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We obtain prompt diphoton cross section in proton-nucleus collisions in Hamiltonian light-cone approach within a hybrid approximation, treating the projectile proton in the parton model and the target nucleus in the color-glass-condensate approach. We study in detail the diphoton correlations in quarknucleus and proton-nucleus collisions at RHIC and the LHC. We show that the single fragmentation diphoton produces the away-side correlations peak, and the double-fragmentation component of prompt diphoton is responsible for the near-side peak, and the long-range in rapidity near-side azimuthal collimation, the so-called "ridge" structure. We study the transverse momentum, density and energy dependence of the diphoton ridge and show that it strongly depends on the kinematics and saturation dynamics. We show that while diphoton ridge exists at the LHC in quark-nucleus collisions, the effect disappears in proton-nucleus collisions at the LHC. At RHIC the ridge-type structure persists at low transverse momenta of diphoton even in proton-nucleus collisions. We argue that diphoton correlation measurements in p + A collisions can help to discriminate among models and understand the true origin of the observed dihadron ridge in p + A collisions. We also show that in addition to the ridge structure, prompt diphoton correlation also exhibits some distinct novel features, including the emergence of away-side double-peak structure at intermediate transverse momenta. [ABSTRACT FROM AUTHOR]

We systematically study exclusiv2 diffractive (photo)production of vector mesons (J/ψ, ψ(2s), ɸ, and ρ) off protons in high-energy collisions and investigate whether the production is a sensitive probe of gluon saturation. We confront saturation-based results for diffractive ψ(2s) and ρ production at HERA and J/ψ photoproduction with all available data, including recent work from HERA, ALICE, and LHCb, finding good agreement. In particular, we show that the t distribution of differential cross sections of photoproduction of vector mesons offers a unique opportunity to discriminate among saturation and nonsaturation models. This is due to the emergence of a pronounced dip (or multiple dips) in the t distribution of diffractive photoproduction of vector mesons at relatively large, but potentially accessible |t| that can be traced back to the unitarity features of color dipole amplitude in the saturation regime. We show that in saturation models the dips in t distribution recede towards lower |t| with decreasing mass of the vector meson, increasing energy or decreasing Bjorken-x, and decreasing virtuality Q. We provide various predictions for exclusive (photo)production of different vector mesons including the ratio of ψ(2s)/J/ψ at HERA, the LHC, and future colliders. [ABSTRACT FROM AUTHOR]

We consider semi-inclusive diphoton+jet and inclusive diphoton production in high-energy proton-nucleus collisions, treating the target nucleus as a color-glass condensate and the projectile proton in the parton model. We obtain the prompt diphoton production cross section in terms of fragmentation and direct contributions. The fragmentation part is given in terms of single-photon and double-photon fragmentation functions. We study prompt, direct, and fragmentation diphoton correlations in p+p and p+A collisions at the LHC and show that at low values of transverse momenta of the produced photon pair these correlations are sensitive to saturation effects. We show that back-to-back (de)correlations in prompt diphoton production are stronger in the fragmentation part than in the direct one. [ABSTRACT FROM AUTHOR]

The impact-parameter dependent color glass condensate (b-CGC) dipole model is based on the Balitsky-Kovchegov nonlinear evolution equation and improves the Iancu-Itakura-Munier dipole model by incorporating the impact-parameter dependence of the saturation scale. Here we confront the model to the recently released high precision combined Hadron Electron Ring Accelerator (HERA) data and obtain its parameters. The b-CGC results are then compared to data at small x for the structure function, the longitudinal structure function, the charm structure function, exclusive vector meson (J/, Ψ, ɸ and ρ) production and deeply virtual Compton scattering. We also compare our results with the impact-parameter dependent saturation (IP-Sat) model. We show that most features of inclusive deep inelastic scattering and exclusive diffractive data, including the Q2, W, |i|, and x dependence, are correctly reproduced in both models. Nevertheless, the b-CGC and the impact-parameter dependent saturation (IP-Sat) models give different predictions beyond the current HERA kinematics, namely for the structure functions at very low x and high virtualities Q2, and for the exclusive diffractive vector meson and deeply virtual Compton scattering production at high t. This can be traced back to the different power-law behavior of the saturation scale in x and to a different impact-parameter b dependence of the saturation scale in these models. Nevertheless, both models give approximately similar saturation scales Qs < 1 GeV for the proton in HERA kinematics, and also both models lead to the same conclusion that the typical impact parameter probed in the total y*p cross section is about b ~ 2-3 GeV-1. Our results provide a benchmark for further investigation of QCD at small x in heavy ion collisions at RHIC and the LHC and also at future experiments such as an electron-ion collider and the LHeC. [ABSTRACT FROM AUTHOR]

The impact-parameter dependent saturation model (IP-Sat) is a simple dipole model that incorporates key features of the physics of gluon saturation and matches smoothly to the perturbative QCD dipole expression at large Q2 for a given x. It was previously shown that the model gives a good description of HERA data suggesting evidence for gluon saturation effects at small x. The model has also been applied to proton-proton and proton-nucleus collisions and provides the basis for the IP-Glasma model of initial conditions in heavy ion collisions. Here we present a reanalysis of available data in electron-proton collisions at small Bjorken x, including the recently released combined data from the ZEUS and H1 collaborations. We first confront the model to the high precision combined data for the reduced cross section and obtain its parameters. With these parameters fixed, we compare model results to the data for the structure function F2, the longitudinal structure function FL, the charm structure function F2cc , exclusive vector meson (J/ψ, ∅ and ρ) production, and deeply virtual Compton scattering. Excellent agreement is obtained for the processes considered at small x in a wide range of Q2. Our results strongly hint at universality of the IP-Sat dipole amplitude and the extracted impact-parameter distribution of the proton. They also provide a benchmark for further refinements in studies of QCD saturation at collider. [ABSTRACT FROM AUTHOR]

We investigate semi-inclusive photon-hadron production in the color-glass-condensate framework at RHIC and the LHC energies in proton-proton (pp) and proton-nucleus (pA) collisions. We calculate the coincidence probability for azimuthal correlation of pairs of photon-hadron production and show that the away-side correlations have a double-peak or a single-peak structure depending on trigger particle selection and kinematics. This novel feature is unique for semi-inclusive photon-hadron production compared to a similar measurement for double inclusive dihadron production in pA collisions. We obtain necessary conditions between kinematic variables for the appearance of a double-peak or a single-peak structure for the away-side photon-hadron correlations in pp and pA collisions at forward rapidities and show that this feature is mainly controlled by the ratio zT = phadronT/ppphotonT oton. Decorrelation of away-side photon-hadron production by increasing the energy, rapidity and density, and appearance of double-peak structure can be understood by QCD saturation physics. We also provide predictions for the ratio of single inclusive prompt photon to hadron production, and the two-dimensional nuclear modification factor for the semi-inclusive photon-hadron pair production at RHIC and the LHC at forward rapidities. [ABSTRACT FROM AUTHOR]

We investigate inclusive prompt photon and semi-inclusive prompt photon-hadron production in high energy proton-nucleus collisions using the color glass condensate formalism, which incorporates nonlinear dynamics of gluon saturation at small x via the Balitsky-Kovchegov equation with a running coupling. For inclusive prompt photon production, we rewrite the cross section in terms of direct and fragmentation contributions and show that the direct photon (and isolated prompt photon) production is more sensitive to gluon saturation effects. We then analyze azimuthal correlations in photon-hadron production in high energy proton-nucleus collisions and obtain a strong suppression of the away-side peak in photon-hadron correlations at forward rapidities, similar to the observed mono-jet production in deuteron-gold collisions at forward rapidity at the Relativistic Heavy Ion Collider. We make predictions for the nuclear modification factor Rp(d)A and photon-hadron azimuthal correlations in proton(deuteron)-nucleus collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider at various rapidities. [ABSTRACT FROM AUTHOR]

The forthcoming LHC measurement of the hadron multiplicity in proton-nucleus collisions is a crucial test of the kt, factorization and gluon saturation based models. Here, we provide quantitative predictions for the pseudorapidity distribution of charged particles produced in minimum-bias proton-nucleus collisions at the LHC based on the idea of gluon saturation in the color glass condensate framework. Our formulation gives good descriptions of the LHC and the Relativistic Heavy Ion Collider (RHIC) data for the charged hadron multiplicities in both proton-proton and nucleus-nucleus collisions, and also the deep inelastic scattering in the Hadron Electron Ring Accelerator (HERA) at small Bjorken-x. [ABSTRACT FROM AUTHOR]

We investigate the contribution of inelastic and elastic processes to single inclusive hadron production in proton-proton and proton (deuteron)-nucleus collisions at RHIC and the LHC. Using the hybrid formulation which includes both elastic and inelastic contributions, supplemented with the running-coupling Balitsky-Kovchegov equation, we get a good description of RHIC data. It is shown that inclusion of the inelastic terms makes the transverse momentum dependence of the production cross section steeper in the midrapidity region but does not affect the cross section in the very forward region. The inelastic processes also lead to a sharper increase of the nuclear modification factor RpA with increasing pT. We also make predictions for the nuclear modification factor in proton-nucleus collisions at the LHC (√s = 4.4 and 8.8 TeV) at various rapidities using the color glass condensate framework. [ABSTRACT FROM AUTHOR]

We show that the long-range rapidity correlations between the produced charged-hadron pairs from two Balitsky-Fadin-Kuraev-Lipatov parton showers generate considerable azimuthal angle correlations. These correlations have no 1/Nc suppression. The effect of gluon saturation on these correlations are discussed and we show that it is important. We show that a pronounced ridgelike structure emerges by going from the Balitsky-Fadin-Kuraev-Lipatov to the saturation region. We show that the ridge structure at high-energy proton-proton and nucleus-nucleus collisions has the same origin and its main feature can be understood due to initial-state effects. Although the effects of final-state interactions in the latter case can be non-negligible. [ABSTRACT FROM AUTHOR]

The recent results in √s=2.76??TeV Pb+Pb collisions at the Large Hadron Collider (LHC) reported by the ALICE collaboration shows that the power-law energy-dependence of charged hadron multiplicity in Pb+Pb collisions is significantly different from p+p collisions. We show that this different energy-dependence can be explained by inclusion of a strong angular-ordering in the gluon-decay cascade within the color glass condensate (or gluon saturation) approach. This effect is more important in nucleus-nucleus collisions where the saturation scale is larger than 1 GeV. Our prescription gives a good description of the LHC data both in p+p and Pb+Pb collisions. [ABSTRACT FROM AUTHOR]

We consider forward inclusive production of several quarks in the high energy p-A collisions in the CGC formalism. For three particle production we provide a complete expression in terms of multipole scattering amplitudes on the nucleus and multiparticle generalized TMD's of the proton. We then calculate all the terms that are not suppressed by the factor of the area in four particle production, and generalize this result up to terms of order 1/N2c for arbitrary number of produced particles. Our results include the contribution of quantum interference effects both in the final state radiation (HBT) and in the initial projectile wave function (Pauli blocking). [ABSTRACT FROM AUTHOR]

We consider forward inclusive production of two quarks in the high energy p-A collisions in the color-glass-condensate formalism. We demonstrate that the production cross section is determined by the convolution of the proton generalized double transverse momentum-dependent distribution functions with two independent eikonal scattering amplitudes: the product of two dipoles and a quadrupole. We explicitly demonstrate that the quadrupole amplitude term accounts for all the (initial and final state) effects of quantum statistics for identical fermions, and the correlations due to these effects. We also demonstrate that the effects due to quantum statistics (entirely encoded in the quadrupole) are parametrically leading contributions to the correlated particle production at large Nc. For nonidentical quarks the quadrupole term also leads to correlated production which (barring accidental cancellations) has characteristics similar to the Hanbury Brown-Twiss effect. [ABSTRACT FROM AUTHOR]

We introduce a technique to study double parton scattering (DPS) in the color-glass-condensate (CGC) approach. We show that the cross section of the DPS in the CGC approach is calculable in terms of new nonperturbative objects, generalized double transverse momentum-dependent parton distribution (2GTMD) functions. We investigate the production of pairs of prompt photons from two partons in the projectile hadron in high-energy proton-nucleus collisions. We show that even for independent partons in the projectile, the prompt photon correlation function exhibits Hanbury Brown and Twiss (HBT) correlations. The width of the HBT peak is controlled by the transverse distance between the partons of the pair, which is of the order of the proton size. Thus, the HBT measurements in two-particle production such as prompt photon pairs provide useful information about the nonperturbative 2GTMDs. [ABSTRACT FROM AUTHOR]

For ultrarelativistic proton-proton and proton-nucleus collisions, we perform an exploratory study of the contribution to the elliptic flow v2 coming from the orientation of the momentum of the produced particles with respect to the reaction plane. Via the color glass condensate factorization valid at high energies, this contribution is related to the orientation of a color dipole with respect to its impact parameter, which in turn probes the transverse inhomogeneity in the target. Using the McLerran-Venugopalan model (with impact-parameter dependence) as an effective description for the soft gluon distribution in the (proton or nuclear) target, we present a semianalytic calculation of the dipole-scattering amplitude, including its angular dependence. We find that the angular dependence is controlled by soft gluon exchanges and hence is genuinely nonperturbative. The effects of multiple scattering turn out to be essential (in particular, they change the sign of v2). We find that sizable values for v2, comparable to those observed in the LHC data and having a similar dependence upon the transverse momenta of the produced particles, can be easily generated via peripheral collisions. In particular, v2 develops a peak at a transverse momentum that scales with the saturation momentum in the target. [ABSTRACT FROM AUTHOR]