Nuclear theory summer meeting on ERHIC

Abstract

PrefaceThe eRHIC BNL summer meeting was held at BNL from June 26 to July 14, 2000. The meeting was very informal with only two talks a day and with ample time for discussions and collaborations.Several of the theory talks focused on the issue of saturation of par-ton distributions at small x -whether screening effects have already been seen at HERA, the relation of saturation to shadowing, and on the various signatures of a proposed novel state of matter -the Colored Glass Condensate -that may be observed at eRHIC. A related topic that was addressed was that of quantifying twist four effects, and on the relevance of these for studies of energy loss. Other issues addressed were coherence effects in vector meson production, anti-quark distributions in nuclei, and the relevance of saturation for heavy ion cbllisions. There were, also, talks on the Pomeronthe relevance of instantons and the non-perturbative gluon condensate to constructing a Pomeron.On the spin physics side, there were talks on predictions for inclusive distributions at small x. There were, also, talks on Skewed Parton Distributions and Deeply Virtual Compton Scattering.Though most of the talks were theory talks, there were, also, several important experimental contributions.A preliminary detector design for eRHIC was presented. Studies for semi-inclusive measurements at eRHIC were also presented. .The current status of pA scattering studies at RHIC was also discussed.The eRHIC summer meeting provided a vigorous discussion of the current status of eRHIC studies. It is hoped that this document summarizing these discussions will be of use to all those interested in electron nucleus and polarized electron-polarized proton studies. A deep connection between diffraction in electron-proton scattering and the nuclear shadowing phenomenon is emphasized. We argue that results of the diffractive studies at HERA allow us to make reliable predictions for shadowing and coherent diffractive cross sections in inclusive eA scattering in a wide range of x. Model-independent leading twist expressions for shadowing for the interaction with two nucleons are given. Large shadowing in the gluon channel is predicted based on the current evidence of the dominance of gluons in the "perturbative Pomeron". We demonstrate that so called eikonal models of nuclear shadowing grossly underestimate the am.ount of shadowing for the interaction of small dipoles, primarily due to the inadequacy of the fixed number of particle approximation. L. McLerran R. Venugopalan iiPredictions for the fraction of events in small x electron-nucleus scattering due to coherent and incoherent diffraction are also given. I. SHADOWING AND DIFFRACTIONA long time ago V.Gribov [l] investigated the analytic structure of the amplitude for the interaction of highenergy hadrons (photons) with nuclei and established an unambiguous connection between the cross section of small t diffraction of a (virtual) photon or a hadron off the nucleon and the amount of shadowing in the interaction of the projectile with a n...

d+Au at PHENIX: Insights on the Cronin effect, shadowing and saturation

“…The suppression at forward rapidity is large, exceeding predictions from gluon shadowing as parameterized in EPS09 [2] and a J/ψ breakup cross section in the nuclear medium of 4 mb. A larger suppression, very similar to that observed in the data, is expected from gluon saturation, where non-linear gluonic interactions limit, in effect, the gluon density [3,4]. Both the centrality and the rapidity dependence of the suppression magnitude are closer to saturation expectations.…”

Scale evolution of nuclear parton distributions

“…5 Also the coordinate space description of nuclear parton densities [43] could be studied in more detail by using the scale evolved nuclear ratios we have presented here. Related to the coordinate space description and scale evolution of nuclear parton distributions, the connection of our results with those in the region of very small x and very large A where the higher twist terms can be expected to be more important [26], should also be studied in more detail.…”

Smallxgluons in nuclei and hadrons

“…To do this, one would need to include the relevant higher twist effects not only in the evolution of gluon distribution (as is done here) but also in the relation between the cross section (or F 2 ) and gluon distribution function. In [26], the all twist structure function, F 2 , was computed in the infinite momentum frame (see also [7,27] for a similar calculation in the lab frame) so that all one has to do is to merge the two results [28].…”