Measurement of the Nuclear Modification Factor and Prompt Charged Particle Production in p-Pb and pp Collisions at √sNN =5 TeV

The production of prompt charged particles in proton-lead collisions and in proton-proton collisions at the nucleon-nucleon center-of-mass energy √𝑠𝑁⁢𝑁=5  TeV is studied at LHCb as a function of pseudorapidity (𝜂) and transverse momentum (𝑝𝑇) with respect to the proton beam direction. The nuclear modification factor for charged particles is determined as a function of 𝜂 between −4.8<𝜂<−2.5 (backward region) and 2.0<𝜂<4.8 (forward region), and 𝑝𝑇 between 0.2<𝑝𝑇<8.0  GeV/⁢𝑐. The results show a suppression of charged particle production in proton-lead collisions relative to proton-proton collisions in the forward region and an enhancement in the backward region for 𝑝𝑇 larger than 1.5  GeV/⁢𝑐. This measurement constrains nuclear PDFs and saturation models at previously unexplored values of the parton momentum fraction down to 10−6.