Measurement of the nuclear modification factor and prompt charged particle production in $p\mathrm{Pb}$ and $pp$ collisions at $\sqrt{s_{\mathrm{NN}}}=5\mathrm{TeV}$

Abstract

The production of prompt charged particles in proton-lead collisions and in proton-proton collisions at the nucleon-nucleon centre-of-mass energy $\sqrt{s_{\mathrm{NN}}}=5\mathrm{TeV}$ is studied at LHCb as a function of pseudorapidity ($\hat{I}\cdot$) and transverse momentum ($p_{\mathrm{T}}$) with respect to the proton beam direction. The nuclear modification factor for charged particles is determined as a function of $\hat{I}\cdot$ between $-4.8<\hat{I}\cdot <-2.5$ (backward region) and $2.0<\hat{I}\cdot <4.8$ (forward region), and $p_{\mathrm{T}}$ between $0.2<p_{\mathrm{T}}<8.0\mathrm{GeV}/c$. 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 $p_{\mathrm{T}}$ larger than $1.5\mathrm{GeV}/c$. This measurement constrains nuclear PDFs and saturation models at previously unexplored values of the parton momentum fraction down to $10^{-6}$.

All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2021-015.html (LHCb public pages)