Mark Strikman

Color screening and color transparency in hard nuclear processes

Abstract We explain why color screening is expected to be essential in the interaction of hadrons in QCD both in perturbative and nonperturbative domains. Current ideas and approximations characteristic for the color transparency phenomenon in QCD are discussed. Space-time development of hard processes at large and moderate energies is considered. We show that Bjorken scaling in deep inelastic processes at small x Bj is another form of the color transparency. We argue that the color transparency phenomenon is an extension of the known similarity between QCD and QED for hard processes to the exclusive processes. The possible role of the color screening in the microscopic nuclear structure is also discussed. It is shown that color transparency phenomenon should be manifested in the wide range of phenomena including nuclear shadowing in the deep inelastic processes at small x Bj , large angle quasielastic lepton (hadron) high energy processes off nuclei, the EMC effect at x Bj > 0.3, diffractive electro and photo production of charmonium states, production of nucleon spectators in high-energy central nucleus-nucleus collisions, etc.

Intrinsic transverse momentum and parton correlations from nonperturbative short-range interactions

We summarize recent progress in understanding the effects of nonperturbative short-range interactions in QCD on the nucleons partonic structure at a low scale: (a) Sea quarks have intrinsic transverse momenta up to the chiral symmetry-breaking scale rho^{-1} ~ 0.6 GeV, much larger than those of valence quarks. (b) Sea quarks in the nucleon light-cone wave function exist partly in correlated in pairs of transverse size rho with sigma and pi-like quantum numbers and a distinctive spin structure (L = 1 components). The effects are demonstrated in an effective model of the low-energy dynamics resulting from chiral symmetry breaking in QCD. They have numerous implications for the P_T distribution of hadrons in semi-inclusive DIS and multiparton processes in high-energy pp collisions.

LEADING TWIST SHADOWING, ONSET OF BLACK DISK REGIME AND POST-SELECTION PHENOMENON

We summarize results of the Generalization of the Gribov picture of nuclear shadowing in high energy processes o nuclei for calculatin g shadowing of the parton distributions in nuclei and some properties of the final states in small x processes in DIS are described. It is explained how account of the restrictions due to the energy-momentum conservation allows to choose approximations eective for the description of small x phenomena. Estimates of proximity of the pQCD interaction of small dipoles to the black disk limit are provided and the phenomenon of post-selection suppression of leading parton spectrum (eective fractional energy losses) in the proximity of the black disk regime in DIS is described. We argue that the resulting suppression of the the leading hadron production provides one of the most sensitive signals of the onset of new QCD dynamics. Evidence for importance of the post-selection phenomenon in the production of the leading pions in the d Au collisions studied at RHIC is presented. We also outline briefly impact of rapid increase of parton distributions within pQCD at small x on the interplay of soft and hard QCD dynamics at the LHC.

Pion cloud and sea quark flavor asymmetry in the impact parameter representation

We study large-distance contributions to the nucleon parton densities in the transverse coordinate (impact parameter) representation based on generalized parton distributions (GPDs). Chiral dynamics generates a distinct component of the partonic structure, located at momentum fractions x ~ 0, and the universal large-distance pion cloud at b > b_core -- could naturally account for the x-dependence of the measured asymmetry.

Imaging the proton via hard exclusive production in diffractive pp scattering

We discuss the prospects for probing Generalized Parton Distributions (GPDs) via exclusive production of a high-mass system (H = heavy quarkonium, di-photon, di-jet, Higgs boson) in diffractive pp scattering, pp -> p + H + p. In such processes the interplay of hard and soft interactions gives rise to a diffraction pattern in the final-state proton transverse momenta, which is sensitive to the transverse spatial distribution of partons in the colliding protons. We comment on the plans for diffractive pp measurements at RHIC and LHC. Such studies could complement future measurements of GPDs in hard exclusive ep scattering (JLab, COMPASS, EIC).

Color transparency in nucleus-nucleus collisions

We predict that probability for a nucleon to traverse nucleus in the central collisions should noticeably increase with energy increase between BNL and CERN energies due to color screening phenomenon. At RHIC energies this effect should practically disappear if perturbative QCD scenario is applicable.

Short-range nucleon correlations and neutrino emission by neutron stars at low temperatures

We argue that significant probability of protons with momenta above their Fermi surface leads for proton concentrations p/n > 1/8 to the enhancement of termally excited direct and modified URCA processes within a cold neutron star, and to a nonzero probability of direct URCA processes for small proton concentrations (p/n < 1/8). We evaluate high momentum tails of neutron, proton and electrons distributions within a neutron star. We expect also significantly faster neutrino cooling of hyperon stars.

High Q2 structure functions of nuclei-open questions

Abstract The aim of this talk to explain existence of a number open problems in investigations of parton distributions at x e (0.01 – 1.5) in nuclei as well as in the studies of higher twist contributions to the high Q 2 structure functions of nuclei which are important for understanding of the structure of bound states in QCD. Main emphasis will be on the measurements which would be feasible using high intensity electron machine in the 20 GeV energy range.