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

TL;DR

The paper addresses constraining nuclear PDFs (nPDFs) using Z and W^{\pm} production in LHC p+Pb and Pb+Pb collisions. It adopts a NLO pQCD framework with nuclear modifications entering via $f^A=(Z/A)f^{p,A}+(N/A)f^{n,A}$ and $f^A=R_f f^{free}$, and analyzes rapidity spectra and asymmetries to isolate nuclear effects, reporting uncertainties from EPS09. The results show that rapidity asymmetries in p+Pb provide strong sensitivity to nPDFs and can constrain their modifications, while Pb+Pb data mainly test factorization and Glauber scaling due to competing proton-PDF uncertainties. Overall, Z/W production at the LHC emerges as a practical probe of nuclear parton distributions in the high-energy regime, guiding experimental measurements and global fits.

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 nucleus-nucleus case. We comment on how such studies will help to improve our knowledge of the nuclear PDFs.

Figures (10)

• Figure 1: The typical shape of the nuclear modifications as a function of $x$, and the kinematical reach for Z-production at three different center-of-mass energies and $M^2=M_Z^2$ for some values of rapidity.
• Figure 2: The calculated cross-sections for Z-boson production in pPb collisions at $\sqrt{s} = 8.8 \, {\rm TeV}$ at the Z-pole $M^2=M_Z^2$. The dashed line represents the central prediction calculated without applying the nuclear corrections to PDFs, and the green band is the uncertainty range derived from CTEQ6.6. The solid line is the prediction computed by CTEQ6.6 applying the nuclear effects from EPS09. The error bars quantify the uncertainty derived from EPS09 uncertainty sets. The red dashed-dotted curve is the prediction with only QED couplings (first term in Eq. \ref{['eq:coup']}) multiplied by 1100 with no nuclear corrections to PDFs. The lower panel shows the relative uncertainties with the same color codes.
• Figure 3: The predicted ratio between forward and backward yields of Z bosons in pPb collisions at $\sqrt{s} = 8.8 \, {\rm TeV}$ at the Z-pole, $M^2=M_Z^2$. The green solid band represents the prediction calculated with CTEQ6.6 without applying the nuclear effects, and the solid black barred line with gray shade is the prediction computed by CTEQ6.6 applying the nuclear effects from EPS09. The error bars quantify the uncertainties resulting from the EPS09 uncertainty sets.
• Figure 4: The calculated cross-sections for W$^\pm$-production in pPb collisions at $\sqrt{s} = 8.8 \, {\rm TeV}$ at the W-pole, $M^2=M_W^2$. The dashed line represents the central prediction calculated with CTEQ6.6 without applying the nuclear effects, and the green band is the uncertainty range derived from CTE6.6 PDFs. The solid line is the prediction computed by CTEQ6.6 applying the nuclear effects from EPS09. The error bars quantify the uncertainties resulting from the EPS09 uncertainty sets. The lower panels show the relative uncertainties with the same color codes.
• Figure 5: The predicted ratio between forward and backward yields of and W$^+$ (right panel) and and W$^-$ (left panel) bosons in pPb collisions at $\sqrt{s} = 8.8 \, {\rm TeV}$. The green solid band represents the prediction calculated with CTEQ6.6 without applying the nuclear effects, and the solid black barred line with gray shade is the prediction computed by CTEQ6.6 applying the nuclear effects from EPS09. The error bars quantify the uncertainties resulting from the EPS09 uncertainty sets.