Novel hard semiexclusive processes and color singlet clusters in hadrons

Hard scattering to a three cluster final state is suggested as a method to probe configurations in hadrons containing small size color singlet cluster and a residual quark-gluon system of a finite mass. Examples of such processes include $e + N \to e+ p +M_X (\Lambda+M_X'), p+p \to p+p+M_X(p+\Lambda+M_X')$ where $M_X(M_X')$ could be a pion(kaon) or other state of finite mass which does not increase with momentum transfer ($Q^2$). We argue that different models of the nucleon may lead to very different qualitative predictions for the spectrum of states $M_X$. We find that in the pion model of nonperturbative $q \bar q$ sea in a nucleon the cross section of these reactions is comparable to the cross section of the corresponding two-body reaction. Studies of these reactions are feasible using both fixed target detectors (EVA at BNL, HERMES at DESY) and collider detectors with a good acceptance in the forward direction.Comment: 9 pages, 2 figures, to be published in the proceedings of the Workshop: Exclusive Processes at High Momentum Transfer, Newport News, Virginia, 15-18 May 200

Rapidity and momentum transfer distributions of coherent J/ψJ/\psi photoproduction in ultraperipheral pPb collisions at the LHC

Based on accurate calculations of the flux of equivalent photons of the proton and heavy nuclei and the pQCD framework for the gluon distribution in the proton and nuclei, we analyze the rapidity and momentum transfer distributions of coherent $J/\psi$ photoproduction in ultraperipheral proton-Pb collisions at the LHC. We demonstrate that unlike the case of proton-proton UPCs marred by certain theoretical uncertainties and experimental limitations, after a cut excluding the region of small momentum transfers, ultraperipheral proton-Pb collisions offer a clean way to study the gluon distribution in the proton down to $x \approx 10^{-5}$. Our analysis of the momentum transfer distributions shows that an interplay of $J/\psi$ production by low-energy photons on the nucleus and by high-energy photons on the proton in proton-Pb UPCs can result in some excess of events at small $p_t$ in a definite region of the rapidity y.Comment: 18 pages, 8 figures. The final published versio

Neutron tagging of quasielastic J/psi photoproduction off nucleus in ultraperipheral heavy ion collisions at RHIC energies

We compare the coherent and quasielastic J/psi photoproduction in the peripheral heavy ion collisions in kinematics of RHIC. Our improved estimate of the total coherent cross section is a factor of two smaller than the earlier ones. We find that the counting rate of quasielastic J/psi photoproduction tagged by neutrons emitted due to cascading of the recoiled nucleon within the residual nucleus exceeds the rate of the coherent events. We argue that measurements of this process can be used to learn about the dynamics of color dipole-nucleon interactions in nuclei in the wide effective range of energies of gamma A interactions.Comment: 12 pages, 5 figure

Soft Neutron Production in DIS: a Window to the Final State Interactions

Recently E-665 reported the first measurement of soft ($E_n \le 10$MeV) neutron production in deep inelastic scattering (DIS) off nuclei. We report the first theoretical analysis of the data. We find that the observed cross section can be quantitatively explained as due to the final state interactions (FSI) of low energy nucleons ($E_N \sim 200-400 MeV$) produced in the elementary $\mu N$ interactions. We argue that the data indicate strong a suppression of the FSI's of fast partons (hadrons) in DIS at high energies, and that studies of the soft neutron production would provide a new sensitive probe of the dynamics of FSI's in DIS both at fixed target energies and at the HERA $eA$ collider.Comment: 14 pages, 4 figure