Prompt Photon and Inclusive π0\pi^0 Production at RHIC and LHC

We present results for prompt photon and inclusive $\pi^0$ production in p-p and A-A collisions at RHIC and LHC energies. We include the full next-to-leading order radiative corrections and nuclear effects, such as nuclear shadowing and parton energy loss. We find the next-to-leading order corrections to be large and $p_T$ dependent. We show how measurements of $\pi^0$ production at RHIC and LHC, at large $p_T$, can provide valuable information about the nature of parton energy loss. We calculate the ratio of prompt photons to neutral pions and show that at RHIC energies this ratio increases with $p_T$ approaching one at $p_T \sim 10$ GeV, due to the large suppression of $\pi^0$ production. We show that at the LHC, this ratio has steep $p_T$ dependence and approaches 10% effect at $p_T \sim 20$ GeV.Comment: Talk presented by I. Sarcevic, to appear in the Proceedings of Quark Matter 2002; 4 pages including 4 color figure

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.Comment: 25 pages, 16 postscript figure

Shadowing of gluons in perturbative QCD: A comparison of different models

We investigate the different perturbative QCD-based models for nuclear shadowing of gluons. We show that in the kinematic region appropriate to RHIC experiment, all models give similar estimates for the magnitude of gluon shadowing. At scales relevant to LHC, there is a sizable difference between predictions of the different models.Comment: 11 pages, 4 figure

Total gluon shadowing due to fluctuation effects

We show a new physical phenomenon expected for the ratio R_{pA} of the unintegrated gluon distribution of a nucleus over the unintegrated gluon distribution of a proton scaled up by the atomic factor A^{1/3} in the fluctuation-dominated (diffusive scaling) region at high energy. We calculate the dependence of R_{pA} on the atomic number A, the rapidity Y and the transverse gluon momentum k_{\perp}. We find that R_{pA} exhibits an increasing gluon shadowing with growing rapidity, approaching 1/A^{1/3} at asymptotic rapidities which means total gluon shadowing, due to the effect of gluon number fluctuations or Pomeron loops. The increase of R_{pA} with rising gluon momentum decreases as the rapidity grows. In contrast, in the geometric scaling region where the effect of fluctuations is negligible, the ratio R_{pA} shows only partial gluon shadowing in the fixed-coupling case, basically independent on the rapidity and the gluon momentum.Comment: 12 pages, 1 figure; two plots and further explanations added; matches version accepted for publicatio

Photon production in high energy proton-nucleus collisions

We calculate the photon production cross-section in $pA$ collisions under the assumption that the nucleus has reached the saturation regime, while the proton can be described by the standard parton distribution functions. We show that due to the strong classical field $O(1/g)$ of the nucleus, bremsstrahlung diagrams become dominant over the direct photon diagrams. In particular, we show that $\gamma-$jet transverse momentum spectrum and correlations are very sensitive to gluon saturation effects in the nucleus.Comment: 15 pages, 2 figure

Properties of inclusive hadron production in Deep Inelastic Scattering on heavy nuclei at low x

In this paper we present a comprehensive study of inclusive hadron production in DIS at low $x$. Properties of the hadron spectrum are different in different kinematic regions formed by three relevant momentum scales: photon virtuality $Q^2$, hadron transverse momentum $k_T$ and the saturation momentum $Q_s(x)$. We investigate each kinematic region and derive the corresponding asymptotic formulas for the cross section at the leading logarithmic order. We also analyze the next-leading-order (NLO) corrections to the BFKL kernel that are responsible for the momentum conservation. In particular, we establish the asymptotic behavior of the forward elastic dipole--nucleus scattering amplitude at high energies deeply in the saturation regime and a modification of the pomeron intercept. We study the nuclear effect on the inclusive cross section using the nuclear modification factor and its logarithmic derivative. We argue that the later is proportional to the difference between the anomalous dimension of the gluon distribution in nucleus and in proton and thus is a direct measure of the coherence effects. To augment our arguments and present quantitative results we performed numerical calculations in the kinematic region that may be accessible by the future DIS experiments.Comment: 29 pages, 8 figure

Effective field theory for the small-x evolution

The small-x behavior of structure functions in the saturation region is determined by the non-linear generalization of the BFKL equation. I suggest the effective field theory for the small-x evolution which solves formally this equation. The result is the 2+1 functional integral for the structure functions at small x.Comment: 5 pages, latex, 2 eps figure

The Wilson renormalization group for low x physics: towards the high density regime

We continue the study of the effective action for low $x$ physics based on a Wilson renormalization group approach. We express the full nonlinear renormalization group equation in terms of the average value and the average fluctuation of extra color charge density generated by integrating out gluons with intermediate values of $x$. This form clearly exhibits the nature of the phenomena driving the evolution and should serve as the basis of the analysis of saturation effects at high gluon density at small $x$.Comment: 14 pages, late

Dilepton production from the Color Glass Condensate

We consider dilepton production in high energy proton-nucleus (and very forward nucleus-nucleus) collisions. Treating the target nucleus as a Color Glass Condensate and describing the projectile proton (nucleus) as a collection of quarks and gluons as in the parton model, we calculate the differential cross section for dilepton production in quark-nucleus scattering and show that it is very sensitive to the saturation scale characterizing the target nucleus.Comment: 9 pages LaTeX document, 1 postscript figur