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Register a new userThe small-x gluon distribution in centrality biased pA and pp collisions
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The nuclear modification factor $R_{pA}(p_T)$ provides information on the small-$x$ gluon distribution of a nucleus at hadron colliders. Several experiments have recently measured the nuclear modification factor not only in minimum bias but also for central $pA$ collisions. In this paper we analyze the bias on the configurations of soft gluon fields introduced by a centrality selection via the number of hard particles. Such bias can be viewed as reweighting of configurations of small-$x$ gluons. We find that the biased nuclear modification factor ${cal Q}_{pA}(p_T)$ for central collisions is above $R_{pA}(p_T)$ for minimum bias events, and that it may redevelop a Cronin peak even at small $x$. The magnitude of the peak is predicted to increase approximately like $1/{A_{perp}}^ u$, $ usim0.6pm0.1$, if one is able to select more compact configurations of the projectile proton where its gluons occupy a smaller transverse area $A_perp$. We predict an enhanced ${cal Q}_{pp}(p_T)-1 sim 1/(p_T^2)^ u$ and a Cronin peak even for central $pp$ collisions.
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Quarkonium production mechanism in high multiplicity small collision systems has recently been pursued in the color-glass-condensate (CGC) effective theory combined with non-relativistic QCD (NRQCD) factorization, allowing to study initial state interactions. Quarkonium polarization, potentially measured in future experiments, would help elucidate the quarkonium production mechanism at high multiplicities. In this paper, we provide predictions on $J/psi$ polarization parameters in high multiplicity proton-proton ($pp$) and proton-nucleus ($pA$) collisions within the CGC+NRQCD framework. Theoretical predictions are given for $J/psi$ rapidity $2.5< y_{J/psi}<4$, charged-particle multiplicity pseudorapidity $|eta_{ch} | <1$ and energies $sqrt{S}=13mathrm{~TeV}$ for $pp$, $sqrt{S}=8.16mathrm{~TeV}$ for $pA$ collisions. Considering two leptonic frame choices (Collins - Soper and helicity) we find a weak polarization of $J/psi$ that additionally decreases with growing event activities. No significant system size dependence between $pp$ and $pA$ collisions is obtained - this could be a new constraint to initial state interactions in small collision systems.
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We show that the diffractive forward production of two quarkonia, especially the $chi_{cJ}$ and $eta_c$ states, in proton-proton or proton-ion collisions can access the Weizsacker-Williams gluon Wigner distribution of the proton. We use the hybrid factorization approach where the collinear, double gluon PDF is applied for one of the protons and the $k_T$-dependent (Wigner or GTMD) distribution for the other. The production of quarkonia is treated in the NRQCD framework. A particularly concise formula is obtained for double $chi_{J=1}$ production.
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This manuscript is the outcome of the subgroup ``PDFs, shadowing and $pA$ collisions from the CERN workshop ``Hard Probes in Heavy Ion Collisions at the LHC. In addition to the experimental parameters for $pA$ collisions at the LHC, the issues discussed are factorization in nuclear collisions, nuclear parton distributions (nPDFs), hard probes as the benchmark tests of factorization in $pA$ collisions at the LHC, and semi-hard probes as observables with potentially large nuclear effects. Also, novel QCD phenomena in $pA$ collisions at the LHC are considered. The importance of the $pA$ program at the LHC is emphasized.
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