Abstract
The first study of W boson production in pPb collisions is presented, for bosons decaying to a muon or electron, and a neutrino. The measurements are based on a data sample corresponding to an integrated luminosity of 34.6 nb−1 at a nucleon–nucleon centre-of-mass energy of sNN=5.02 TeV, collected by the CMS experiment. The W boson differential cross sections, lepton charge asymmetry, and forward–backward asymmetries are measured for leptons of transverse momentum exceeding 25 GeV/c, and as a function of the lepton pseudorapidity in the |ηlab|<2.4 range. Deviations from the expectations based on currently available parton distribution functions are observed, showing the need for including W boson data in nuclear parton distribution global fits.
Highlights
Electroweak boson production in proton–nucleus and nucleus– nucleus collisions at the CERN LHC offers a unique opportunity to probe nuclear parton distribution functions [1,2,3,4]
Precise measurements of W boson production in heavy ion collisions might lead to an improved determination of the nuclear parton distribution functions (nPDFs)
The differential cross sections as a function of the lepton pseudorapidity agree with theoretical predictions assuming both unmodified (CT10) and modified (CT10+EPS09) nPDFs, except in the most backward region (Pb ion beam direction), where a hint of an enhancement is seen for the W− bosons
Summary
Electroweak boson production in proton–nucleus and nucleus– nucleus collisions at the CERN LHC offers a unique opportunity to probe nuclear parton distribution functions (nPDFs) [1,2,3,4]. Leptonic decays of electroweak bosons are of particular interest since leptons do not interact strongly with the medium produced in these collisions [5,6]. As compared to those in a proton, the nPDFs are expected to be depleted (shadowing) for partons carrying small momentum fractions x 10−2, and enhanced (anti-shadowing) in the 5 × 10−2 x 10−1 range [7]. Precise measurements of W boson production in heavy ion collisions might lead to an improved determination of the nPDFs. asymmetries in the individual yields of W+ and W− should permit the flavour decomposition of u and d quark distributions in nuclei
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