W and Z Boson Production Cross Section Measurements in ATLAS

The electroweak boson inclusive production cross sections and their ratios are described in the electron and muon decay channels. The observation of W and Z bosons in the tau decay channel is also presented. The data from pp collisions at 7 TeV were collected during 2010 using the ATLAS detector at the Large Hadron Collider and the results are compared to predictions at NNLO in perturbative QCD.Comment: 4 pages, 2 figures, Proceedings for the XIX International Workshop on Deep-Inelastic Scattering and Related Subjects, Newport News, VA USA, April 201

Extrapolation of Multiplicity distribution in p+p(\bar(p)) collisions to LHC energies

The multiplicity (N_ch) and pseudorapidity distribution (dN_ch/d\eta) of primary charged particles in p+p collisions at Large Hadron Collider (LHC) energies of \sqrt(s) = 10 and 14 TeV are obtained from extrapolation of existing measurements at lower \sqrt(s). These distributions are then compared to calculations from PYTHIA and PHOJET models. The existing \sqrt(s) measurements are unable to distinguish between a logarithmic and power law dependence of the average charged particle multiplicity () on \sqrt(s), and their extrapolation to energies accessible at LHC give very different values. Assuming a reasonably good description of inclusive charged particle multiplicity distributions by Negative Binomial Distributions (NBD) at lower \sqrt(s) to hold for LHC energies, we observe that the logarithmic \sqrt(s) dependence of are favored by the models at midrapidity. The dN_ch/d\eta versus \eta for the existing measurements are found to be reasonably well described by a function with three parameters which accounts for the basic features of the distribution, height at midrapidity, central rapidity plateau and the higher rapidity fall-off. Extrapolation of these parameters as a function of \sqrt(s) is used to predict the pseudorapidity distributions of charged particles at LHC energies. dN_ch/d\eta calculations from PYTHIA and PHOJET models are found to be lower compared to those obtained from the extrapolated dN_ch/d\eta versus \eta distributions for a broad \eta range.Comment: 11 pages and 13 figures. Substantially revised and accepted for publication in Journal of Physics

Top quark production at the LHC

Twenty years past its discovery, the top quark continues attracting great interest as experiments keep unveiling its properties. An overview of the latest measurements in the domain of top quark production, performed by the ATLAS and CMS experiments at the CERN LHC, is given. The latest measurements of top quark production rates via strong and electroweak processes are reported and compared to different perturbative QCD predictions. Fundamental properties, such as the mass or the couplings of the top quark, as well as re-interpretations seeking for beyond the standard model contributions in the top quark sector, are extracted from these measurements. In each case an attempt to highlight the first results and main prospects for the on-going Run 2 of the LHC is made.Comment: Contribution to the proceedings of the 50th Rencontres de Moriond Electroweak session - 201

Measurement of the production of a W boson in association with a charm quark in pp collisions at √s = 7 TeV with the ATLAS detector

The production of a W boson in association with a single charm quark is studied using 4.6 fb−1 of pp collision data at s√ = 7 TeV collected with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea-quark distributions is determined to be 0.96+0.26−0.30 at Q 2 = 1.9 GeV2, which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally, the cross-section ratio σ(W + +c¯¯)/σ(W − + c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the s−s¯¯¯ quark asymmetry

Probing Light Stops with Stoponium

We derive new limits on light stops from diboson resonance searches in the $\gamma\gamma$, $Z \gamma$, $ZZ$, $WW$ and $hh$ channels from the first run of the LHC. If the two-body decays of the light stop are mildly suppressed or kinematically forbidden, stoponium bound states will form in $pp$ collisions and subsequently decay via the pair annihilation of the constituent stops to diboson final states, yielding striking resonance signatures. Remarkably, we find that stoponium searches are highly complementary to direct collider searches and indirect probes of light stops such as Higgs coupling measurements. Using an empirical quarkonia potential model and including the first two $S$-wave stoponium states, we find that in the decoupling limit $m_{\widetilde t_1} \lesssim 130$ GeV is excluded for any value of the stop mixing angle and heavy stop mass by the combination of the latest resonance searches and the indirect constraints. The $\gamma \gamma$ searches are the most complementary to the indirect constraints, probing the stop "blind spot" parameter region in which the $h^0 \tilde t_1 \tilde t_1^*$ trilinear coupling is small. Interestingly, we also find that the $Z\gamma$ searches give a stronger constraint, $m_{\widetilde t_1} \lesssim 170$ GeV, if the stop is primarily left-handed. For a scenario with a bino LSP and stop NLSP, several gaps in the direct collider searches for stops can unambiguously be filled with the next run of the LHC. For a stop LSP decaying through an R-parity violating $UDD$ coupling, the stoponium searches can fill the gap 100 GeV $\lesssim m_{\tilde t_1} \lesssim 200$ GeV in the direct searches for couplings $\lambda" \lesssim 10^{-2}$.Comment: 35 pages, 33 figures. v2: references adde

Measurement of the WZ Production Cross Section in Proton-Proton Collisions at CME of 7 TeV with the ATLAS Detector

This document presents a measurement of WZ production in 1.02 fb-1 of pp collision data at CME of 7 TeV collected by the ATLAS experiment at LHC in 2011. The production cross-section is measured to be consistent with the Standard Model (SM) prediction. Limits on anomalous triple gauge boson couplings have been derived.Comment: Proceeding for DPF 2011 meetin

First Results from the TOTEM Experiment

The first physics results from the TOTEM experiment are here reported, concerning the measurements of the total, differential elastic, elastic and inelastic pp cross-section at the LHC energy of $\sqrt{s}$ = 7 TeV, obtained using the luminosity measurement from CMS. A preliminary measurement of the forward charged particle $\eta$ distribution is also shown.Comment: Conference Proceeding. MPI@LHC 2010: 2nd International Workshop on Multiple Partonic Interactions at the LHC. Glasgow (UK), 29th of November to the 3rd of December 201