High Energy pA Collisions Research Articles

D-meson production in high energy pA collisions within the QCD color dipole transverse momentum representation

The D-meson production is investigated by considering the unintegrated gluon distribution within the dipole approach in the momentum representation. We analyze the D-meson spectrum accounting for the effects of nonlinear behavior of the QCD dynamics which can be accordingly addressed in the dipole framework. The unintegrated gluon distribution is obtained by using geometric scaling property and the results are compared to the Glauber–Gribov framework. The absolute transverse momentum spectra and the nuclear modification ratios are investigated. Predictions are compared with the experimental measurements by the ALICE and LHCb Collaborations in pA collisions for different rapidity bins.

Azimuthal asymmetries and the emergence of “collectivity” from multi-particle correlations in high-energy pA collisions

We show how angular asymmetries ∼cos⁡2ϕ can arise in dipole scattering at high energies. We illustrate the effects due to anisotropic fluctuations of the saturation momentum of the target with a finite correlation length in the transverse impact parameter plane, i.e. from a domain-like structure. We compute the two-particle azimuthal cumulant in this model including both one-particle factorizable as well as genuine two-particle non-factorizable contributions to the two-particle cross section. We also compute the full BBGKY hierarchy for the four-particle azimuthal cumulant and find that only the fully factorizable contribution to c2{4} is negative while all contributions from genuine two, three and four-particle correlations are positive. Our results may provide some qualitative insight into the origin of azimuthal asymmetries in p+Pb collisions at the LHC which reveal a change of sign of c2{4} in high-multiplicity events.

Coulomb corrections to photon and dilepton production in high-energypAcollisions

We consider particle production in high energy pA collisions. In addition to the coherent interactions with the nuclear color field, we take into account coherent interactions with the nuclear electromagnetic Coulomb field. Employing the dipole model, we sum up the leading multiple color and electromagnetic interactions and derive inclusive cross sections for photon and dilepton production. We found that the Coulomb corrections are up to $10\%$ at $\sqrt{s}=200$~GeV per nucleon.

Heavy quark pair production in high energy pA collisions: Open heavy flavors

We study open heavy flavor meson production in proton-nucleus (pA) collisions at RHIC and LHC energies within the Color Glass Condensate framework. We use the unintegrated gluon distribution at small Bjorken's x in the proton obtained by solving the Balitsky-Kovchegov equation with running coupling correction and constrained by global fitting of HERA data. We change the initial saturation scale of the gluon distribution for the heavy nucleus. The gluon distribution with McLerran-Venugopalan model initial condition is also used for comparison. We present transverse momentum spectra of single D and B productions in pA collisions, and the so-called nuclear modification factor. The azimuthal angle correlation of open heavy flavor meson pair is also computed to study the modification due to the gluon saturation in the heavy nucleus at the LHC.

Sudakov Resummation in the Small-xSaturation Formalism

Through an explicit calculation of massive scalar particle (e.g., Higgs boson) production in high energy pA collisions up to one-loop order, we demonstrate, for the first time, that the Sudakov-type logarithms in hard processes in the small-x saturation formalism can be systematically separated from the small-x logarithms. The generic feature of the Sudakov logarithms and all order resummation is derived. This calculation shall provide us deep insights into the understanding of factorizations in the saturation formalism. We further comment on the phenomenological implications in the LHC energy regime and extension to other hard processes in small-x calculations.

Beyond the proton collinear factorization in heavy quark production in pA collisions at low x

We consider heavy quark production in high energy pA collisions and investigate the contribution of interactions of valence quarks of proton with the nucleus. The often made assumption that valence quarks of proton can be factored out is justified only if the nucleus saturation momentum is much smaller than the heavy quark mass. This is not the case in phenomenologically relevant situations. Breakdown of factorization manifests itself in substantial decrease of the cross section at large total and small relative transverse momenta of the heavy quark -antiquark pair.

Cronin effect for protons and pions in high-energy pA collisions

Pions and protons production cross sections are analyzed in proton–proton and proton–nucleus collisions at the RHIC energy at midrapidity. We employ the pQCD factorization scheme supplemented with the color-dipole formalism to investigate the Cronin effect. We calculate the broadening in the color-dipole approach for different centralities. Our main goal is to investigate, in a parameter-free manner within a unified framework, how much of the cronin effect for both pions and baryons comes from the transverse momentum broadening due to initial partons multi-scatterings. We conclude that final-state effects in pA collisions are important. Uncertainties in nuclear shadowing of various parton distributions and parton fragmentation functions are also discussed.

Forward hadron production in high energy pA collisions: From RHIC to LHC

We present a calculation of inclusive π, D and B mesons production at RHIC and LHC energies based upon the KKT model of gluon saturation. We discuss dependence of the nuclear modification factor on rapidity, centrality and transverse momentum.

High Energy pA Collisions in the Color Glass Condensate Approach

We present a brief review of phenomenological applications of the gluon saturation approach to the proton-nucleus collisions at high energies.

Quark pair production in high energy pA collisions: General features

A consistent treatment of both multiple scattering and small x quantum evolution effects on pair production in high energy pA collisions is feasible in the framework of the color glass condensate (CGC) [J.P. Blaizot, F. Gelis, R. Venugopalan, Nucl. Phys. A 743 (2004) 57]. We first discuss the properties of quark pair production in the classical effective theory where only multiple scattering effects are included. Explicit results are given for pair production as a function of the invariant mass of pairs, the pair momenta, the atomic mass number A and the quark mass. We relate the logarithms that appear in our formulation of pair production to logarithms that appear in the limit of collinear factorization in QCD. Violations of k ⊥ factorization and medium modifications, as represented by the Cronin effect, are also investigated. We next consider how small x quantum evolution (shadowing) effects modify the results for pair production. In particular, we provide results for the rapidity distribution of pairs and the dependence of the Cronin effect on rapidity. We discuss the dependence of our results on the initial conditions for small x evolution and comment on its implications for pair production at RHIC and the LHC.

High energy pA collisions in the color glass condensate approach II: quark pair production

We compute the production of quark–antiquark pairs in high energy collisions between a small and a large projectile, as in proton–nucleus collisions, in the framework of the color glass condensate. We derive a general expression for quark pair-production, which is not k ⊥ -factorizable. However, k ⊥ -factorization is recovered in the limit of large mass pairs or large quark–antiquark momenta. Our results are amenable to a simple interpretation and suggest how multi-parton correlations at small x can be quantified in high-energy proton/deuteron–nucleus collisions.

High energy pA collisions in the color glass condensate approach I: gluon production and the Cronin effect

We study gluon production in high energy proton–nucleus collisions in the semi-classical framework of the color glass condensate. We develop a general formalism to compute gluon fields in covariant gauge to lowest order in the classical field of the proton and to all orders in the classical field of the nucleus. The use of the covariant gauge makes the diagrammatic interpretation of the solution more transparent. k ⊥ -factorization holds to this order for gluon production—our results for the gluon distribution are equivalent to the prior diagrammatic analysis of Kovchegov and Mueller. We also show that these results are equivalent to the computation of gluon production by Dumitru and McLerran in the Fock–Schwinger gauge. We demonstrate how the Cronin effect arises in this approach, and examine its behavior in the two extreme limits of (a) no small- x quantum evolution, and (b) fully saturated quantum evolution. In both cases, the formalism reduces to Glauber's formalism of multiple scatterings. We comment on the possible implications of this study for the interpretation of the recent results on deuteron–gold collisions at the Relativistic Heavy Ion Collider (RHIC).

Proton breakup in high-energy pA collisions from perturbative QCD.

We argue that the distribution of hadrons near the longitudinal light cone in central high-energy pA collisions is computable in weak coupling QCD. This is because the density of gluons per unit transverse area in the dense target at saturation provides an intrinsic semihard momentum scale, Q(s). We predict that the longitudinal distribution of (anti)baryons and mesons steepens with increasing energy and atomic number of the target and that the transverse momentum distribution broadens. We show that the evolution of high moments of the longitudinal net baryon distribution with Q(s) is determined by the anomalous dimension gamma(qq).

Excess dileptons in high-energy nucleus-nucleus collisions

It has been observed recently that while continuum dilepton production and open charm production in high-energy pA collisions can be described in terms of the superposition of pp collisions, dilepton yields in S+U collisions are in excess of similar extrapolations. This feature can be explained as arising from the interaction of gluons produced in different soft baryon-baryon collisions, leading to additional open-charm pairs in nucleus-nucleus collisions but not in pA collisions.