Observation of nuclear modifications in W$^\pm$ boson production in pPb collisions at $\sqrt{s_\mathrm{NN}} =$ 8.16 TeV

The production of W± bosons is studied in proton-lead (pPb) collisions at a nucleon-nucleon centre-of-mass energy of sNN=8.16TeV . Measurements are performed in the W±→μ±νμ channel using a data sample corresponding to an integrated luminosity of 173.4±6.1nb−1 , collected by the CMS Collaboration at the LHC. The number of positively and negatively charged W bosons is determined separately in the muon pseudorapidity region in the laboratory frame |ηlabμ| 25GeV/c . The W± boson differential cross sections, muon charge asymmetry, and the ratios of W± boson yields for the proton-going over the Pb-going beam directions are reported as a function of the muon pseudorapidity in the nucleon-nucleon centre-of-mass frame. The measurements are compared to the predictions from theoretical calculations based on parton distribution functions (PDFs) at next-to-leading-order. The results favour PDF calculations that include nuclear modifications and provide constraints on the nuclear PDF global fits.