2,485 research outputs found
Search for physics beyond the standard model with top quarks at atlas
No full textSeveral extensions of the Standard Model predict the presence of new particles that couple to the top quark. With the dataset of proton-proton collisions collected by the ATLAS detector at the Large Hadron Collider we present searches for fourth generation quarks that decay in top quarks or a similar topology, as well as searches for resonances decaying to top-quark pairs or top-quark partners producing an excess of missing transverse momentum
Search for Pair Production of Second Generation Scalar Leptoquarks in ppbar Collisions at the Tevatron
No full textAs predicted by numerous extensions of the Standard Model, leptoquarks (LQ) are hypothetical bosons allowing lepton-quark transitions. Under the assumption that they couple only to quarks and leptons of the same generation, three generations of leptoquarks can be distinguished. The search for the pair production of second generation scalar leptoquarks has been carried out in p{bar p} collisions at {radical}s = 1.96TeV, using an integrated luminosity of 1 fb{sup -1} collected by the D0 experiment at the Tevatron collider between August 2002 and February 2006. Topologies arising from the LQ{ovr LQ} {yields} {mu}q{nu}q and LQ{ovr LQ} {yields} {mu}q{mu}q decay modes have been investigated. In order to maximize the available statistics, a method for the combination of various prescaled triggers with an inclusive OR has been developed. Since no excess of data over the Standard Model prediction has been observed, upper limits on the leptoquark pair production cross section have been derived at 95% confidence level as function of the leptoquark mass and the branching fraction {beta} = Br(LQ {yields} {mu}q), and are interpreted as lower limits on the leptoquark mass as function of {beta}. For {beta} = 1, {beta} = 1/2 and {beta} = 0.1, the combination of the two channels excludes scalar second generation leptoquarks with masses up to 309GeV, 262GeV, and 174GeV, respectively. The lower bounds on the scalar second generation leptoquark mass obtained for {beta} {ge} 0.1 are the best exclusion limits to date
W, Z, and QCD
No full textThe analyses of W and Z vector bosons, photon, and jets production have been carried out by the ATLAS and CMS collaborations using datasets collected during LHC Run 2, comprising from 36.3 fb−1 to 140 fb−1. Unfolded differential production cross sections as well as event shape observables have been measured, allowing to achieve tests of perturbative quantum chromodynamics, extractions of fundamental parameters of the Standard Model, and constraints on parton density functions
Dark Matter searches at the LHC
No full textMultiple cosmological observations indicate the existence of Dark Matter, which may be a weakly interacting massive particle (WIMP). In this case, Dark Matter could be produced in proton-proton collisions at the LHC, but would escape the detector without interacting. Final states consisting in pair-produced Dark Matter candidates would however be balanced by radiated particles from colliding partons. ATLAS and CMS experiments can therefore search for Dark Matter signal in events involving large amount of missing transverse energy in the detector. Analyses have been carried out in the context of the mono-jet, mono-photon, mono-W and mono-Z signatures, including both hadronic and leptonic W and Z decays. No evidence of physics beyond the Standard Model expectation has been observed, and the pair production of Dark Matter particles has been interpreted in the context of an effective field theory and simplified models. Limits on the suppression scale of the effective theory have been translated into bounds on the WIMP-nucleon scattering and WIMP annihilation cross sections. Results were derived from datasets of collisions at a center-of-mass energy of 7 and 8TeV, with an integrated luminosity of up to 20/fb
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Search for scalar top and bottom quarks at the Tevatron
No full textThis document reviews recent results on the search for scalar top and scalar bottom quarks in p{bar p} collisions at {radical}s = 1.96 TeV. The analyses presented are based on data samples with integrated luminosities from 1.0 to 1.9 fb{sup -1} recorded at the Tevatron with the D0 and CDF detectors
Usage of Message Queueing Technologies in the ATLAS Distributed Data Management System
No full textThe ATLAS Distributed Data Management system is composed of semi-autonomous, heterogeneous, and independently designed subsystems. To achieve successful operation of such a system, the activities of the agents controlling the subsystems have to be coordinated. In addition, external applications can require to synchronize on events relative to data availability. A common way to proceed is to implement polling strategies within the distributed components, which leads to an increase of the load in the overall system. We describe an alternative based on notifications using standard message queuing. The application of this technology in the distributed system has been exercised
Author Correction: A detailed map of Higgs boson interactions by the ATLAS experiment ten years after the discovery
No full textIn the version of this article initially published, the ATLAS Collaboration
author names, affiliations and acknowledgements were omitted and
have now been included in the HTML and PDF versions of the article
Measurement of differential production cross-sections and forward-backward ratio in p+Pb collisions with the ATLAS detector
No full textSee paper for full list of authors - 13 pages plus author list + cover pages (26 pages total), 8 figures, 8 tables, submitted to Phys. Rev. C. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2013-07/International audienceMeasurements of differential cross-sections for production in p+Pb collisions at = 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. The mesons are reconstructed in the dimuon decay channel over the transverse momentum range GeV and over the center-of-mass rapidity range . Prompt are separated from resulting from -hadron decays through an analysis of the distance between the decay vertex and the event primary vertex. The differential cross-section for production of nonprompt 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 -quark production in p+Pb collisions
Strong constraints on jet quenching in centrality-dependent +Pb collisions at 5.02 TeV from ATLAS
No full textJet quenching is the process of color-charged partons losing energy via interactions with quark-gluon plasma droplets created in heavy-ion collisions. The collective expansion of such droplets is well described by viscous hydrodynamics. Similar evidence of collectivity is consistently observed in smaller collision systems, including and +Pb collisions. In contrast, while jet quenching is observed in Pb+Pb collisions, no evidence has been found in these small systems to date, raising fundamental questions about the nature of the system created in these collisions. The ATLAS experiment at the Large Hadron Collider has measured the yield of charged hadrons correlated with reconstructed jets in 0.36 nb of +Pb and 3.6 pb of collisions at 5.02 TeV. The yields of charged hadrons with GeV near and opposite in azimuth to jets with or 60 GeV, and the ratios of these yields between +Pb and collisions, , are reported. The collision centrality of +Pb events is categorized by the energy deposited by forward neutrons from the struck nucleus. The values are consistent with unity within a few percent for hadrons with GeV at all centralities. These data provide new, strong constraints which preclude almost any parton energy loss in central +Pb collisions
Transverse momentum, rapidity, and centrality dependence of inclusive charged-particle production in root s(NN)=5.02 TeV p+Pb collisions measured by the ATLAS experiment
No full textMeasurements of the per-event charged-particle yield as a function of the charged-particle transverse momentum and rapidity are performed using p + Pbcollision data collected by the ATLAS experiment at the LHC at a centre-of-mass energy of root sNN= 5.02 TeV. Charged particles are reconstructed over pseudorapidity |eta| < 2.3and transverse momentum between 0.1GeVand 22GeVin a dataset corresponding to an integrated luminosity of 1 mu b(-1). The results are presented in the form of chargedparticle nuclear modification factors, where the p + Pbcharged-particle multiplicities are compared between central and peripheral p + Pbcollisions as well as to charged-particle cross sections measured in ppcollisions. The p + Pbcollision centrality is characterized by the total transverse energy measured in -4.9<eta<-3.1, which is in the direction of the outgoing lead beam. Three different estimations of the number of nucleons participating in the p + Pbcollision are carried out using the Glauber model and two Glauber-Gribov colour-fluctuation extensions to the Glauber model. The values of the nuclear modification factors are found to vary significantly as a function of rapidity and transverse momentum. Abroad peak is observed for all centralities and rapidities in the nuclear modification factors for chargedparticle transverse momentum values around 3GeV. The magnitude of the peak increases for more central collisions as well as rapidity ranges closer to the direction of the outgoing lead nucleus. (C) 2016 The Author(s). Published by Elsevier B.V
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