Abstract
Ultra-peripheral heavy ion collisions provide a unique opportunity to study the parton distributions in the colliding nuclei via the measurement of photo-nuclear jet production. An analysis of jet production in ultra-peripheral Pb+Pb collisions at sNN=5.02 TeV performed using data collected with the ATLAS detector in 2015 is described. The data set corresponds to a total Pb+Pb integrated luminosity of 0.38 nb−1. The ultra-peripheral collisions are selected using a combination of forward neutron and rapidity gap requirements. The cross-sections, not unfolded for detector response, are compared to results from Pythia Monte Carlo simulations re-weighted to match a photon spectrum obtained from the STARlight model. Qualitative agreement between data and these simulations is observed over a broad kinematic range suggesting that using these collisions to measure nuclear parton distributions is experimentally realisable.
Highlights
In the past few decades significant interest has developed on the subject of nuclear parton distribution functions (PDFs) and their modifications relative to the proton PDFs
The photon-emitting nucleus is expected to remain intact resulting in no neutrons along the beam direction, while multiple neutrons are expected in hadronic collisions due to the nuclear breakup. These lisions at aprpoecreneudcinlegosnprceesnetnetr-aofm-meaassusreemneerngtyooffph√ostNoN-nu=cl5e.a0r2jeTtepVr,odreuccotriodnedcrions2s-0s1e5cti[o8n].s in Pb+Pb colPhoto-nuclear events are identified by requirements on the number of neutrons and the presence of rapidity gaps
Systematic uncertainties on the cross-sections due to event selection efficiency and jet response are shown on the figures with shaded bands
Summary
In the past few decades significant interest has developed on the subject of nuclear parton distribution functions (PDFs) and their modifications relative to the proton PDFs. Angerami / Nuclear Physics A 967 (2017) 277–280 where the interplay between the direct and resolved contributions was exploited to study the partonic structure of both the proton and the photon [6, 7] This process is one of many types of photon-induced reactions that are referred to as “ultra-peripheral collisions” (UPCs) because they can occur when the impact parameters between the incoming nuclei are large such that there is no hadronic interaction between the nuclei. The photon-emitting nucleus is expected to remain intact resulting in no neutrons along the beam direction, while multiple neutrons are expected in hadronic collisions due to the nuclear breakup These lisions at aprpoecreneudcinlegosnprceesnetnetr-aofm-meaassusreemneerngtyooffph√ostNoN-nu=cl5e.a0r2jeTtepVr,odreuccotriodnedcrions2s-0s1e5cti[o8n].s in Pb+Pb colPhoto-nuclear events are identified by requirements on the number of neutrons and the presence of rapidity gaps. For direct processes xγ is unity, while for resolved events it is the fraction of the photon’s energy carried by the resolved parton entering the hard scattering
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