Puzzles of J/Psi production off nuclei

Nuclear effects for J/Psi production in pA collisions are controlled by the coherence and color transparency effects. Color transparency onsets when the time of formation of the charmonium wave function becomes longer than the inter-nucleon spacing. In this energy regime the effective break-up cross section for a c-cbar dipole depends on energy and nuclear path length, and agrees well with data from fixed target experiments, both in magnitude and energy dependence. At higher energies of RHIC and LHC coherence in c-cbar pair production leads to charm quark shadowing which is a complement to the high twist break up cross section. These two effects explain well with no adjusted parameters the magnitude and rapidity dependence of nuclear suppression of J/Psi observed at RHIC in dAu collisions, while the contribution of leading twist gluon shadowing is found to be vanishingly small. A novel mechanism of double color filtering for c-cbar dipoles makes nuclei significantly more transparent in AA compared to pA collisions. This is one of the mechanisms which make impossible a model independent "data driven" extrapolation from pA to AA. This effect also explains the enhancement of nuclear suppression observed at forward rapidities in AA collisions at RHIC, what hardly can be related to the produced dense medium. J/Psi is found to be a clean and sensitive tool measuring the transport coefficient characterizing the dense matter created in AA collisions. RHIC data for pT dependence of J/Psi production in nuclear collisions are well explained with the low value of the transport coefficient q_0-hat<0.5 GeV^2/fm.Comment: 15 pages, 8 figures, Invited talk at the Workshop "Saturation, the Color Glass Condensate and Glasma: What Have we Learned from RHIC?", BNL, May 10-12, 201

Nuclear Hadronization: Within or Without?

Nuclei are unique analyzers for the early stage of the space-time development of hadronization. DIS at medium energies is especially suitable for this task being sensitive to hadronization dynamics, since the production length is comparable with the nuclear size. This was the driving motivation to propose measurements at HERMES using nuclear targets, and to provide predictions based on a pQCD model of hadronization [1]. Now when the first results of the experiment are released [2,3], one can compare the predictions with the data. The model successfully describes with no adjustment the nuclear effects for various energies, zh, pT, and Q2, for different flavors and different nuclei. It turns out that the main source of nuclear suppression of the hadron production rate is attenuation of colorless pre-hadrons in the medium. An alternative model [4] is based upon an ad hoc assumption that the colorless pre-hadron is produced outside the nucleus. This model has apparent problems attempting to explain certain features of the results from HERMES. A good understanding of the hadronization dynamics is important for proper interpretation of the strong suppression of high-pT hadrons observed in heavy ion collisions at RHIC. We demonstrate that the production length is even shorter in this case and keeps contracting with rising pT.Comment: Latex 34 p. Based on talks given by B.Z.K. at the Fourth International Conference on Perspectives in Hadronic Physics, Trieste, Italy, May 12-16, 2003; and at the EuroConference on Hadron Structure Viewed with Electromagnetic Probes, Santorini, Greece, October 7-12, 200

Nuclear bound states of antikaons, or quantized multiskyrmions?

The spectrum of strange multibaryons is considered within the chiral soliton model using one of several possible SU(3$ quantization models (the bound state rigid oscillator version). The states with energy below that of antikaon and corresponding nucleus can be interpreted as antikaon-nucleus bound states. In the formal limit of small kaon mass the number of such states becomes large, for real value of this mass there are at least several states. For large values of binding energies interpretation of such states just as antikaon-nuclear bound states becomes more ambiguous.Comment: Corrections, amendments and additions made, references adde

Time Evolution of Hadronization and Grey Tracks in DIS off Nuclei

The analysis of the grey tracks produced in Deep Inelastic Scattering (DIS) off nuclei provides important information on the space-time development of hadronization in nuclear medium. This method is complementary to the measurement of nuclear attenuation of leading inclusive hadrons. While the latter is focused on the hadronization dynamics for the quite rare process of leading hadrons production, the former covers the main bulk of inelastic events, and its Q^2 dependence is a very sensitive tool to discriminate between different models of hadronization. Employing the model of perturbative hadronization developed earlier, we calculate the Q^2 and x_{Bj} dependences of the number of collisions and relate it to the mean number of grey tracks, using an empirical relation obtained from the analysis of data from the Fermilab E665 experiment on DIS of muons off the Xe nucleus. We found the number of grey tracks to rise steeply with Q^2 in good agreement with the experimental data.Comment: 10 pages, 2 figure

Azimuthal Asymmetry of Prompt Photons in Nuclear Collisions

The azimuthal elliptic asymmetry v2 observed in heavy ion collisions, is usually associated with properties of the medium created in the final state. We compute the azimuthal asymmetry which is due to multiple interactions of partons at the initial stage of nuclear collisions, and which is also present in $pA$ collisions. In our approach the main source of azimuthal asymmetry is the combination of parton multiple interactions with the steep variation of the nuclear density at the edge of nuclei. We apply the light-cone dipole formalism to compute the azimuthal asymmetry of prompt photons yield from parton-nucleus, proton-nucleus and nucleus-nucleus collisions at the RHIC energy.Comment: 13 pages, 4 figures, Eq.(4) corrected, figures and references updated. The version to appear in Nucl. Phys.

Propagation of Fast Partons in the Nuclear Medium

The color dipole approach has been applied in the target rest frame to address the issues of transverse momentum broadening and energy loss of a fast quark propagating in the nuclear medium. A recent application of the theory to the FermiLab E772/E866 experimental data, determining the rate of energy loss of a quark propagating in the medium to be 2 to 3 GeV/fm, will be reviewed. Calculations for the transverse momentum distribution will be presented, and the results will be compared to the E866 data. The theory will be shown to compare favorably to the data, and these results will be shown to suggest that the momentum broadening of a quark is about twice the generally accepted size.Comment: 6 pages, 4 figures, Invited talk given at 4th International Conference on Perspectives in Hadronic Physic

Direct photons at forward rapidities in high-energy pp collisions

We investigate direct photon production in pp collisions at the energies of RHIC, CDF and LHC, at different rapidities employing various color-dipole models. The cross section peaks at forward rapidities due to the abelian dynamics of photon radiation. This opens new opportunities for measurement of direct photons at forward rapidities, where the background from radiative hadronic decays is strongly suppressed. Our model calculations show that photon production is sensitive to the gluon saturation effects, and strongly depends on the value of the anomalous dimension.Comment: 7 pages, 8 figures, minor clarifications added. The version to appear in PL

Jet lag effect and leading hadron production

We propose a solution for the long standing puzzle of a too steeply falling fragmentation function for a quark fragmenting into a pion, calculated by Berger [1] in the Born approximation. Contrary to the simple anticipation that gluon resummation worsens the problem, we find good agreement with data. Higher quark Fock states slow down the quark, an effect which we call jet lag. It can be also expressed in terms of vacuum energy loss. As a result, the space-time development of the jet shrinks and the $z$-dependence becomes flatter than in the Born approximation. The space-time pattern is also of great importance for in-medium hadronization.Comment: 11 pages, 5 figure

From Hard to Soft Diffraction and Return

The long standing mystery of smallness of diffractive dissociation of hadrons to large effective masses (the Pomeron-proton cross section is only 2 mb) witnesses that the gluonic clouds of valence quarks are so small (r_0=0.3fm) that soft interaction hardly resolves those gluons (diffraction is \propto r_0^4). A color-dipole light-cone (LC) approach is developed which incorporates a strong nonperturbative interaction of the LC gluons. The energy dependent part of the total hadronic cross section is calculated in a parameter-free way employing the nonperturbative LC wave functions of the quark-gluon Fock states. It rises with energy as s^\Delta, and we predict \Delta=0.17 +/- 0.01, as well as the normalization. However, the energy independent part of the cross section related to inelastic collisions with no gluon radiated (gluons are not resolved) cannot be calculated reliably and we treat it as an adjustable parameter which is fixed fitting just one experimental point for total cross section. Then the energy dependence of the total cross section (the Pomeron part) and the elastic slope are fully predicted, as well as the effective Pomeron trajectory in impact parameter space, in a good agreement with data. These results naturally explain the x-dependence of the diffractive DIS observed at HERA. Although diffraction is expected to be dominated by soft interactions the observed effective \Delta is about twice as large as one (0.08) known for total cross sections. Diffractive excitations of large effective mass correspond to diffractive gluon radiation and should be associated with our calculated \Delta.Comment: Latex 8 pages including 4 figures. Invited talk at the Workshop Diffraction'2000, Cetraro, Italy, Sept. 200

Glue drops inside hadrons

We present experimental evidences for the existence of a semi-hard scale in light hadrons. This includes the suppression of gluon radiation that is seen in high mass hadron diffraction; the weak energy dependence of hadronic total cross sections; the small value of the Pomeron trajectory slope measured in photoproduction of J/Psi; the weakness of gluon shadowing in nuclei; shortage of gluons in the proton revealed by an unusual behavior of the proton structure function in the soft limit, and the enhanced intrinsic transverse momentum of quarks and gluons, which considerably exceeds the inverse hadronic size. All these observations suggest that gluons in hadrons are located within spots of a small size relative to the confinement radius.Comment: Based on talks given by B.P at the Fifth International Conference on Perspectives In Hadronic Physics, Trieste, May 2006, and by B.K. at the Workshop on Future Prospects in QCD at High Energies, Brookhaven, Jly 200