Single-Transverse Spin Asymmetry in Dijet Correlations at Hadron Colliders

We present a phenomenological study of the single-transverse spin asymmetry in azimuthal correlations of two jets produced nearly "back-to-back" in pp collisions at RHIC. We properly take into account the initial- and final-state interactions of partons that can generate this asymmetry in QCD hard-scattering. Using distribution functions fitted to the existing single-spin data, we make predictions for various weighted single-spin asymmetries in dijet correlations that are now readily testable at RHIC.Comment: 14 pages, 2 figure

Fusion of light proton-rich exotic nuclei at near-barrier energies

We study theoretically fusion of the light proton-rich exotic nuclei $^{17}$F and $^8$B at near-barrier energies in order to investigate the possible role of breakup processes on their fusion cross sections. To this end, coupled channel calculations are performed considering the couplings to the breakup channels of these projectiles. In case of $^{17}$F, the coupling arising out of the inelastic excitation from the ground state to the bound excited state and its couplings to the continuum have also been taken into consideration. It is found that the inelastic excitation/breakup of $^{17}$F affect the fusion cross sections very nominally even for a heavy target like Pb. On the other hand, calculations for fusion of the one-proton halo nucleus $^8$B on a Pb target show a significant suppression of the complete fusion cross section above the Coulomb barrier. This is due to the larger breakup probability of $^8$B as compared to that of $^{17}$F. However, even for $^8$B, there is little change in the complete fusion cross sections as compared to the no-coupling case at sub-barrier energies.Comment: 11 pages, 3 figures, Revtex.st

Single Transverse-Spin Asymmetry in Drell-Yan Production at Large and Moderate Transverse Momentum

We study the single-transverse spin asymmetry for the Drell-Yan process. We consider production of the lepton pair at large transverse momentum, $q_\perp\sim Q$, where $Q$ is the pair's mass. The spin asymmetry is then of higher twist and may be generated by twist-three quark-gluon correlation functions. Expanding the result for $q_\perp\ll Q$, we make contact with the transverse-momentum-dependent QCD factorization involving the so-called Sivers functions. We find that the two mechanisms, quark-gluon correlations on one hand and the Sivers effect on the other, contain the same physics in this region. This ties the two together and imposes an important constraint on phenomenological studies of single transverse spin asymmetries.Comment: 29 pages, 10 figure

Single Transverse-Spin Asymmetries at Large-x

The large-$x$ behavior of the transverse-momentum dependent quark distributions is analyzed in the factorization-inspired perturbative QCD framework, particularly for the naive time-reversal-odd quark Sivers function which is responsible for the single transverse-spin asymmetries in various semi-inclusive hard processes. By examining the dominant hard gluon exchange Feynman diagrams, and using the resulting power counting rule, we find that the Sivers function has power behavior $(1-x)^4$ at $x \to 1$, which is one power of $(1-x)$ suppressed relative to the unpolarized quark distribution. These power-counting results provide important guidelines for the parameterization of quark distributions and quark-gluon correlations.Comment: 20 pages, 4 figure

Semi-classical Characters and Optical Model Description of Heavy Ion Scattering, Direct Reactions, and Fusion at Near-barrier Energies

An approach is proposed to calculate the direct reaction (DR) and fusion probabilities for heavy ion collisions at near-Coulomb-barrier energies as functions of the distance of closest approach D within the framework of the optical model that introduces two types of imaginary potentials, DR and fusion. The probabilities are calculated by using partial DR and fusion cross sections, together with the classical relations associated with the Coulomb trajectory. Such an approach makes it possible to analyze the data for angular distributions of the inclusive DR cross section, facilitating the determination of the radius parameters of the imaginary DR potential in a less ambiguous manner. Simultaneous $\chi^{2}$-analyses are performed of relevant data for the $^{16}$O+$^{208}$Pb system near the Coulomb-barrier energy

Fusion of light exotic nuclei at near-barrier energies : effect of inelastic excitation

The effect of inelastic excitation of exotic light projectiles (proton- as well as neutron-rich) $^{17}$F and $^{11}$Be on fusion with heavy target has been studied at near-barrier energies. The calculations have been performed in the coupled channels approach where, in addition to the normal coupling of the ground state of the projectile to the continuum, inelastic excitation of the projectile to the bound excited state and its coupling to the continuum have also been taken into consideration. The inclusion of these additional couplings has been found to have significant effect on the fusion excitation function of neutron-rich $^{11}$Be on $^{208}$Pb whereas the effect has been observed to be nominal for the case of proton-rich $^{17}$F on the same target. The pronounced effect of the channel coupling on the fusion process in case of $^{11}$Be is attributed to its well-developed halo structure.Comment: 9 pages, 3 figures, Revtex.st

Is Strangeness still interesting at RHIC ?

With the advent of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL), Heavy Ion Physics will enter a new energy regime. The question is whether the signatures proposed for the discovery of a phase transition from hadronic matter to a Quark Gluon Plasma (QGP), that were established on the basis of collisions at the BEVALAC, the AGS, and the SPS, respectively, are still useful and detectable at these high incident energies. In the past two decades, measurements related to strangeness formation in the collision were advocated as potential signatures and were tested in numerous fixed target experiments at the AGS and the SPS. In this article I will review the capabilities of the RHIC detectors to measure various aspects of strangeness, and I will try to answer the question whether the information content of those measurements is comparable to the one at lower energies.Comment: 12 pages, 7 figures, Invited Talk at the IV International Conference on Strangeness in Quark Matter, Padova (Italy), July 20-24, 199

Evolution of Baryon-Free Matter Produced in Relativistic Heavy-Ion Collisions

A 3-fluid hydrodynamic model is introduced for simulating heavy-ion collisions at incident energies between few and about 200 AGeV. In addition to the two baryon-rich fluids of 2-fluid models, the new model incorporates a third, baryon-free (i.e. with zero net baryonic charge) fluid which is created in the mid-rapidity region. Its evolution is delayed due to a formation time $\tau$, during which the baryon-free fluid neither thermalizes nor interacts with the baryon-rich fluids. After formation it thermalizes and starts to interact with the baryon-rich fluids. It is found that for $\tau$=0 the interaction strongly affects the baryon-free fluid. However, at reasonable finite formation time, $\tau$=1 fm/c, the effect of this interaction turns out to be substantially reduced although still noticeable. Baryonic observables are only slightly affected by the interaction with the baryon-free fluid.Comment: 17 pages, 3 figures, submitted to the issue of Phys. of Atomic Nuclei dedicated to S.T. Belyaev on the occasion of his 80th birthday, typos correcte

Density and expansion effects on pion spectra in relativistic heavy-ion collisions

We compute the pion inclusive momentum distribution in heavy-ion collisions at AGS energies, assuming thermal equilibrium and accounting for density and expansion effects at the time of decoupling. We compare to data on mid rapidity charged pions produced in central Au + Au collisions and find a very good agreement. The shape of the distribution at low $m_t-m$ is explained in part as an effect arising from the high mean pion density achieved in these reactions. The difference between the positive and negative pion distributions in the same region is attributed in part to the different average yields of each kind of charged pions.Comment: Minor changes, typo in Fig. 2b corrected, version to appear in Phys. Rev.

Centrality dependence of charged-particle pseudorapidity distributions from d+Au collisions at sqrt(s_{NN})=200 GeV

Charged-particle pseudorapidity densities are presented for the d+Au reaction at sqrt{s_{NN}}=200 GeV with -4.2 <= eta <= 4.2$. The results, from the BRAHMS experiment at RHIC, are shown for minimum-bias events and 0-30%, 30-60%, and 60-80% centrality classes. Models incorporating both soft physics and hard, perturbative QCD-based scattering physics agree well with the experimental results. The data do not support predictions based on strong-coupling, semi-classical QCD. In the deuteron-fragmentation region the central 200 GeV data show behavior similar to full-overlap d+Au results at sqrt{s_{NN}}=19.4 GeV.Comment: 4 pages, 3figures; expanded discussion of uncertainties; added 60-80% centrality range; added additional discussion on centrality selection bia