Person:

Ippolito, Victoria

Measurements of the total and differential cross sections of Higgs boson production are performed using 20.3 fb−1 of pp collisions produced by the Large Hadron Collider at a center-of-mass energy of √s=8 TeV and recorded by the ATLAS detector. Cross sections are obtained from measured H→γγ and H→ZZ∗→4ℓ event yields, which are combined accounting for detector efficiencies, fiducial acceptances, and branching fractions. Differential cross sections are reported as a function of Higgs boson transverse momentum, Higgs boson rapidity, number of jets in the event, and transverse momentum of the leading jet. The total production cross section is determined to be σpp→H= 33.0 ± 5.3(stat)±1.6 (syst) pb. The measurements are compared to state-of-the-art predictions.

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 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′ 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 states, respectively. The results are also interpreted as limits on the threshold mass for quantum black hole production.

A search is presented for pair production of a new heavy quark (Q) that decays into a W boson and a light quark (q) in the final state where one W boson decays leptonically (to an electron or muon plus a neutrino) and the other W boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb−1 of pp collisions at √s=8 TeV collected by the ATLAS detector at the LHC. No evidence of Q¯Q production is observed. New chiral quarks with masses below 690 GeV are excluded at 95% confidence level, assuming BR(Q→Wq)=1. Results are also interpreted in the context of vectorlike quark models, resulting in the limits on the mass of a vectorlike quark in the two-dimensional plane of BR (Q→Wq) versus BR (Q→Hq).

The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from pp collisions at ��√=8 TeV. The dataset corresponds to an integrated luminosity of 20.3 fb−1 collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the ����⎯⎯�� and ����⎯⎯�� processes simultaneously yields a significance of 5.0σ (4.2σ) over the background-only hypothesis for ����⎯⎯�� (����⎯⎯��) production. The measured cross sections are ������⎯⎯��=369+100−91 fb and ������⎯⎯��=176+58−52 fb. The background-only hypothesis with neither ����⎯⎯�� nor ����⎯⎯�� production is excluded at 7.1σ. All measurements are consistent with next-to-leading-order calculations for the ����⎯⎯�� and ����⎯⎯�� processes.

A search for supersymmetry in events with large missing transverse momentum, jets, and at least one hadronically decaying tau lepton has been performed using 3.2 fb−1 of proton–proton collision data at ��√=13 TeV recorded by the ATLAS detector at the Large Hadron Collider in 2015. Two exclusive final states are considered, with either exactly one or at least two tau leptons. No excess over the Standard Model prediction is observed in the data. Results are interpreted in the context of gauge-mediated supersymmetry breaking and a simplified model of gluino pair production with tau-rich cascade decays, substantially improving on previous limits. In the GMSB model considered, supersymmetry-breaking scale (Λ) values below 92 TeV are excluded at the 95% confidence level, corresponding to gluino masses below 2000 GeV. For large values of tan��, values of Λ up to 107 TeV and gluino masses up to 2300 GeV are excluded. In the simplified model, gluino masses are excluded up to 1570 GeV for neutralino masses around 100 GeV. Neutralino masses below 700 GeV are excluded for all gluino masses between 800 and 1500 GeV, while the strongest exclusion of 750 GeV is achieved for gluino masses around 1450 GeV.

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 22.7 fb-1 of pp collision data delivered to ATLAS at s√= 8 TeV in 2012.

The normalized differential cross section for top-quark pair production in association with at least one jet is studied as a function of the inverse of the invariant mass of the ����⎯⎯+ 1-jet system. This distribution can be used for a precise determination of the top-quark mass since gluon radiation depends on the mass of the quarks. The experimental analysis is based on proton-proton collision data collected by the ATLAS detector at the LHC with a centre-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.6 fb−1. The selected events were identified using the lepton+jets top-quark-pair decay channel, where lepton refers to either an electron or a muon. The observed distribution is compared to a theoretical prediction at next-to-leading-order accuracy in quantum chromodynamics using the pole-mass scheme. With this method, the measured value of the top-quark pole mass, m pole t , is: ��pole��=173.7±1.5(stat.)±1.4(syst.)+1.0−0.5(theory)GeV. This result represents the most precise measurement of the top-quark pole mass to date.