Production In Proton-nucleus Collisions Research Articles

CGC factorization for forward particle production in proton-nucleus collisions at next-to-leading order

Within the Color Glass Condensate effective theory, we reconsider the next-to-leading order (NLO) calculation of the single inclusive particle production at forward rapidities in proton-nucleus collisions at high energy. Focusing on quark production for definiteness, we establish a new factorization scheme, perturbatively correct through NLO, in which there is no `rapidity subtraction'. That is, the NLO correction to the impact factor is not explicitly separated from the high-energy evolution. Our construction exploits the skeleton structure of the (NLO) Balitsky-Kovchegov equation, in which the first step of the evolution is explicitly singled out. The NLO impact factor is included by computing this first emission with the exact kinematics for the emitted gluon, rather than by using the eikonal approximation. This particular calculation has already been presented in the literature [1,2], but the reorganization of the perturbation theory that we propose is new. As compared to the proposal in [1,2], our scheme is free of the fine-tuning inherent in the rapidity subtraction, which might be the origin of the negativity of the NLO cross-section observed in previous studies.

Forward J/ψ production at high energy: Centrality dependence and mean transverse momentum

Forward rapidity $J/\psi$ meson production in proton-nucleus collisions can be an important constraint of descriptions of the small-$x$ nuclear wavefunction. In an earlier work we studied this process using a dipole cross section satisfying the Balitsky-Kovchegov equation, fit to HERA inclusive data and consistently extrapolated to the nuclear case using a standard Woods-Saxon distribution. In this paper we present further calculations of these cross sections, studying the mean transverse momentum of the meson and the dependence on collision centrality. We also extend the calculation to backward rapidities using nuclear parton distribution functions. We show that the parametrization is overall rather consistent with the available experimental data. However, there is a tendency towards a too strong centrality dependence. This can be traced back to the rather small transverse area occupied by small-$x$ gluons in the nucleon that is seen in the HERA data, compared to the total inelastic nucleon-nucleon cross section.

Nuclear modification of forward J/ψ production in proton-nucleus collisions at the LHC

We re-evaluate the nuclear suppression of forward J/ψ production at high energy in the Color Glass Condensate framework. We use the collinear approximation for the projectile proton probed at large x and an up to date dipole cross section fitted to HERA data to describe the target in proton-proton collisions. We show that using the Glauber approach to generalize the proton dipole cross section to the case of a nucleus target leads to a nuclear modification factor much closer to LHC data than previous estimates using the same framework.

Heavy quarks in proton-nucleus collisions: The hybrid formalism

We explore the quark mass effects on inclusive hadron production in proton-nucleus collisions at high energies. We consider two processes. First, we compute the single inclusive cross-section for production of hadrons with open heavy flavour in the proton forward direction at leading order. Next, in the same kinematics, we calculate the heavy-quark contribution to single inclusive production of light or unidentified hadrons at next-to-leading-order. For both studies we exploit the hybrid formalism, that is the collinear factorisation on the proton side while high-density and high-energy effects are resummed on the side of the nucleus.

Theory status of quarkonium production in proton-nucleus collisions

I give a brief overview of the recent theoretical progress in the study of quarkonium production in proton-nucleus collisions in view of the recent LHC and RHIC results. A special emphasis is put on the excited states such as the Ψ', ϒ(2S) and ϒ (3S).

Kaon pair production in proton-nucleus collisions at 2.83 GeV kinetic energy

The production of non-ϕK+K− pairs by protons at 2.83 GeV kinetic energy on C, Cu, Ag, and Au targets has been investigated by using the COSY-ANKE magnetic spectrometer. The K− momentum dependence of the differential cross section has been measured for laboratory polar angles θK±≤12∘ over the 0.2–0.9 GeV/c range. The comparison of the data with detailed model calculations indicates an attractive K−-nucleus potential of about −60 MeV at normal nuclear matter density at a mean momentum of 0.5 GeV/c. However, this approach has difficulty in reproducing the smallness of the observed cross sections at low-K− momenta.Received 9 September 2015DOI:https://doi.org/10.1103/PhysRevC.92.065201©2015 American Physical Society

Single inclusive particle production in proton-nucleus collisions at next-to-leading order in the hybrid formalism

We reconsider the perturbative next-to-leading calculation of the single inclusive hadron production in the framework of the hybrid formalism, applied to hadron production in proton-nucleus collisions. Our analysis, performed in the wave function approach, differs from the previous works in three points. First, we are careful to specify unambiguously the rapidity interval that has to be included in the evolution of the leading-order eikonal scattering amplitude. This is important, since varying this interval by a number of order unity changes the next-to-leading order correction that the calculation is meant to determine. Second, we introduce the explicit requirement that fast fluctuations in the projectile wave function which only exist for a short time are not resolved by the target. This Ioffe time cutoff also strongly affects the next-to-leading order terms. Third, our result does not employ the approximation of a large number of colors. Our final expressions are unambiguous and do not coincide at next-to-leading order with the results available in the literature.

Quarkonium Suppression from Coherent Energy Loss in Fixed-Target Experiments Using LHC Beams

Quarkonium production in proton-nucleus collisions is a powerful tool to disentangle cold nuclear matter effects. A model based on coherent energy loss is able to explain the available quarkonium suppression data in a broad range of rapidities, from fixed-target to collider energies, suggesting coherent energy loss in cold nuclear matter to be the dominant effect in quarkonium suppression in p-A collisions. This could be further tested in a high-energy fixed-target experiment using a proton or nucleus beam. The nuclear modification factors ofJ/ψandΥas a function of rapidity are computed in p-A collisions ats=114.6 GeV, and in p-Pb and Pb-Pb collisions ats=72 GeV. These center-of-mass energies correspond to the collision on fixed-target nuclei of 7 TeV protons and 2.76 TeV (per nucleon) lead nuclei available at the LHC.

Erratum to: “Self-similarity of low-pT cumulative pion production in proton-nucleus collisions at U70”

Erratum to: “Self-similarity of low-pT cumulative pion production in proton-nucleus collisions at U70”

Self-similarity of Low-p T cumulative pion production in proton-nucleus collisions at U70

The data on inclusive momentum spectra of π± mesons produced in p + A collisions over the range pL =15–65 GeV/c and θlab = 159° at U70 are analyzed in the framework of z-scaling approach. Self-similarity of the pion production in the low-pT cumualtive region is verified. Scaling function ψ(z) for Be, C, Al, Ti, Mo, W nuclei is constructed. It is expressed via the invariant cross section and the average multiplicity density of charged particles. Results of the analysis are compared with data obtained by J. Cronin, R. Sulyaev, and D. Jaffe groups at pL = 70, 400 GeV/c, pT > 1 GeV/c and θcms − 90°. A microscopic scenario of p + A interactions in terms of momentum fractions x1, x2 is discussed. Indication on self-similarity of the low-pT cumulative pion production in p + A collisions was found. Universality of the shape of the scaling function is used to predict inclusive cross sections of π+ and π− pions produced in p + A collisions on Be, C, Al, Ti, Mo, W targets in the deep-cumulative region (x2 ≫ 1/A).

ALICE measurements in p–Pb collisions: Charged particle multiplicity, centrality determination and implications for binary scaling

Measurements of particle production in proton-nucleus collisions provide a reference to disentangle final state effects, i.e. signatures of the formation of a deconfined hot medium, from initial state effects, already present in cold nuclear matter. Since many initial state effects are expected to vary as a function of the number of collisions suffered by the incoming proton, it is crucial to estimate the centrality of the collision. In p-Pb collisions categorization of events into different centrality classes using a particle multiplicity distribution is complicated by the low particle multiplicities and the large multiplicity fluctuations. We present ALICE measurements of particle production in p-Pb collisions at sqrt(sNN) = 5.02$ TeV, including the pseudo-rapidity and transverse momentum dependence, we discuss the event classification in centrality classes and its implications for the measurements of nuclear modification factors.

Self-similarity of low-p T cumulative pion production in proton-nucleus 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 cumulative region is verified. Scaling function ψ(z) for the Li, Be, C, Al, Cu, Ta nuclei is constructed. It is expressed via the invariant cross section and average multiplicity density 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. A microscopic scenario of p + A interactions in terms of momentum fractions x1, x2 is discussed. Indication on self-similarity of the cumulative pion production in p + A collisions over a wide kinematical range has been found. Based on the universality of the shape of the scaling function the inclusive cross sections of the π+ mesons produced in p + A collisions on the Li, Be, C, Al, Cu, Ta targets in deep-cumulative region (x2 ≫ 1/A) are predicted.

Fractal structure of near-threshold quarkonium production off cold nuclear matter

We investigate near-threshold production of quarkonium resonances in cold nuclear matter through a phenomenologically motivated scaling theory with two exponents, which are fixed by existing data on near-threshold $J/\ensuremath{\psi}$ production in proton-nucleus collisions. Interestingly, it seems possible to extend one of the exponents to the production of other mesons in cold nuclear matter. The scaling theory can be tested and refined in experiments at the upcoming high-intensity Facility for Antiproton and Ion Research (FAIR) accelerator complex at GSI.

Forward particle production in proton-nucleus collisions at the LHC

We present the current theory status on the particle production in the forward region in proton-nucleus (p+A) collisions based on the Color Glass Condensate framework. The cross-section expressed in a factorized form with the dipole amplitude which is related to the unintegrated gluon distribution. Energy dependence of the dipole amplitude is controlled by the nonlinear x-evolution equation and constrained by e+p scattering data at HERA. The transverse geometry of the target is modeled as an assembly of the nucleons distributed with the Woods-Saxon profile. We show our prediction for the nuclear modification factor of the hadron multiplicity in p+A reactions at the LHC energy.

Azimuthal asymmetries for quark pair production inpAcollisions

We study the azimuthal asymmetries for quark pair production in proton-nucleus collisions using a hybrid approach in which the nucleus is treated in the Color Glass Condensate (CGC) framework while the Lipatov approximation is applied on the proton side. Our treatment goes beyond the large N_c limit. We particularly focus on the so-called correlation limit where the imbalance of the transverse momentum of the quark pair is much smaller than the out-going individual quark transverse momenta. In this kinematic region, a matching between the hybrid approach and a factorization in terms of transverse momentum dependent parton distributions(TMDs) has been found. It is shown which of the various unpolarized and linearly polarized gluon TMD distributions contribute to $\cos 2 \phi$ and $\cos 4 \phi$ modulations of quark pair production.

Initial-state quark energy loss from Drell–Yan production in proton-proton and proton-nucleus collisions

Drell-Yan production cross sections were measured by the NA3 and E866 experiments, in p-Pt and pp collisions, as a function of the dimuon mass and x F . We compare these measurements to next-to-leading order Drell-Yan calculations, made with the CTEQ6M parton densities modified (or not) by nuclear effects, using the EPS09 parameterization. The analysis of the data allows us to evaluate the initial-state quark energy loss. Drell-Yan measurements are ideally suited to isolate the initial-state parton energy loss, given the absence of final-state effects on the produced dimuon. Our study shows that these data indicate negligible quark energy loss and allow us to derive rather strict upper limits. For completeness, our study has been repeated using the less accurate measurements of Drell-Yan cross section ratios between heavy and light nuclear targets, provided by the E772 and E866 experiments. Our results provide an additional constraint on the models trying to explain quarkonium production in proton-nucleus collisions, as a function of quarkonium rapidity and collision energy, where initial- and final-state energy loss has frequently been assumed to play an important role, convoluted with several other complex mechanisms, including final-state quarkonium breakup, formation time effects, etc.

Low mass dimuon production in proton-nucleus collisions at 400 GeV/ c

The NA60 experiment has studied low-mass muon pair production in proton-nucleus collisions with a system of Be, Cu, In, W, Pb and U targets using a 400 GeV/ c proton beam at the CERN SPS. The mass spectrum is well described by the superposition of the two-body and Dalitz decays of the light neutral mesons η, ρ, ω, η ′ and ϕ new measurement of the electromagnetic transition form factors of the η and ω was performed. The values found are Λ η − 2 = 1.950 ± 0.059 (stat.) ± 0.042 (syst.) ( GeV / c 2 ) − 2 and Λ ω − 2 = 2.241 ± 0.025 (stat.) ± 0.028 (syst.) ( GeV / c 2 ) − 2 . They agree with the previous available measurements, improving their uncertainty thanks to the higher statistics, and confirm the discrepancy with the prediction of the Vector Meson Dominance model in the case of the electromagnetic form factor of the ω meson. The comparison of the data to an improved calculation including also a direct point-like term is discussed. The nuclear dependence of the production cross sections of the ω, ρ and ϕ mesons was also studied. An increase of the ϕ production with respect to the ω is observed from p-Be to p-U collisions. Parameterizing the nuclear dependence of the production cross sections with the power law σ pA ∝ A α , one finds α ω = 0.841 ± 0.014 (stat.) ± 0.030 (syst.) and α ϕ = 0.906 ± 0.011 (stat.) ± 0.025 (syst.) . A possible bias induced by the use of the light Be target is found. If it is excluded, the results are α ω = 0.801 ± 0.037 (stat.) ± 0.041 (syst.) and α ϕ = 0.963 ± 0.026 (stat.) ± 0.017 (syst.) , showing an increase of α ϕ and a decrease of α ω .

Two-Hadron Correlations in the Color Glass Condensate Formalism

Two-hadron correlations are a sensitive probe of the dynamics of gluon saturation and the Color Glass Condensate formalism where the degrees of freedom are Wilson lines of the gluon field. It is shown that unlike structure functions in DIS and single hadron production in proton-nucleus collisions, higher point functions of Wilson lines appear in two-hadron production cross section. We derive equations for the energy ($x$) evolution of these higher point functions using the JIMWLK Hamiltonian and show that dipole approximation, employed in the literature to fit the di-hadron data measured by the STAR collaboration in the forward rapidity region, breaks down. This necessitates an investigation of the full hierarchy of the JIMWLK evolution equations for these higher point functions and their solutions. This can then be used to make a quantitative analysis of the di-hadron correlations in the forward rapidity region in deuteron-gold collisions at RHIC and in the long range rapidity correlations observed in proton-proton collisions at the LHC.

Measurement of the η and ω Dalitz decays transition form factors in p-A collisions at 400 GeV/c with the NA60 apparatus

The NA60 experiment has studied low-mass muon pairs production in proton-nucleus collisions at 400 GeV/c at the CERN SPS. The mass spectrum is well described by the superposition of the two-body and Dalitz decays of the light neutral mesons η, ρ, ω, ή and ϕ. The large collected statistics allows to isolate the contributions due to the Dalitz decays of the η and ω mesons, from which the electromagnetic transition form factors of the two mesons have been extracted. The found values agree with the previous available measurements, improving their uncertainty thanks to the higher statistics. The results thus confirm the discrepancy with the prediction of the Vector Meson Dominance model in the case of the electromagnetic form factor of the ω meson.

Nuclear effects on J/ψ production in proton–nucleus collisions

The study of nuclear effects for J/{\psi} production in proton-nucleus collisions is crucial for a correct interpretation of the J/{\psi} suppression patterns experimentally observed in heavy-ion collisions. By means of three representative sets of nuclear parton distribution, the energy loss effect in the initial state and the nuclear absorption effect in the final state are taken into account in the uniform framework of the Glauber model. A leading order phenomenological analysis is performed on J/{\psi} production cross-section ratios RW/Be(xF) for the E866 experimental data. The J/{\psi} suppression is investigated quantitatively due to the different nuclear effects. It is shown that the energy loss effect with resulting in the suppression on RW/Be(xF) is more important than the nuclear effects on parton distributions in high xF region. The E866 data in the small xF keep out the nuclear gluon distribution with a large anti-shadowing effect. However, the new HERA-B measurement is not in support of the anti-shadowing effect in the nuclear gluon distribution. It is found that the J/{\psi}-nucleon inelastic cross section {\sigma} J/{\psi} abs depends on the kinematical variable xF, and increases as xF in the region xF > 0.2. 1 Introduction