Transparent Nuclei and Deuteron-Gold Collisions at RHIC

Abstract

The current normalization of the cross section of inclusive high-pT particle production in deuteron-gold collisions measured RHIC relies on Glauber calculations for the inelastic d-Au cross section. These calculations should be corrected for diffraction. Moreover, they miss the Gribov's inelastic shadowing which makes nuclei more transparent (color transparency). The magnitude of this effect rises with energy and it may dramatically affect the normalization of the RHIC data. We evaluate these corrections employing the light-cone dipole formalism and found a rather modest corrections for the current normalization of the d-Au data. The results of experiments insensitive to diffraction (PHENIX, PHOBOS) should be renormalized by about 20% down, while those which include diffraction (STAR), by only 10%. Such a correction completely eliminates the Cronin enhancement in the PHENIX data for pions. The largest theoretical uncertainty comes from the part of the inelastic shadowing which is related to diffractive gluon radiation, or gluon shadowing. Our estimate is adjusted to data for the triple-Pomeron coupling, however, other models do not have such a restrictions and predict much stronger gluon shadowing. Therefore, the current data for high-pT hadron production in d-Au collisions at RHIC cannot exclude in a model independent way the possibility if initial state suppression proposed by Kharzeev-Levin-McLerran. Probably the only way to settle this uncertainty is a direct measurement of the inelastic d-Au cross sections at RHIC. Also d-Au collisions with a tagged spectator nucleon may serve as a sensitive probe for nuclear transparency and inelastic shadowing. We found an illuminating quantum-mechanical effect: the nucleus acts like a lens focusing spectators into a very narrow cone.