I. K. Potashnikova

Breakdown of QCD factorization at large Feynman x

Recent measurements by the BRAHMS Collaboration of high-p{sub T} hadron production at forward rapidities at the BNL Relativistic Heavy Ion Collider found the relative production rate (d-Au)/(p-p) to be suppressed rather than enhanced. Examining other known reactions (forward production of light hadrons, the Drell-Yan process, heavy flavor production, etc.), one notes that all of these display a similar property, namely, their cross sections in nuclei are suppressed at large x{sub F}. Since this is the region where x{sub 2} is minimal, it is tempting to interpret this as a manifestation of coherence or of a color glass condensate, whereas it is actually a simple consequence of energy conservation and takes place even at low energies. We demonstrate that in all these reactions there is a common suppression mechanism that can be viewed, alternatively, as a consequence of a reduced survival probability for large rapidity gap processes in nuclei, a Sudakov suppression, an enhanced resolution of higher Fock states by nuclei, or an effective energy loss that rises linearly with energy. Our calculations agree with the data.

Polarization measurements in π+p, K+p and pp elastic scattering at 45 GeV/c and comparison with Regge phenomenology

Abstract The polarization parameter P has been measured for elastic π + p, K + p and pp scattering at 45 GeV/c. Four-momentum transfer ranges from −0.08 to −1.1 (GeV/) 2 for pp, and from −0.08 to −0.9 (GeV/) 2 for π + p and K + p. The energy dependence of the polarization P ( t ) in π + p and in K + p above 6 GeV/c incident momentum is compatible with interference between pomeron and Regge poles. On the other hand, the polarization in p p elastic scattering decreases faster than ordinary Regge model predictions. This result can be explained by interference between non flip and flip amplitudes of the pomeron, leading to negative values for the polarization.

Quenching of high-pThadrons: Energy loss versus color transparency

High-pT hadrons produced in hard collisions and detected inclusively bear peculiar features: (i) they originate from jets whose initial virtuality and energy are of the same order; (ii) such jets are rare and have a very biased energy sharing among the particles, namely, the detected hadron carries the main fraction of the jet energy. The former feature leads to an extremely intensive gluon radiation and energy dissipation at the early stage of hadronization, either in vacuum or in a medium. As a result, a leading hadron must be produced on a short length scale. Evaluation within a model of perturbative fragmentation confirms the shortness of the production length. This result is at variance with the unjustified assumption of long production length, made within the popular energy loss scenario. Thus we conclude that the main reason of suppression of high-pT hadrons in heavy ion collisions is the controlled by color transparency attenuation of a high-pT dipole propagating through the hot medium. Adjusting a single parameter, the transport coefficient, we describe quite well the data from LHC and RHIC for the suppression factor R_{AA} as function of pT, collision energy and centrality. We observe that the complementary effect of initial state interaction causes a flattening and even fall of R_{AA} at large pT. The azimuthal anisotropy of hadron production, calculated with no further adjustment, also agrees well with data at different energies and centralities.

Damping of forward neutrons in pp collisions

We calculate absorptive corrections to single pion exchange in the production of leading neutrons in pp collisions. Contrary to the usual procedure of convolving the survival probability with the cross section, we apply corrections to the spin amplitudes. The nonflip amplitude turns out to be much more suppressed by absorption than the spin-flip one. We identify the projectile proton Fock state responsible for the absorptive corrections as a color octet-octet 5-quarks configuration. Calculations within two very different models, color-dipole light-cone description, and in hadronic representation, lead to rather similar absorptive corrections. We found a much stronger damping of leading neutrons than in some of previous estimates. Correspondingly, the cross section is considerably smaller than was measured at ISR. However, comparison with recent measurements by the ZEUS collaboration of neutron production in deep-inelastic scattering provides a strong motivation for challenging the normalization of the ISR data. This conjecture is also supported by preliminary data from the NA49 experiment for neutron production in pp collisions at SPS.

Determining the saturation scale in nuclei

The saturation momentum seen in the nuclear infinite-momentum frame is directly related to transverse momentum broadening of partons propagating through the medium in the nuclear rest frame. Calculation of broadening within the color dipole approach, including the effects of saturation in the nucleus, gives rise to an equation which describes well the data on broadening in the Drell-Yan reaction and heavy quarkonium production.

Single transverse spin asymmetry of forward neutrons

We calculate the single transverse spin asymmetry

Large rapidity gap processes in proton-nucleus collisions

A_N(t)

Nuclear suppression at large forward rapidities in d-Au collisions at relativistic and ultrarelativistic energies

, for inclusive neutron production in

Nuclear suppression of J/Ψ: From RHIC to the LHC

pp

J/{Psi} production in nuclear collisions: Theoretical approach to measuring the transport coefficient

collisions at forward rapidities relative to the polarized proton in the energy range of RHIC. Absorptive corrections to the pion pole generate a relative phase between the spin-flip and non-flip amplitudes, leading to a transverse spin asymmetry which is found to be far too small to explain the magnitude of