Transparent Nuclei and Deuteron-Gold Collisions at RHIC

TL;DR

This work reevaluates the RHIC deuteron-gold high-pT cross-section normalization by incorporating Gribov inelastic shadowing, diffraction, and gluon-shadowing effects via a light-cone dipole formalism. It extends the Glauber model to dA collisions, analyzes spectator-tagged channels, and sums inelastic corrections using both intermediate-state diffraction and an eigenstate approach, yielding modest overall corrections with notable model dependence for gluon Shadowing. The study shows that diffractive sensitivity of the experimental trigger strongly influences N_coll and that gluon shadowing at RHIC is relatively weak, which together suggest that existing dAu data cannot decisively rule out initial-state suppression scenarios without direct inelastic cross-section measurements or tagged spectator studies. Consequently, a careful renormalization of RHIC data (with K factors around 0.8–0.95) can eliminate the observed Cronin enhancement in pT spectra, underscoring the need for direct cross-section measurements to settle the underlying physics of cold nuclear matter effects. The results highlight quantum-mechanical focusing of spectator nucleons and propose experimental tests that can constrain nuclear transparency and shadowing models in high-energy nuclear collisions.

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.

Figures (11)

• Figure 1: Data and calculations murthy for the total neutron-lead cross section as function of energy. The dashed curve corresponds to the Glauber model, while the solid curve is corrected for Gribov's inelastic shadowing.
• Figure 2: The impact parameter distribution of inelastic deuteron-gold collisions (three upper curves) including diffractive excitations (STAR trigger). Impact parameter $\vec{b}$ corresponds to center of gravity of the deuteron. The dashed curve corresponds to the Glauber approximation Eq. (\ref{['39']}). The thin solid curve include inelastic shadowing related to excitation of the valence quark skeleton, Eq. (\ref{['63']}). The thick solid curve is final, it includes gluon shadowing as well. The bottom solid thick curve shows the difference between the Glauber and final curves. The dotted curve shows the range of model uncertainty and corresponds to gluon shadowing with $R_G=03$ (see Sect. \ref{['models']}). All curves are calculated with total cross section $\tilde{\sigma_{tot}^{NN}}=51\,\hbox{mb}$.
• Figure 3: The impact parameter distribution of interacting protons in tagged deuteron-gold collisions with spectator neutrons. The calculation includes diffractive excitations (STAR trigger). Impact parameter $\vec{b}$ corresponds to the proton. The dashed curve represents the Glauber approximation Eq. (\ref{['39']}). The thin solid curve include inelastic shadowing related to excitation of the valence quark skeleton, Eq. (\ref{['63']}). The thick solid curve includes gluon shadowing as well. All curves are calculated with total cross section $\tilde{\sigma_{tot}^{NN}}=51\,\hbox{mb}$.
• Figure 4: The impact parameter distribution of spectator neutrons in tagged deuteron-gold collisions with interacting protons. The calculation includes diffractive excitations (STAR trigger). Impact parameter $\vec{b}$ corresponds to the proton. The dashed curve represents the Glauber approximation Eq. (\ref{['39']}). The thin solid curve include inelastic shadowing related to excitation of the valence quark skeleton, Eq. (\ref{['63']}). The thick solid curve includes gluon shadowing as well. All curves are calculated with total cross section $\tilde{\sigma_{tot}^{NN}}=51\,\hbox{mb}$.
• Figure 5: Transverse momentum distribution of spectator neutrons in the tagged reaction $d+Au\to n+X$ (solid curve), and in the projectile deuteron (dashed curve). The inelastic reaction $p+Au\to X$ is assumed to include diffraction (STAR experiment). The calculations are performed in the Glauber approximation, Eq. (\ref{['47']}).