Constraints for the nuclear parton distributions from Z and W ± production at the LHC

Abstract

The LHC is foreseen to finally bring also the nuclear collisions to the TeV scale thereby providing new possibilities for physics studies, in particular related to the electro-weak sector of the Standard Model. We study here the Z and W ± production in proton-lead and lead-lead collisions at the LHC, concentrating on the prospects of testing the factorization and constraining the nuclear modifications of the parton distribution functions (PDFs). Especially, we find that the rapidity asymmetries in proton-nucleus collisions, arising from the differences in the PDFs between the colliding objects, provide a decisive advantage in comparison to the rapidity-symmetric nucleusnucleus case. We comment on how such studies will help to improve our knowledge of the nuclear PDFs.

Study ofZBoson Production in PbPb Collisions atsNN=2.76 TeV

“…At 7 TeV center-of-mass energy, measurements in pp collisions at the LHC [5,6] are well described by calculations based on higher-order perturbative quantum chromodynamics (pQCD), using recent parton distribution functions (PDFs). In AA collisions, Z boson production can be affected by various initial-state effects, though predictions indicate that these contributions are rather small [3,[7][8][9][10]. First, the mix of protons and neutrons in AA collisions (the so-called isospin effect) is estimated to modify the Z yield by less than 3% compared to pp collisions [9].…”

“…In AA collisions, Z boson production can be affected by various initial-state effects, though predictions indicate that these contributions are rather small [3,[7][8][9][10]. First, the mix of protons and neutrons in AA collisions (the so-called isospin effect) is estimated to modify the Z yield by less than 3% compared to pp collisions [9]. Second, energy loss and multiple scattering of the initial partons can also alter the Z production, by about 3% [10].…”

Z and W boson production in pPb collisions with CMS

“…The measured differential cross section is consistent with the theory predictions within uncertainties that are dominated by the statistical uncertainties. The forward-backward ratio, defined as dσ(+y)/dσ(−y), is expected to be more sensitive to nuclear effects [6], because normalization uncertainties cancel both in theory and in experiment. The right hand-side of Figure 2 shows the measured forward-backward ratio as a function of |y c.m.…”

“…The left hand-side of Figure 3 shows the lepton charge asymmetry, (dσ + − dσ − )/(dσ + + dσ − ), as a function of η lab compared to theoretical predictions. The predictions use the CT10 PDF set for the nucleons with or without the modification from EPS09 nPDF set as described in [6]. The W charge asymmetry is a sensitive probe of the down to up quark PDF ratio.…”

“…Both Z and W bosons were measured by the ATLAS [1,2] and the CMS [3,4,5] experiments using PbPb collision data taken in 2010 and 2011 at √ s NN = 2.76 TeV, confirming with a precision of about 10%, that the production cross section scales with the number of elementary nucleon-nucleon collisions. However, in heavy-ion collisions, electroweak boson production can be affected by initial state conditions [6,7].…”