Measurement of W(+/-)Z production in proton-proton collisions at root s=7 TeV with the ATLAS detector

A study of W(+/-)Z production in proton-proton collisions at root s = 7 TeV is presented using data corresponding to an integrated luminosity of 4.6 fb(-1) collected with the ATLAS detector at the Large Hadron Collider in 2011. In total, 317 candidates, with a background expectation of 68 +/- 10 events, are observed in double-leptonic decay final states with electrons, muons and missing transverse momentum. The total cross-section is determined to be sigma(tot)(WZ) = 19.0(-1.3)(+1.4)(stat.) +/- 0.9(syst.) +/- 0.4(lumi.) pb, consistent with the Standard Model expectation of 17.6(-1.0)(+1.1) pb. Limits on anomalous triple gauge boson couplings are derived using the transverse momentum spectrum of Z bosons in the selected events. The cross-section is also presented as a function of Z boson transverse momentum and diboson invariant mass

Measurement of the relative width difference of the B0B^0-Bˉ0\bar B^0 system with the ATLAS detector

20 pages plus author list + cover page (38 pages total), 5 figures, 2 tables, submitted to Journal of High Energy Physics, All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/BPHY-2015-01/ - See paper for full list of authorsInternational audienceThis paper presents the measurement of the relative width difference $\Delta \Gamma_d / \Gamma_d$ of the $B^0$-$\bar B^0$ system using the data collected by the ATLAS experiment at the LHC in $p p$ collisions at $\sqrt{s} = 7$ TeV and $\sqrt{s} = 8$ TeV and corresponding to an integrated luminosity of 25.2 fb$^{-1}$. The value of $\Delta \Gamma_d / \Gamma_d$ is obtained by comparing the decay-time distributions of $B^0 \to J/\psi K_S$ and $B^0 \to J/\psi K^{*0}(892)$ decays. The result is \Delta \Gamma_d / \Gamma_d = (-0.1 \pm 1.1~\mbox{(stat.)} \pm 0.9~\mbox{(syst.)}) \times 10^{-2}. Currently, this is the most precise single measurement of $\Delta \Gamma_d / \Gamma_d$. It agrees with the Standard Model prediction and the measurements by other experiments

Measurement of the ttˉZt\bar{t}Z and ttˉWt\bar{t}W production cross sections in multilepton final states using 3.2 fb−1^{-1} of pppp collisions at s\sqrt{s} =13 TeV with the ATLAS detector

A measurement of the $t\bar{t}Z$ and $t\bar{t}W$ production cross sections in final states with either two same-charge muons, or three or four leptons (electrons or muons) is presented. The analysis uses a data sample of proton-proton collisions at $\sqrt{s} = 13$ 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 $\sigma_{t\bar{t}Z} = 0.9 \pm 0.3$ pb and $\sigma_{t\bar{t}W} = 1.5 \pm 0.8$ pb, in agreement with the Standard Model predictions

Measurement of tau polarization in W -> tau nu decays with the ATLAS detector in pp collisions at root s=7 TeV

In this paper, a measurement of tau polarization in W -> tau nu decays is presented. It is measured from the energies of the decay products in hadronic tau decays with a single final state charged particle. The data, corresponding to an integrated luminosity of 24 pb(-1), were collected by the ATLAS experiment at the Large Hadron Collider in 2010. The measured value of the tau polarization is P-tau = -1.06 +/- 0.04 (stat)(-0.07)(+0.05) (syst), in agreement with the Standard Model prediction, and is consistent with a physically allowed 95 % CL interval [-1,-0.91]. Measurements of tau polarization have not previously been made at hadron colliders. RI Fazio, Salvatore /G-5156-2010; Fabbri, Laura/H-3442-2012; Villa, Mauro/C-9883-2009; Jones, Roger/H-5578-2011; valente, paolo/A-6640-2010; Takai, Helio/C-3301-2012; Delmastro, Marco/I-5599-2012; Livan, Michele/D-7531-201

Search for heavy neutrinos and right-handed W bosons in events with two leptons and jets in pp collisions at root s=7 TeV with the ATLAS detector

This letter reports on a search for hypothetical heavy neutrinos, N, and right-handed gauge bosons, W-R, in events with high transverse momentum objects which include two reconstructed leptons and at least one hadronic jet. The results were obtained from data corresponding to an integrated luminosity of 2.1 fb(-1) collected in proton-proton collisions at root s = 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. No excess above the Standard Model background expectation is observed. Excluded mass regions for Majorana and Dirac neutrinos are presented using two approaches for interactions that violate lepton and lepton-flavor numbers. One approach uses an effective operator framework, the other approach is guided by the Left-Right Symmetric Model. The results described in this letter represent the most stringent limits to date on the masses of heavy neutrinos and W-R bosons obtained in direct searches