R. Lafaye

Measurement of W gamma and Z gamma production in proton-proton collisions at root s=7 TeV with the ATLAS detector

We present studies of W and Z bosons with associated high energy photons produced in pp collisions at √ s = 7 TeV. The analysis uses 35 pb−1 of data collected by the ATLAS experiment in 2010. The event selection requires W and Z bosons decaying into high pT leptons (electrons or muons) and a photon with ET > 15 GeV separated from the lepton(s) by a distance ∆R(l, γ) > 0.7 in η-φ space. A total of 95 (97) pp→ e±νγ+X (pp→ μ±νγ +X) and 25 (23) pp→ e+e−γ +X (pp→ μ+μ−γ +X) event candidates are selected. The kinematic distributions of the leptons and photons and the production cross sections are measured. The data are found to agree with Standard Model predictions that include next-to-leading-order O(ααs) contributions.

Measuring Higgs couplings from LHC data.

Following recent ATLAS and CMS publications we interpret the results of their Higgs searches in terms of standard model operators. For a Higgs boson mass of 125 GeV we determine several Higgs couplings from published 2011 data and extrapolate the results towards different scenarios of LHC running. Even though our analysis is limited by low statistics we already derive meaningful constraints on modified Higgs sectors.

Measuring the Higgs sector

If we find a light Higgs boson at the LHC, there should be many observable channels which we can exploit to measure the relevant parameters in the Higgs sector. We use the SFitter framework to map these measurements on the parameter space of a general weak-scale effective theory with a light Higgs state of mass 120 GeV. Our analysis benefits from the parameter determination tools and the error treatment used in new-physics searches, to study individual parameters and their error bars as well as parameter correlations.

Energy linearity and resolution of the ATLAS electromagnetic barrel calorimeter in an electron test-beam

A module of the ATLAS electromagnetic barrel liquid argon calorimeter was exposed to the CERN electron test-beam at the H8 beam line upgraded for precision momentum measurement. The available energies of the electron beam ranged from 10 to 245 GeV. The electron beam impinged at one point corresponding to a pseudo-rapidity of eta=0.687 and an azimuthal angle of phi=0.28 in the ATLAS coordinate system. A detailed study of several effects biasing the electron energy measurement allowed an energy reconstruction procedure to be developed that ensures a good linearity and a good resolution. Use is made of detailed Monte Carlo simulations based on Geant which describe the longitudinal and transverse shower profiles as well as the energy distributions. For electron energies between 15 GeV and 180 GeV the deviation of the measured incident electron energy over the beam energy is within 0.1%. The systematic uncertainty of the measurement is about 0.1% at low energies and negligible at high energies. The energy resolution is found to be about 10% sqrt(E) for the sampling term and about 0.2% for the local constant term.

Measuring Hidden Higgs and Strongly-Interacting Higgs Scenarios

Higgs couplings can be affected by physics beyond the Standard Model. We study modifications through interactions with a hidden sector and in specific composite Higgs models accessible at the LHC. Both scenarios give rise to congruent patterns of universal, or partially universal, shifts. In addition, Higgs decays to the hidden sector may lead to invisible decay modes which we also exploit. Experimental bounds on such potential modifications will measure the concordance of an observed Higgs boson with the Standard Model.

Precision measurement and interpretation of inclusive W+, W- and Z/gamma* production cross sections with the ATLAS detector

High-precision measurements by the ATLAS Collaboration are presented of inclusive W + → +ν, W − → −ν̄ and Z/γ ∗ → ( = e, μ) Drell–Yan production cross sections at the LHC. The data were collected in proton–proton collisions at √ s = 7 TeV with an integrated luminosity of 4.6 fb−1. Differential W + and W − cross sections are measured in a lepton pseudorapidity range |η | < 2.5. Differential Z/γ ∗ cross sections are measured as a function of the absolute dilepton rapidity, for |y | < 3.6, for three intervals of dilepton mass, m , extending from 46 to 150 GeV. The integrated and differential electronand muon-channel cross sections are combined and compared to theoretical predictions using recent sets of parton distribution functions. The data, together with the final inclusive e± p scattering cross-section data from H1 and ZEUS, are interpreted in a next-to-next-to-leading-order QCD analysis, and a new set of parton distribution functions, ATLAS-epWZ16, is obtained. The ratio of strange-to-light sea-quark densities in the proton is determined more accurately than in previous determinations based on collider data only, and is established to be close to unity in the sensitivity range of the data. A new measurement of the CKM matrix element |Vcs | is also provided.High-precision measurements by the ATLAS Collaboration are presented of inclusive

Measurement of the

W^+ \rightarrow \ell ^+\nu

cross section in

W+→ℓ+ν,