Electroweak–boson production processes (W, Z and photon) provide access to the earliest moments of heavy–ion collisions. Furthermore, because they do not undergo strong interactions, they are ...sensitive to the initial–state geometry of the collision and potentially the details of the nuclear parton distribution functions. ATLAS results on vector–boson yields have demonstrated binary collision scaling in Pb+Pb collisions. In p + Pb collisions, the measurement of vector bosons provides possible constraints on the nuclear parton distribution functions and insights into the details of the initial collision geometry. We report on the latest results of vector–boson production in p + Pb collisions at sNN=5.02 TeV. Production yields of Z and W bosons and lepton charge asymmetry of W bosons are presented as a function of pseudo-rapidity and centrality. The vector–boson yields are compared to calculations incorporating different parton distribution functions, as well as different centrality calculations.
The cross sections for deeply virtual Compton scattering in the reaction has been measured with the ZEUS detector at HERA using integrated luminosities of 95 pb-1 of e + p and 17 pb-1 of e-p ...collisions. Cross sections are presented as a function of the exchanged photon virtality, Q2, and the centre-of-mass energy, W, of the system in the region and . The obtained results are compared to QCD-based calculations.
This report provides an overview of the new results obtained by the ATLAS Collaboration at the LHC, which were presented at the Quark Matter 2018 conference. Furthermore these measurements were ...covered in 12 parallel talks, one flash talk and 11 posters. In this document, a discussion of results is grouped into four areas: electromagnetic interactions, jet quenching, quarkonia and heavy-flavour production, and collectivity in small and larger systems. Measurements from the xenon-xenon collisions based on a short run collected in October 2017 are reported for the first time.
This note represents an attempt to gather the input related to light-by-light scattering (\(\gamma\gamma\)) cross-section measurements at LHC with the aim of checking the consistency with different ...standard model predictions. For the first time, we also consider the contribution from the \(\eta_b(1S)\) meson production to the diphoton invariant mass distribution, by calculating its inclusive photoproduction cross-section. Using a simplified set of assumptions, we find a result of \(115\pm 19\,\,\text{nb}\), consistent with standard model predictions within two standard deviations. Although an improved determination of the integrated fiducial \(\textrm{PbPb}\,(\gamma\gamma)\to \textrm{Pb}^{(\ast)}\textrm{+}\textrm{Pb}^{(\ast)}\,\gamma\gamma\) cross-section by approximately 10\% could be potentially achieved relative to current measurements, further improvements are expected with the inclusion of existing or forthcoming LHC nuclear data.
Overview of ATLAS results Grabowska-Bold, Iwona
Nuclear and particle physics proceedings,
08/2017, Letnik:
289-290
Journal Article
Recenzirano
Odprti dostop
The heavy-ion programme in the ATLAS experiment at the Large Hadron Collider aims to probe and characterise hot and dense matter created in relativistic lead-lead collisions. Moreover, smaller ...collision systems involving nuclei and hadrons are of interest to disentangle initial- from final-state effects. This report presents new results based on lead-lead and proton-proton data collected at sNN=5.02 TeV in 2015, including measurements of bulk collectivity, charged-particle production, electroweak bosons, photon-jet correlations, and quarkonium suppression. First attempts to measure electromagnetic processes in ultra-peripheral collisions are also discussed.
Photons and weak bosons do not interact strongly and, thus, their production yields provide direct tests of scaling with a number of binary nucleon–nucleon collisions in the heavy-ion environment. In ...addition, they should be sensitive to the nuclear modification of parton distribution functions (nPDF). Proton-lead collisions also provide an excellent opportunity to test nPDF in a less dense environment than lead–lead via looking at forward–backward production of weak bosons. The ATLAS detector has proven to be an excellent apparatus in measurements involving photons, electrons and muons, the latter being products of weak boson decays, in the high occupancy environment produced in heavy-ion collisions. The experiment has recorded 30nb−1 of proton–lead data and 140μb−1 of lead–lead data, both of which have similar integrated partonic luminosities. We present the prompt photon, Z and W boson yields as a function of centrality, and also differentially in transverse momentum and rapidity, in lead–lead and proton–lead collisions from the ATLAS experiment. For W± bosons, a lepton charge asymmetry has also been studied, which may also shed light on nPDF.
Direct photons are a powerful tool to study heavy ion collisions. Their production rates provide access to the parton distribution functions, which are expected to be modified by nuclear effects. ...They also provide a mean to calibrate the energy of jets that are produced in the medium, and thus are a tool to probe the physics of jet quenching more precisely both through jet rates and fragmentation properties. The ATLAS detector measures photons with its hermetic, longitudinally segmented calorimeter, which gives excellent spatial and energy resolution, and detailed information about the shower shape of each measured photon. This gives powerful rejection against backgrounds from neutral pions produced in jets. Rejection against jet fragmentation products is further enhanced by isolation criteria, which can be based on energy flow in the calorimeter. First results on the rates of isolated direct photons from approximately 0.13nb−1 of lead–lead data will be shown, as a function of transverse momentum, pseudorapidity and centrality.
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