Probing gluon pdf at x->0 with ultraperipheral PbPb collisions at 5.02 TeV in CMS

<!--HTML--><p><span style="background-color:rgb(255,255,255);color:rgb(75,75,75);"><span style="caret-color:rgb(0, 0, 0);font-family:Verdana;font-size:11pt;font-variant-alternates:normal;font-variant-east-asian:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-position:normal;vertical-align:baseline;white-space:pre-wrap;">We will describe a new measurement of coherent J/Psi photoproduction in LHC collisions with PbPb collisions at 5.02 TeV collected by CMS. In UPCs, the coherent heavy-flavor vector meson production via photon-nuclear interactions is of particular interest, as its cross section can directly probe the nuclear gluon density function. Since&nbsp; the two nuclei in symmetric UPCs can serve both as a photon-emitter projectile and a target, one has to disentangle contributions involving high- and low-energy photon-nucleus interactions to probe extremely small-x regimes. In this study, a new forward neutron tagging technique is utilized to solve this two-way ambiguity. We first review the technique of controlling the impact parameter of UPCs via forward neutron tagging established in the measurement of photon-photon QED processes. Then, we present the coherent J/psi photoproduction cross section as a function of photon-Pb center-of-mass energy in UPCs up to about a record energy of 400 GeV, corresponding to x ~ 5x10^{-5}. We discuss the physics implication of this new result, as well as opportunities in future LHC heavy ion runs.</span></span></p><p>&nbsp;</p>