Understanding the initial state effects by the measurement of the Drell-Yan process in pPb collisions with CMS

The Drell-Yan process is considered as an essential probe to understand initial-state protons, usually described by the parton distribution functions (PDFs) for stand-alone protons and by the nuclear PDFs (nPDFs) for protons confined in the nucleus. In the LHC era, Z and W boson production in pPb and PbPb collisions have been used to investigate the initial-state effects. In this presentation, we report results for the Drell-Yan process in pPb collisions at a center-of-mass energy of 8.16 TeV with the CMS detector. Differential cross sections are presented as functions of dimuon $p_\mathrm{T}$, rapidity and $\phi^{*}$ in a wider dimuon mass region which includes not only the Z boson mass range but also the lower mass region down to 15 GeV. In addition, the forward-backward asymmetries are shown in both mass regions, and the uncertainties are found to be smaller than in the model calculations. The results in the Z mass region are the most precise to date, while the measurements in the lower mass region allow access to a new phase space of nPDF studies at a lower longitudinal momentum fraction $x$ and lower energy scale $Q^{2}$. The results are compared to the EPPS16 and nCTEQ15WZ nPDFs, and the free-proton PDF CT14. An improved understanding of the nuclear PDF and the modeling of pPb collisions is achieved.