Jamal Jalilian-Marian

Saturation physics and deuteron-Gold collisions at RHIC

Abstract We present a review of parton saturation/Color Glass Condensate physics in the context of deuteron–gold ( d + Au ) collisions at RHIC. Color Glass Condensate physics is a universal description of all high energy hadronic and nuclear interactions. It comprises classical (McLerran–Venugopalan model and Glauber–Mueller rescatterings) and quantum evolution (JIMWLK and BK equations) effects both in small- x hadronic and nuclear wave functions and in the high energy scattering processes. Proton–nucleus (or d + A ) collisions present a unique opportunity to study Color Glass Condensate predictions, since many relevant observables in proton–nucleus collisions are reasonably well-understood theoretically in the Color Glass Condensate approach. In this article we review the basics of saturation/Color Glass Condensate physics and reproduce derivations of many important observables in proton (deuteron)–nucleus collisions. We compare the predictions of Color Glass physics to the data generated by d + Au experiments at RHIC and observe an agreement between the data and the theory, indicating that Color Glass Condensate has probably been discovered at RHIC. We point out further experimental measurements which need to be carried out to test the discovery.

The color glass condensate and hadron production in the forward region

Abstract We consider one loop corrections to single inclusive particle production in parton–nucleus scattering at high energies, treating the target nucleus as a color glass condensate. We prove by explicit computation that in the leading log Q 2 approximation, these corrections lead to collinear factorization and DGLAP evolution of the projectile parton distribution and hadron fragmentation functions. In single-inclusive cross sections, only two-point functions of Wilson lines in the adjoint and fundamental representations (Muellers dipoles) arise, which can be obtained from the solution of the JIMWLK equations. The application of our results to forward-rapidity production shows that, in general, recoil effects are large. Hence, the forward rapidity region at RHIC is rather different from the central region at LHC, despite comparable gluon densities in the target. We show that both the quantum x-evolution of the high-density target as well as the DGLAP Q 2 -evolution of the parton distribution and fragmentation functions are clearly seen in the BRAHMS data. This provides additional strong evidence for the color glass condensate at RHIC.

Geometric scaling violations in the central rapidity region of d+Au collisions at RHIC

Abstract We show that geometric scaling is satisfied to good accuracy in the forward region of d + Au collisions at RHIC. Scaling violations do show up, however, at mid-rapidity, and the anomalous dimension of the small- x gluon distribution evolves to near its DGLAP limit for transverse momenta of a few GeV. This represents a first consistency check of RHIC deuteron–nucleus and HERA DIS phenomenology, and of the universality of the underlying color glass condensate (CGC) theory, which describes both phenomena. It also reconciles successful leading-twist LO and NLO perturbative QCD computations of mid-rapidity particle production with small- x evolution. Finally, we introduce a new parameterization for the anomalous dimension of the small- x gluon distribution which properly reproduces known theoretical limits at large rapidity, at large virtuality, and on the saturation boundary, and still fits the available data from d + Au collisions at RHIC. We find indications that sub-asymptotic terms in the rapidity-evolution of the anomalous dimension are large.

Forward rapidity hadron production in deuteron–gold collisions from valence quarks

Abstract We consider hadron production in deuteron–gold collisions at RHIC in the forward rapidity region. Treating the target nucleus as a color glass condensate and the projectile deuteron as a dilute system of valence quarks, we obtain good agreement with the BRAHMS minimum bias data on charged hadron production in the forward rapidity ( y = 3.2 ) and low p t region. We provide predictions for neutral pion production in minimum bias deuteron–gold collisions in the forward rapidity region, y = 3.8 , measured by the STAR Collaboration at RHIC.

Electromagnetic signatures of the color glass condensate: dileptons

Abstract We evaluate the invariant cross section for production of dileptons in forward rapidities at RHIC and LHC, using the color glass condensate formalism and present results for the nuclear modification factor R d ( p ) A as a function of dilepton invariant mass for the most central deuteron (proton)–nucleus collisions.

Production of forward rapidity photons in high energy heavy ion collisions

Abstract We consider production of prompt photons in high energy gold–gold and deuteron–gold collisions in the forward rapidity region of RHIC ( y ∼ 3.8 ). In this kinematics, the projectile partons typically have large x b j while the target partons are mostly at very small x b j , so that the primary partonic collisions involve valence quarks from the projectile and gluons from the target. We take the target nucleus to be a color glass condensate while the projectile deuteron or nucleus is treated as a dilute system of partons. We show that the photon production cross section can be written as a convolution of a quark–nucleus scattering cross section, involving a quark–antiquark dipole, with the leading order quark–photon fragmentation function. We consider different models of the quark–antiquark dipole and show that measurement of photons in the forward rapidity region at RHIC can distinguish between different parameterizations of the dipole cross section as well as help clarify the role of parton coalescence models in hadron production at RHIC.

Drell-Yan production and Lam-Tung relation in the color glass condensate formalism

We study the Drell-Yan production cross section and structure functions in proton (deuteron)-nucleus collisions using the color glass condensate formalism. The nucleus is treated in the color glass condensate framework which includes both higher twist effects due to the inclusion of multiple scatterings and leading twist pQCD shadowing due to the small x resummation, while the proton (or deuteron) is treated within the DGLAP improved parton model. In particular, the Drell-Yan structure functions are used in order to investigate the Lam-Tung relation at small x, which is known to be identically zero at leading twist up to next-to-leading order, and is thus a good playground for studying higher twist effects. In agreement with this, we find that violations of this relation are more important for low momentum and invariant mass of the Drell-Yan pair, and also in the region of rapidity that corresponds to smaller values of x in the nucleus.

Photon + hadron production in high energy deuteron (proton)–nucleus collisions

Abstract We apply the color glass condensate formalism to photonxa0+xa0hadron production cross section in high energy deuteron (proton)–gold collisions at RHIC. We investigate the dependence of the production cross section on the angle between the produced hadron and photon for various rapidities and transverse momenta. It is shown that the angular correlation between the produced hadron and photon is a sensitive probe of the saturation dynamics.

Colour glass condensate at RHIC

The colour glass condensate formalism and its application to high-energy heavy-ion collisions at RHIC are discussed. We argue that the RHIC data support the view that the colour glass condensate provides the initial conditions for gold–gold collisions at RHIC while final-state (quark–gluon–plasma) effects are responsible for the high pt physics in mid-rapidity. At forward-rapidities in deuteron–gold collisions, however, the colour glass condensate is the underlying physics of the observed suppression of the particle spectra and their centrality dependence INT-PUB 04-07.

Ultra-High Energy Neutrino-Nucleon Scattering and Parton Distributions at Small x

The cross section for ultra‐high energy neutrino‐nucleon scattering is very sensitive to the parton distributions at very small values of Bjorken x (x ⩽ 10−4). We numerically investigate the effects of modifying the behavior of the gluon distribution function at very small x in the DGLAP evolution equation. We then use the Color Glass Condensate formalism to calculate the neutrino‐nucleon cross section at ultra‐high energies and compare the result with those based on modification of DGLAP evolution equation.