K. Bendtz

Measurement of differential J/psi production cross sections and forward-backward ratios in p plus Pb collisions with the ATLAS detector

Measurements of differential cross-sections for J/ψ production in p+Pb collisions at √ sNN = 5.02 TeV at the LHC with the ATLAS detector are presented. The data set used corresponds to an integrated luminosity of 28.1 nb−1. The J/ψ mesons are reconstructed in the dimuon decay channel over the transverse momentum range 8 < pT < 30 GeV and over the center-of-mass rapidity range −2.87 < y∗ < 1.94. Prompt J/ψ are separated from J/ψ resulting from b-hadron decays through an analysis of the distance between the J/ψ decay vertex and the event primary vertex. The differential cross-section for production of nonprompt J/ψ is compared to a FONLL calculation that does not include nuclear effects. Forward-backward production ratios are presented and compared to theoretical predictions. These results constrain the kinematic dependence of nuclear modifications of charmonium and b-quark production in p+Pb collisions. c

Search for magnetic monopoles with the MoEDAL prototype trapping detector in 8 TeV proton-proton collisions at the LHC

A bstractThe MoEDAL experiment is designed to search for magnetic monopoles and other highly-ionising particles produced in high-energy collisions at the LHC. The largely passive MoEDAL detector, deployed at Interaction Point 8 on the LHC ring, relies on two dedicated direct detection techniques. The first technique is based on stacks of nucleartrack detectors with surface area ~18m2, sensitive to particle ionisation exceeding a high threshold. These detectors are analysed offline by optical scanning microscopes. The second technique is based on the trapping of charged particles in an array of roughly 800 kg of aluminium samples. These samples are monitored offline for the presence of trapped magnetic charge at a remote superconducting magnetometer facility. We present here the results of a search for magnetic monopoles using a 160 kg prototype MoEDAL trapping detector exposed to 8TeV proton-proton collisions at the LHC, for an integrated luminosity of 0.75 fb–1. No magnetic charge exceeding 0:5gD (where gD is the Dirac magnetic charge) is measured in any of the exposed samples, allowing limits to be placed on monopole production in the mass range 100 GeV≤ m ≤ 3500 GeV. Model-independent cross-section limits are presented in fiducial regions of monopole energy and direction for 1gD ≤ |g| ≤ 6gD, and model-dependent cross-section limits are obtained for Drell-Yan pair production of spin-1/2 and spin-0 monopoles for 1gD ≤ |g| ≤ 4gD. Under the assumption of Drell-Yan cross sections, mass limits are derived for |g| = 2gD and |g| = 3gD for the first time at the LHC, surpassing the results from previous collider experiments.

Z boson production in p plus Pb collisions at root S-NN=5.02 TeV measured with the ATLAS detector

The ATLAS Collaboration has measured the inclusive production of Z bosons via their decays into electron and muon pairs in p+Pb collisions at sNN = 5.02 TeV at the Large Hadron Collider. The measurements are made using data corresponding to integrated luminosities of 29.4 nb−1 and 28.1 nb−1 for Z → ee and Z → μμ, respectively. The results from the two channels are consistent and combined to obtain a cross section times the Z → `` branching ratio, integrated over the rapidity region |yZ | < 3.5, of 139.8 ± 4.8 (stat.) ± 6.2 (syst.) ± 3.8 (lumi.) nb. Differential cross sections are presented as functions of the Z boson rapidity and transverse momentum, and compared with models based on parton distributions both with and without nuclear corrections. The centrality dependence of Z boson production in p+Pb collisions is measured and analyzed within the framework of a standard Glauber model and the model’s extension for fluctuations of the underlying nucleon-nucleon scattering cross section. c

Measurement of jet p(T) correlations in Pb+Pb and pp collisions at root s(NN)=2.76 TeV with the ATLAS detector

Measurements of dijet pT correlations in Pb+Pb and pp collisions at a nucleon–nucleon centre-of-mass energy of √ sNN = 2.76 TeV are presented. The measurements are performed with the ATLAS detector at the Large Hadron Collider using Pb+Pb and pp data samples corresponding to integrated luminosities of 0.14 nb−1 and 4.0 pb−1, respectively. Jets are reconstructed using the anti-kt algorithm with radius parameter values R = 0.3 and R = 0.4. A background subtraction procedure is applied to correct the jets for the large underlying event present in Pb+Pb collisions. The leading and sub-leading jet transverse momenta are denoted pT1 and pT2 . An unfolding procedure is applied to the two-dimensional (pT1 , pT2) distributions to account for experimental effects in the measurement of both jets. Distributions of (1/N)dN/dxJ, where xJ = pT2/pT1 , are presented as a function of pT1 and collision centrality. The distributions are found to be similar in peripheral Pb+Pb collisions and pp collisions, but highly modified in central Pb+Pb collisions. Similar features are present in both the R = 0.3 and R = 0.4 results, indicating that the effects of the underlying event are properly accounted for in the measurement. The results are qualitatively consistent with expectations from partonic energy loss models.

Measurement of the t(t)over-bar production cross section in the tau plus jets final state in pp collisions at root s=8 TeV using the ATLAS detector

A measurement of the inclusive pp→ tt̄+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 σtt̄ = 239± 29 pb, which is in agreement with the measurements in other final states and the theoretical predictions at this center-of-mass energy.

Search for top quark decays t -g qH,with H -g gamma gamma, in root s=13 TeV pp collisions using the ATLAS detector

A bstractThis article presents a search for flavour-changing neutral currents in the decay of a top quark into an up-type (q = c, u) quark and a Higgs boson, where the Higgs boson decays into two photons. The proton-proton collision data set analysed amounts to 36.1 fb−1 at s=13

Simultaneous measurements of the t(t)over-bar, W+W-, and Z/gamma* -> tau tau production cross-sections in pp collisions at root s=7 TeV with the ATLAS detector

\sqrt{s}=13

Search for direct pair production of a chargino and a neutralino decaying to the 125 GeV Higgs boson in root s=8 TeV pp collisions with the ATLAS detector

TeV collected by the ATLAS experiment at the LHC. Top quark pair events are searched for, where one top quark decays into qH and the other decays into bW . Both the hadronic and leptonic decay modes of the W boson are used. No significant excess is observed and an upper limit is set on the t → cH branching ratio of 2.2 × 10−3 at the 95% confidence level, while the expected limit in the absence of signal is 1.6 × 10−3. The corresponding limit on the tcH coupling is 0.090 at the 95% confidence level. The observed upper limit on the t → uH branching ratio is 2.4 × 10−3.