Polarized Photoproduction of Heavy Quarks in Next-to-Leading Order

The results of a next-to-leading order calculation of heavy quark production in longitudinally polarized photon-nucleon collisions are presented. At c.m. energy $\sqrt{S}=10$ GeV, for $\vec \gamma +\vec p \to c+X$, cross sections differential in the transverse momentum and rapidity of the charmed quark $c$ and the corresponding asymmetries are presented; also, as functions of $\sqrt{S}$, integrated cross sections, $K$-factors and the corresponding asymmetries are given. Errors in the asymmetries are estimated and the possibility to distinguish between three scerarios differing essentially in the polarized gluon distribution is discussed.Comment: 8 pages, 3 figure

Cronin Effect in Hadron Production off Nuclei

Recent data from RHIC for high-$p_T$ hadrons in gold-gold collisions raised again the long standing problem of quantitatively understanding the Cronin effect, i.e. nuclear enhancement of high-$p_T$ hadrons due to multiple interactions in nuclear matter. In nucleus-nucleus collisions this effect has to be reliably calculated as baseline for a signal of new physics in high-$p_T$ hadron production. The only possibility to test models is to compare with available data for $pA$ collisions, however, all existing models for the Cronin effect rely on a fit to the data to be explained. We develop a phenomenological description based on the light-cone QCD-dipole approach which allows to explain available data without fitting to them and to provide predictions for $pA$ collisions at RHIC and LHC. We point out that the mechanism causing Cronin effect drastically changes between the energies of fixed target experiments and RHIC-LHC. High-$p_T$ hadrons are produced incoherently on different nucleons at low energies, whereas the production amplitudes interfere if the energy is sufficiently high.Comment: the final version to appear in Phys. Rev. Let

Transparent Nuclei and Deuteron-Gold Collisions at RHIC

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.Comment: Latex 50 pages. Based on lectures given by the author at Workshop on High-pT Correlations at RHIC, Columbia University, May-June, 2003. The version to appear in PR

Families of N=2 Strings

In a given 4d spacetime bakcground, one can often construct not one but a family of distinct N=2 string theories. This is due to the multiple ways N=2 superconformal algebra can be embedded in a given worldsheet theory. We formulate the principle of obtaining different physical theories by gauging different embeddings of the same symmetry algebra in the same ``pre-theory.'' We then apply it to N=2 strings and formulate the recipe for finding the associated parameter spaces of gauging. Flat and curved target spaces of both (4,0) and (2,2) signatures are considered. We broadly divide the gauging choices into two classes, denoted by alpha and beta, and show them to be related by T-duality. The distinction between them is formulated topologically and hinges on some unique properties of 4d manifolds. We determine what their parameter spaces of gauging are under certain simplicity ansatz for generic flat spaces (R^4 and its toroidal compactifications) as well as some curved spaces. We briefly discuss the spectra of D-branes for both alpha and beta families.Comment: 66+1 pages, 2 tables, latex 2e, hyperref. ver2: typos corrected, reference adde

Next-to-leading order QCD corrections to one hadron-production in polarized pp collisions at RHIC

We calculate the next-to-leading order QCD corrections to the spin-dependent cross section for single-inclusive hadron production in hadronic collisions. This process will be soon studied experimentally at RHIC, providing a tool to unveil the polarized gluon distribution $\Delta g$. We observe a considerably improvement in the perturbative stability for both unpolarized and polarized cross sections. The NLO corrections are found to be non-trivial, resulting in a reduction of the asymmetry.Comment: 8 pages, RevTeX4, 9 figures include

Shadowing in neutrino deep inelastic scattering and the determination of the strange quark distribution

We discuss shadowing corrections to the structure function $F_2$ in neutrino deep-inelastic scattering on heavy nuclear targets. In particular, we examine the role played by shadowing in the comparison of the structure functions $F_2$ measured in neutrino and muon deep inelastic scattering. The importance of shadowing corrections in the determination of the strange quark distributions is explained.Comment: 22 pages, 7 figure

Prompt photons at RHIC

We calculate the inclusive cross section for prompt photon production in heavy-ion collisions at RHIC energies ($\sqrt{s}=130$ GeV and $\sqrt{s}=200$ GeV) in the central rapidity region including next-to-leading order, $O(\alpha_{em}\alpha_s^2)$, radiative corrections, initial state nuclear shadowing and parton energy loss effects. We show that there is a significant suppression of the nuclear cross section, up to $\sim 30$ at $\sqrt{s}=200$ GeV, due to shadowing and medium induced parton energy loss effects. We find that the next-to-leading order contributions are large and have a strong $p_t$ dependence.Comment: 9 pages, 5 figures, expanded discussion of the K facto

An Alternative Method to Obtain the Quark Polarization of the Nucleon

An alternate method is described to extract the quark contribution to the spin of the nucleon directly from the first moment of the deuteron structure function, $g^d_1$. It is obtained without recourse to the use of input on the nucleon wave function from hyperon decays involving the flavor symmetry parameters, F and D. The result for the quark polarization of the nucleon, $\Delta\Sigma_ N,$ is in good agreement with the values of the singlet axial current matrix element, $a_0$, obtained from recent next-to-leading order analyses of current proton, neutron and deuteron data.Comment: 7 pages, 1 figur

Double parton scatterings in b-quark pairs production at the LHC

A sizable rate of events where two pairs of b-quarks are produced contemporarily is foreseen at the CERN LHC, as a consequence of the large parton luminosity. At very high energies both single and the double parton scatterings contribute to the process, the latter mechanisms, although power suppressed, giving the dominant contribution to the integrated cross section.Comment: 17 pages, 6 figure

The fully differential single-top-quark cross section in next-to-leading order QCD

We present a new next-to-leading order calculation for fully differential single-top-quark final states. The calculation is performed using phase space slicing and dipole subtraction methods. The results of the methods are found to be in agreement. The dipole subtraction method calculation retains the full spin dependence of the final state particles. We show a few numerical results to illustrate the utility and consistency of the resulting computer implementations.Comment: 37 pages, latex, 2 ps figure