Nuclear parton density modifications from low-mass lepton pair production at the LHC

TL;DR

Nuclear PDFs, especially gluon modifications at small $x$, are poorly constrained, complicating cold-nuclear matter subtraction in heavy-ion observables. The paper proposes using low-mass lepton pair production from virtual photons in $pA$ collisions at the LHC, where the dominant partonic subprocess is $qg \to \gamma^* q$, and studies cross-section ratios $R^{\rm pPb}$ to cancel scale and bare-proton uncertainties. NLO QCD calculations across ALICE/ATLAS/CMS acceptances show sensitivity to shadowing, antishadowing, EMC, and isospin effects, with distinguishable predictions among EPS09, HKN07, nCTEQ, and DSSZ parametrisations. This approach could tighten nuclear PDFs in global fits and improve subtraction of cold nuclear matter effects in heavy-ion physics.

Abstract

In this article, we investigate the potential of low-mass lepton pair production in proton-ion collisions at the LHC to constrain nuclear modifications of parton densities. Similarly to prompt photon production, the transverse momentum spectrum is shown to be dominated by the QCD Compton process, but has virtually no fragmentation or isolation uncertainties. Depending on the orientation of the proton and ion beams and on the use of central or forward detector components, all interesting regions of nuclear effects (shadowing, antishadowing, isospin and EMC effects) can be probed. Ratios of cross sections allow to eliminate theoretical scale and bare-proton parton density errors as well as many experimental systematic uncertainties.

Figures (8)

• Figure 1: Modifications of up-quark (left), strange-quark (centre) and gluon (right) PDFs in lead nuclei with respect to bare proton PDFs at the factorisation scales 50 GeV (top) and 500 GeV (bottom) and as obtained in global fits by four different groups.
• Figure 2: Total cross sections for central and forward lepton-pair production in proton-lead (full, dashed and dotted lines) and lead-proton (dot-dashed line) collisions.
• Figure 3: Fractional contributions of partonic subprocesses to low (top) and intermediate (bottom) mass electron-positron pair production at central rapidity and $p_T<100$ GeV.
• Figure 4: Fractional contributions of partonic subprocesses to low-mass muon pair production at forward rapidity and $p_T<100$ GeV in proton-lead (top) and lead-proton (bottom) collisions.
• Figure 5: Fractional contributions of partonic subprocesses to intermediate-mass electron-positron pair production at central rapidity and $p_T<1000$ GeV.