Signals for black body limit in coherent ultraperipheral heavy ion collisions

We argue that study of total cross section of photoabsorption and coherent photoproduction of $\rho,\rho^{\prime}$-mesons in ultraperipheral heavy ion collisions (UPC) is effective method to probe onset of black body limit(BBL) in the soft and hard QCD interactions. We illustrate the expected features of the onset of BBL using generalized vector dominance model. We show that this model describes very well $\rho$-meson coherent photoproduction at $6 \leq E_{\gamma} \leq 10 GeV$. In the case of $\rho$-meson production we find a UPC cross section which is a factor $\sim 1.5$ larger than the one found by Klein and Nystrand. The advantages of the process of coherent dijet production to probe onset of BBL in hard scattering regime where decomposition over the twists becomes inapplicable are explained and relative importance of the $\gamma +Pomeron$ and $\gamma +\gamma$ mechanisms is estimated.Comment: 17 pages, 7 figure

Charm production in antiproton-nucleus collisions at the J/ψJ/\psi and the ψ′\psi' thresholds

We discuss the production of charmonium states in antiproton-nucleus collisions at the $\psi'$ threshold. It is explained that measurements in $\bar p A$ collisions will allow to get new information about the strengths of the inelastic $J/\psi N, \psi'N$ interaction, on the production of $\Lambda_c$ and $\bar{D}$ in charmonium-nucleon interactions and for the first time about nondiagonal transitions $\psi' N\to J/\psi N$. The inelastic $J/\psi$-nucleon cross section is extracted from the comparison of hadron-nucleus collisions with hadron-nucleon collisions. We extract the total $J/\psi$ nucleon cross section from photon-nucleon collisions by accounting for the color transparency phenomenon within the frame of the GVDM (Generalized Vector meson Dominance Model). We evaluate also within the GVDM the inelastic $\psi'$-nucleon cross section as well as the cross section for the nondiagonal transitions. Predictions for the ratio of $J/\psi$ to $\psi'$ yields in antiproton-nucleus scatterings close to the threshold of $\psi'$ production for different nuclear targets are presented.Comment: 13 pages, 10 figures, typos corrected, some discussion adde

The matrix Hamiltonian for hadrons and the role of negative-energy components

The world-line (Fock-Feynman-Schwinger) representation is used for quarks in arbitrary (vacuum and valence gluon) field to construct the relativistic Hamiltonian. After averaging the Green's function of the white $q\bar q$ system over gluon fields one obtains the relativistic Hamiltonian, which is matrix in spin indices and contains both positive and negative quark energies. The role of the latter is studied in the example of the heavy-light meson and the standard einbein technic is extended to the case of the matrix Hamiltonian. Comparison with the Dirac equation shows a good agreement of the results. For arbitrary $q\bar q$ system the nondiagonal matrix Hamiltonian components are calculated through hyperfine interaction terms. A general discussion of the role of negative energy components is given in conclusion.Comment: 29 pages, no figure

Correlation Effects in Nuclear Transparency

The Glauber approximation is used to calculate the contribution of nucleon correlations in high-energy $A(e,e'N)$ reactions. When the excitation energy of the residual nucleus is small, the increase of the nuclear transparency due to correlations between the struck nucleon and the other nucleons is mostly compensated by a decrease of the transparency due to the correlations between non detected nucleons. We derive Glauber model predictions for nuclear transparency for the differential cross section when nuclear shell level excitations are measured. The role of correlations in color transparency is briefly discussed.Comment: 24 pages revtex, 4 uuencoded PostScript Figures as separate fil

Revisiting spin alignment of heavy mesons in its inclusive production

In the heavy quark limit inclusive production rate of a heavy meson can be factorized, in which the nonperturbative effect related to the heavy meson can be characterized by matrix elements defined in the heavy quark effective theory. Using this factorization, predictions for the full spin density matrix of a spin-1 and spin-2 meson can be obtained and they are characterized only by one coefficient representing the nonperturbative effect. Predictions for spin-1 heavy meson are compared with experiment performed at $e^+e^-$ colliders in the energy range from $\sqrt{s}=10.5$GeV to $\sqrt{s}=91$GeV, a complete agreement is found for $D^*$- and $B^*$-meson. For $D^{**}$ meson, our prediction suffers a large correction, as indicated by experimental data. There exists another approach by taking heavy mesons as bound systems, in which the total angular momentum of the light degrees of freedom is 1/2 and 3/2 for spin-1 and spin-2 meson respectively, then the diagonal parts of spin density matrices can be obtained. However, there are distinct differences in the predictions from the two approaches and they are discussed in detail.Comment: 14 pages with one figur

Color screening and the suppression of the charmonium state yield in nuclear reactions

We discuss the new data for the production of the psi meson in pA collisions at 450 GeV at CERNSPS (of the NA50-collaboration) [1]. We extract from the CERN data sigma(psi'N) 8 mb under the assumption that the psi is produced as a result of the space-time evolution of a point-like c¯c pair which expands with time to the full size of the charmonium state. In the analysis we assume the existence of a relationship between the distribution of color in a hadron and the cross section of its interaction with a nucleon. However, our result is rather sensitive to the pattern of the expansion of the wave packet and significantly larger values of sigma(psi'N)are not ruled out by the data. We show that recent CERN data confirm the suggestion of ref. [2] that color fluctuations of the strengths in charmonium-nucleon interaction are the major source of suppression of the J/psi yield as observed at CERN in both pA and AA collisions

Color Screening and the Suppression of the Charmonium State Yield in Nuclear Reactions

We discuss the new data for the production of the $\psi'$ meson in pA collisions at 450 GeV at CERN-SPS (of the NA50-collaboration) [1]. We extract from the CERN data $\sigma(\psi' N)\approx 8$ mb under the assumption that the $\psi'$ is produced as a result of the space-time evolution of a point-like $c\bar c$ pair which expands with time to the full size of the charmonium state. In the analysis we assume the existence of a relationship between the distribution of color in a hadron and the cross section of its interaction with a nucleon. However, our result is rather sensitive to the pattern of the expansion of the wave packet and significantly larger values of $\sigma(\psi' N)$ are not ruled out by the data. We show that recent CERN data confirm the suggestion of [2] that color fluctuations of the strengths in charmonium-nucleon interaction are the major source of suppression of the $J/\psi$ yield as observed at CERN in both pA and AA collisions.Comment: 10 pages, 5 figures (one with color