Production In pA Collisions Research Articles

Particle Production in pA Collisions at Mid-Rapidity in the Color Glass Condensate

Particle correlations in small collisions systems, like proton–nucleus, lie at the core of the discussion about whether quark–gluon plasma is produced in small systems. Both initial and final state explanations have been essayed to describe such correlations. In this work, we focus on the initial state explanations provided by the quantum effects in the initial wave function of the incoming hadrons, in the framework of the Color Glass Condensate effective theory. We describe the formalism indicating the different inputs required for phenomenological applications. We compare the results from two different models, finding that the results for azimuthal harmonics agree qualitatively, but show quantitative differences, particularly at transverse momenta above the saturation scale.

QCD resummation of dijet azimuthal decorrelations in pp and pA collisions

We study the azimuthal angular decorrelations of dijet production in both proton-proton (pp) and proton-nucleus (pA) collisions. By utilizing soft-collinear effective theory, we establish the factorization and resummation formalism at the next-to-leading logarithmic accuracy for the azimuthal angular decorrelations in the back-to-back limit in pp collisions. We propose an approach where the nuclear modifications to dijet production in pA collisions are accounted for in the nuclear modified transverse momentum dependent parton distribution functions (nTMDPDFs), which contain both collinear and transverse dynamics. This approach naturally generalizes the well-established formalism related to the nuclear modified collinear parton distribution functions (nPDFs). We demonstrate strong consistency between our methodology and the CMS measurements in both pp and pA collisions, and make predictions for dijet production in the forward rapidity region in pA collisions at LHC kinematics and for mid-rapidity kinematics at sPHENIX. Throughout this paper, we focus on the application of this formalism to a simultaneous fit to both collinear and transverse momentum dependent contributions to the transverse momentum dependent distributions.

Single inclusive jet production in pA collisions at NLO in the small-x regime

We present the first complete next-to-leading-order (NLO) prediction with full jet algorithm implementation for the single inclusive jet production in pA collisions at forward rapidities within the color glass condensate (CGC) effective theory. Our prediction is fully differential over the final state physical kinematics, which allows the implementation of any infra-red safe observable including the jet clustering procedure. The NLO calculation is organized with the aid of the observable originated power counting proposed in [1] which gives rise to the novel soft contributions in the CGC factorization. We achieve the fully-differential calculation by constructing suitable subtraction terms to handle the singularities in the real corrections. The subtraction contributions can be exactly integrated analytically. We present the NLO cross section with the jets constructed using the anti-kT algorithm. The NLO calculation demonstrates explicitly the validity of the CGC factorization in jet production. Furthermore, as a byproduct of the subtraction method, we also derive the fully analytic cross section for the forward jet production in the small-R limit. We show that in the small-R approximation, the forward jet cross section can be factorized into a semi-hard cross section that produces a parton and the semi-inclusive jet functions (siJFs). We argue that this feature holds for generic jet production and jet substructure observables in the CGC framework. Last, we show numerical analyses of the derived formula to validate our calculations. We justify when the small-R approximation is appropriate. Like forward hadron production, the obtained NLO result also exhibits the negativity of the cross section in the large jet transverse regime, which signals the need for the threshold resummation. A sketch of the threshold resummation in the CGC framework is presented based on the multiple emission picture and it is found to agree with the approach using the rapidity renormalization group equation developed in [2].

Impact of fully coherent energy loss on heavy meson production in pA collisions

Hadron production in proton-nucleus (pA) collisions was previously shown to be suppressed by medium-induced fully coherent energy loss (FCEL). We show that the quenching of D and B mesons in pPb collisions at the LHC due solely to FCEL is, at least, on par with other nuclear effects such as gluon shadowing or saturation. This is consistent with previous findings for both quarkonium and light hadron production in pA collisions, emphasising that FCEL effects need to be included for a reliable understanding of hadron production measurements in pA collisions.

Fully coherent energy loss effects on light hadron production in pA collisions

We single out the role of fully coherent induced gluon radiation on light hadron production in pA collisions. The effect has the same general features as for quarkonium production, however with a richer color structure as the induced radiation depends on the global color charge of the partonic subprocess final state. Baseline predictions for light hadron nuclear suppression in pPb collisions at the LHC are provided, taking into account only the effect of fully coherent energy loss, which proves to be of the same order of magnitude as gluon shadowing or saturation. This underlines the need to include fully coherent energy loss in phenomenological studies of hadron production in pA collisions.

Initial-state energy loss in cold QCD matter and the Drell-Yan process

The effects of parton energy loss in nuclear matter on the Drell-Yan process in pA and πA collisions at fixed-target energies are investigated. Calculations are based on the Baier-Dokshitzer-Mueller-Peigné-Schiff (BDMPS) framework embedded in a next-to-leading order calculation, using the transport coefficient extracted from J/ψ measurements. Model calculations prove in good agreement with preliminary measurements by the E906 experiment, despite a slightly different magnitude, supporting a consistent picture between Drell-Yan and J/ψ data. Predictions for the COMPASS future measurements in πA collisions at sqrt{s}=18.9 GeV are also performed. At higher collision energy ( sqrt{s}=38.7 GeV), Drell-Yan measurements are only slightly affected by energy loss effects. On the contrary, the E906 results turn out in clear disagreement with nuclear PDF effects alone. The comparison of E772, E866, and E906 measurements indicates for the first time a clear violation of QCD factorization in Drell-Yan production in pA collisions.

Two-particle azimuthal harmonics in pA collisions

We compute two-particle production in pA collisions and extract azimuthal harmonics, using the dilute-dense formalism in the Color Glass Condensate framework. The multiple scatterings of the partons inside the projectile proton on the dense gluons inside the target nucleus are expressed in terms of Wilson lines. They generate interesting correlations, which can be partly responsible for the signals of collectivity measured at RHIC and at the LHC. Most notably, while gluon Wilson loops yield vanishing odd harmonics, quark Wilson loops can generate sizable odd harmonics for two particle correlations. By taking both quark and gluon channels into account, we find that the overall second and third harmonics lie rather close to the recent PHENIX data at RHIC.

Heavy quark production inpAcollisions: The double parton scattering contribution

In this paper, we estimate the double parton scattering (DPS) contribution for the heavy quark production in pA collisions at the LHC. The cross-sections for the charm and bottom production are estimated using the dipole approach, taking into account the saturation effects, which are important for high energies and for the scattering with a large nucleus. We compare the DPS contribution with the single parton scattering one and demonstrate that in the case of charm production, both are similar in the kinematical range probed by the LHC. Predictions for the rapidity range analyzed by the LHCb Collaboration are also presented. Our results indicate that the study of the DPS contribution for the heavy quark production in pPb collisions at the LHC is feasible and can be useful to probe the main assumptions of the approach.

Production cross-sections of prompt ⋀c+ in pPb collisions at √SNN = 5 TeV with the LHCb detector

The study of the nuclear modification factor and the forward/backward asymmetry in pPb collisions at [see formula in PDF] = 5 TeV is extended to the [see formula in PDF] baryon, providing the first measurement of charmed baryon production in pA collisions. The result is compared to the analogous measurement on charmed mesons, providing an insight on the production mechanism of charmed hadrons.

Particle production in pA collisions beyond leading order

We describe the origin of, and the solution to, the negativity problem which occurs in the perturbative calculation of the cross-section for single-inclusive particle production in proton-nucleus collisions at next-to-leading-order.

Forward hadron production in pA collisions beyond leading order

In this talk, we report our recent progress on pinning down the cause of negativity in the NLO single inclusive hadron production in pA collisions at forward rapidity.

Transverse single spin asymmetry in Drell-Yan production in polarized pA collisions

We study the transverse single spin asymmetry in Drell-Yan production in pA collisions with incoming protons being transversely polarized. We carry out the calculation using a newly developed hybrid approach. The polarized cross section computed in the hybrid approach is consistent with that obtained from the usual TMD factorization at low transverse momentum as expected, whereas at high transverse momentum, color entanglement effect is found to play a role in contributing to the spin asymmetry of Drell-Yan production, though it is a $1/N_c^2$ suppressed effect.

High-pT hadrons in pA and AA collisions: Impact of energy conservation

The Cronin effect, which is nuclear enhancement of high-pT hadron production in pA collisions was successfully predicted prior the measurements at RHIC and LHC. The restrictions imposed by energy conservation lead to spectacular effects. Energy deficit becomes an issue for hadron production in pA collisions at large xL and/or large xT towards the kinematic bounds xL,T = 1. It leads to a suppression, which has been indeed observed for hadrons produced at forward rapidities and large pT. Intensive energy dissipation via gluon radiation by a highly virtual parton produced with large pT, makes impossible this process to continue long. Color neutralization and creation of a colorless dipole must occur promptly. When this happens inside a hot medium created in AA collisions, attenuation of dipoles, rather than induced energy loss, becomes a dominant mechanism for suppression of high-pT hadrons.

Cumulative hadron production in pA collisions in the framework of z-scaling

Data on cumulative particle production in pA collisions with momentum [Formula: see text] are analyzed in the [Formula: see text]-scaling approach. Scaling function [Formula: see text] is constructed for different types of target nuclei and inclusive particle angle. The function is expressed via respective inclusive cross section and average multiplicity density of charged particles. This regime corresponds to low [Formula: see text] (low transverse momentum [Formula: see text]). Validity of the concept was verified for this data. A-dependence of the scaling function is studied for low-[Formula: see text] region. Self-similarity of hadron production in pA collisions is confirmed over a wide kinematic range.

Transverse single spin asymmetry in direct photon production in polarizedpAcollisions

We study the transverse single spin asymmetry in direct photon production in pA collisions with incoming protons being transversely polarized. To facilitate the calculation, we formulate a hybrid approach in which the nucleus is treated in the Color Glass Condensate (CGC) framework while the collinear twist-3 formalism is applied on the proton side. It has been found that an additional term which arises from color entanglement shows up in the spin dependent differential cross section. The fact that this additional term is perturbatively calculable allows us to quantitatively study color entanglement effects.

Next-to-eikonal corrections in the CGC: gluon production and spin asymmetries in pA collisions

We present a new method to systematically include corrections to the eikonal approximation in the background field formalism. Specifically, we calculate the subleading, power-suppressed corrections due to the finite width of the target or the finite energy of the projectile. Such power-suppressed corrections involve Wilson lines decorated by gradients of the background field - thus related to the density - of the target. The method is of generic applicability. As a first example, we study single inclusive gluon production in pA collisions, and various related spin asymmetries, beyond the eikonal accuracy.

Energy conservation in high-pT nuclear reactions

The Cronin effect, which is nuclear enhancement of high-pT hadron production in pA collisions was successfully predicted prior the measurements at RHIC and LHC. The restrictions imposed by energy conservation lead to spectacular effects. Energy deficit becomes an issue for hadron production in pA collisions at large xL and/or large xT toward the kinematic bounds xL, T = 1. It leads to a suppression, which has been indeed observed for hadrons produced at forward rapidities and large pT. Intensive energy dissipation via gluon radiation by a highly virtual parton produced with large pT, makes this process impossible to continue long. Color neutralization and creation of a colorless dipole must occur promptly. When this happens inside a hot medium created in AA collisions, attenuation of dipoles, rather than induced energy loss, becomes a dominant mechanism for suppression of high-pT hadrons.

Towards the Test of Saturation Physics Beyond Leading Logarithm

We present results from the first next-to-leading-order (NLO) numerical analysis of forward hadron production in pA and dA collisions in the small-x saturation formalism. Using parton distributions and fragmentation functions at NLO, as well as the dipole amplitude from the solution to the Balitsky-Kovchegov equation with running coupling, together with the NLO corrections to the hard coefficients, we obtain a good description of the available RHIC data in dAu collisions. In the large p⊥ region beyond the saturation scale, we find that the NLO correction becomes dominant and negative, which indicates that other physics beyond NLO becomes important and should also be taken into account. Furthermore, we make predictions for forward hadron production in pPb collisions at the LHC. This analysis not only incorporates the important NLO corrections for all partonic channels, but also reduces the renormalization scale dependence and helps to significantly reduce the theoretical uncertainties. It therefore provides a precise test of saturation physics beyond the leading logarithmic approximation.

Z-Scaling of cumulative pion production in pA collisions at high energies

The experimental data on inclusive spectra of the π+ mesons produced in p+A collisions at the incident proton momentum of 400 GeV/c obtained by G. Leksin group at FNAL are analyzed in the framework of z-scaling. Self-similarity of the hadron production in the low-pT cumualtive region is verified. The scaling function ψ(z) is expressed via the invariant cross section Ed3σ/dp3 and the average multiplicity density dNch/dη of charged particles. Results of the analysis of the low-pT data are compared with the high-pT data sets obtained by J. Cronin, R. Sulyaev and D. Jaffe groups. Indication on self-similarity of the cumulative pion production over a wide kinematical range has been found. Based on the universality of the shape of ψ(z) the inclusive cross sections of the π+ mesons produced in p+A collisions in deep-cumulatice region (x2≫1/A) are predicted.

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.