboration.The result of a search for pair production of the supersymmetric partner of the Standard Model bottom quark is reported. The search uses 3.2 fb- 1 of pp collisions at s=13 TeV collected by the ATLAS experiment at the Large Hadron Collider in 2015. Bottom squarks are searched for in events containing large missing transverse momentum and exactly two jets identified as originating from b-quarks. No excess above the expected Standard Model background yield is observed. Exclusion limits at 95 % confidence level on the mass of the bottom squark are derived in phenomenological supersymmetric R-parity-conserving models in which the b~1 is the lightest squark and is assumed to decay exclusively via b~1→bχ~10, where χ~10 is the lightest neutralino. The limits significantly extend previous results; bottom squark masses up to 800 GeV are excluded for the χ~10 mass below 360 GeV whilst differences in mass above 100 GeV between the b~1 and the χ~10 are excluded up to a b~1 mass of 500 GeV.

© 2016, CERN for the benefit of the ATLAS collaboration.This paper presents a dedicated search for exotic decays of the Higgs boson to a pair of new spin-zero particles, H→ aa, where the particle a decays to b-quarks and has a mass in the range of 20–60 GeV. The search is performed in events where the Higgs boson is produced in association with a W boson, giving rise to a signature of a lepton, missing transverse momentum, and multiple jets from b-quark decays. The analysis is based on the full dataset of pp collisions at s=13TeV recorded in 2015 by the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 3.2 fb-1. No significant excess of events above the Standard Model prediction is observed, and a 95% confidence-level upper limit is derived for the product of the production cross section for pp→ WH times the branching ratio for the decay H→ aa→ 4 b. The upper limit ranges from 6.2 pb for an a-boson mass ma=20GeV to 1.5 pb for ma=60GeV.

© 2016, The Author.A search for neutral Higgs bosons of the minimal supersymmetric standard model and for a heavneutral Z′ boson is performed using a data sample corresponding to an integrated luminosity of 3.2 fb- 1 from proton–proton collisions at s=13 TeV recorded by the ATLAS detector at the LHC. The heavy resonance is assumed to decay to a τ+τ- pair with at least one τ lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2–1.2 TeV for the MSSM neutral Higgs bosons and 0.5–2.5 TeV for the heavy neutral Z′ boson. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in MSSM and Z′ benchmark scenarios. The most stringent constraints on the MSSM mA–tan β space exclude at 95 % confidence level tan β> 7.6 for mA= 200 GeV in the mhmod+ MSSM scenario. For the Sequential Standard Model, a ZSSM′ mass up to 1.90 TeV is excluded at 95 % CL and masses up to 1.82–2.17 TeV are excluded for a ZSFM′ of the strong flavour model.

© 2017, CERN for the benefit of the ATLAS collaboration.This paper presents a measurement of the polarisation of W bosons from tt¯ decays, reconstructed in events with one high-pT lepton and at least four jets. Data from pp collisions at the LHC were collected at s = 8 TeV and correspond to an integrated luminosity of 20.2 fb- 1. The angle θ∗ between the b-quark from the top quark decay and a direct W boson decay product in the W boson rest frame is sensitive to the W boson polarisation. Two different W decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying W boson, respectively. The most precise measurement of the W boson polarisation via the distribution of cos θ∗ is obtained using the leptonic analyser and events in which at least two of the jets are tagged as b-quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are F0=0.709±0.019, FL=0.299±0.015 and FR=-0.008±0.014, and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.

© 2017, CERN for the benefit of the ATLAS collaboration.A measurement of the tt¯ Z and tt¯ W production cross sections in final states with either two same-charge muons, or three or four leptons is presented. The analysis uses a data sample of proton–proton collisions at s=13 TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015, corresponding to a total integrated luminosity of 3.2 fb- 1. The inclusive cross sections are extracted using likelihood fits to signal and control regions, resulting in σtt¯Z= 0.9 ± 0.3 pb and σtt¯W= 1.5 ± 0.8 pb, in agreement with the Standard Model predictions.

© 2017 CERN.A measurement of the inclusive pp→tt+X production cross section in the τ+jets final state using only the hadronic decays of the τ lepton is presented. The measurement is performed using 20.2 fb-1 of proton-proton collision data recorded at a center-of-mass energy of s=8 TeV with the ATLAS detector at the Large Hadron Collider. The cross section is measured via a counting experiment by imposing a set of selection criteria on the identification and kinematic variables of the reconstructed particles and jets, and on event kinematic variables and characteristics. The production cross section is measured to be σtt=239±29 pb, which is in agreement with the measurements in other final states and the theoretical predictions at this center-of-mass energy.

© 2017, CERN for the benefit of the ATLAS collaboration.Two searches for new phenomena in final states containing a same-flavour opposite-sign lepton 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 s=13 TeV by the ATLAS detector at the large hadron collider, which correspond to an integrated luminosity of 14.7fb-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 of two mechanisms: a leptonically decaying Z boson in the final state, leading to a peak in the dilepton invariant-mass distribution around the Z boson mass; and decays of neutralinos, yielding a kinematic endpoint in the dilepton invariant-mass spectrum. The data are found to be consistent with the Standard Model expectation. Results are interpreted in simplified models of gluino-pair production, and provide sensitivity to gluinos with masses as large as 1.70 TeV.

© 2017, CERN for the benefit of the ATLAS collaboration.The production of two prompt J/ ψ mesons, each with transverse momenta pT> 8.5 GeV and rapidity |y| < 2.1, is studied using a sample of proton-proton collisions at s=8 TeV, corresponding to an integrated luminosity of 11.4 fb- 1 collected in 2012 with the ATLAS detector at the LHC. The differential cross-section, assuming unpolarised J/ ψ production, is measured as a function of the transverse momentum of the lower-pTJ/ ψ meson, di-J/ ψpT and mass, the difference in rapidity between the two J/ ψ mesons, and the azimuthal angle between the two J/ ψ mesons. The fraction of prompt pair events due to double parton scattering is determined by studying kinematic correlations between the two J/ ψ mesons. The total and double parton scattering cross-sections are compared with predictions. The effective cross-section of double parton scattering is measured to be σeff= 6.3 ± 1.6 ± 1.0 mb.

© 2017, CERN for the benefit of the ATLAS collaboration.A measurement of the calorimeter response to isolated charged hadrons in the ATLAS detector at the LHC is presented. This measurement is performed with 3.2 nb- 1 of proton–proton collision data at s=7 TeV from 2010 and 0.1 nb- 1 of data at s=8 TeV from 2012. A number of aspects of the calorimeter response to isolated hadrons are explored. After accounting for energy deposited by neutral particles, there is a 5% discrepancy in the modelling, using various sets of Geant4 hadronic physics models, of the calorimeter response to isolated charged hadrons in the central calorimeter region. The description of the response to anti-protons at low momenta is found to be improved with respect to previous analyses. The electromagnetic and hadronic calorimeters are also examined separately, and the detector simulation is found to describe the response in the hadronic calorimeter well. The jet energy scale uncertainty and correlations in scale between jets of different momenta and pseudorapidity are derived based on these studies. The uncertainty is 2–5% for jets with transverse momenta above 2 TeV, where this method provides the jet energy scale uncertainty for ATLAS.

© 2016, CERN for the benefit of the.The luminosity determination for the ATLAS detector at the LHC during pp collisions at s= 8 TeV in 2012 is presented. The evaluation of the luminosity scale is performed using several luminometers, and comparisons between these luminosity detectors are made to assess the accuracy, consistency and long-term stability of the results. A luminosity uncertainty of δL/ L= ± 1.9 % is obtained for the 22.7fb-1 of pp collision data delivered to ATLAS at s= 8 TeV in 2012.

© 2016, CERN for the benefit of the ATLAS collaboration.A study of the decays Bs0→μ+μ- and B0→ μ+μ- has been performed using data corresponding to an integrated luminosity of 25 fb- 1 of 7 and 8 TeV proton–proton collisions collected with the ATLAS detector during the LHC Run 1. For the B0 dimuon decay, an upper limit on the branching fraction is set at B < 4.2 × 10 - 10 at 95 % confidence level. For Bs0, the branching fraction B=×10-9 is measured. The results are consistent with the Standard Model expectation with a p value of 4.8 %, corresponding to 2.0 standard deviations.

© 2016, CERN for the benefit of the ATLAS collaboration.The algorithms used by the ATLAS Collaboration to reconstruct and identify prompt photons are described. Measurements of the photon identification efficiencies are reported, using 4.9 fb- 1 of pp collision data collected at the LHC at s=7 TeV and 20.3 fb- 1 at s=8 TeV. The efficiencies are measured separately for converted and unconverted photons, in four different pseudorapidity regions, for transverse momenta between 10 GeV and 1.5 TeV. The results from the combination of three data-driven techniques are compared to the predictions from a simulation of the detector response, after correcting the electromagnetic shower momenta in the simulation for the average differences observed with respect to data. Data-to-simulation efficiency ratios used as correction factors in physics measurements are determined to account for the small residual efficiency differences. These factors are measured with uncertainties between 0.5% and 10% in 7 TeV data and between 0.5% and 5.6% in 8 TeV data, depending on the photon transverse momentum and pseudorapidity.

© 2016, CERN for the benefit of the ATLAS collaboration.The dijet production cross section for jets containing a b-hadron has been measured in proton–proton collisions with a centre-of-mass energy of s=7 TeV, using the ATLAS detector at the LHC. The data used correspond to an integrated luminosity of 4.2fb-1. The cross section is measured for events with two identified b-jets with a transverse momentum pT> 20 GeV and a minimum separation in the η–ϕ plane of Δ R= 0.4. At least one of the jets in the event is required to have pT> 270 GeV. The cross section is measured differentially as a function of dijet invariant mass, dijet transverse momentum, boost of the dijet system, and the rapidity difference, azimuthal angle and angular distance between the b-jets. The results are compared to different predictions of leading order and next-to-leading order perturbative quantum chromodynamics matrix elements supplemented with models for parton-showers and hadronization.