Exclusive photoproduction of J/\psi mesons at HERA

The exclusive photoproduction of mesons, , has been studied in ep collisions with the ZEUS detector at HERA, in the kinematic range 20<W<290 GeV, where W is the photon-proton centre-of-mass energy. The mesons were reconstructed in the muon and the electron decay channels using integrated luminosities of 38 and 55,respectively. The helicity structure of production shows that the hypothesis of s-channel helicity conservation is satisfied within two standard deviations. The total cross section and the differential cross-section , where t is the squared four-momentum transfer at the proton vertex, are presented as a function of W, for |t|<1.8 GeV². The t distribution exhibits an exponential shape with a slope parameter increasing logarithmically with W with a value at . The effective parameters of the pomeron trajectory are and . Received: 1 February 2002 / Published online: 22 May 2002     

Measurement of the spin-density matrix elements in exclusive electroproduction of \rho^0 mesons at HERA

Exclusive electroproduction of mesons has been measured using the ZEUS detector at HERA in two ranges, GeV and GeV. The low- data span the range GeV; the high- data cover the GeV interval. Both samples extend up to four-momentum transfers of GeV. The distribution in the azimuthal angle between the positron scattering plane and the production plane shows a small but significant violation of s-channel helicity conservation, corresponding to the production of longitudinally polarised (i.e. helicity zero) mesons from transverse photons. Measurements of the 15 combinations of spin-density matrix elements which completely define the angular distributions are presented and discussed. Received: 30 July 1999 / Revised version: 20 September 1999 / Published online: 8 December 1999     

Elastic electroproduction of \rho mesons at HERA

The elastic electroproduction of mesons is studied at HERA with the H1 detector for a photon virtuality in the range and for a hadronic centre of mass energy in the range GeV. The shape of the () mass distribution in the resonance region is measured as a function of . The full set of spin density matrix elements is determined, and evidence is found for a helicity flip amplitude at the level of of the non-flip amplitudes. Measurements are presented of the dependence of the cross section on , W and t (the four-momentum transfer squared to the proton). They suggest that, especially at large , the cross section develops a stronger W dependence than that expected from the behaviour of elastic and total hadron–hadron cross sections. Received: 22 February 1999 / Published online: 25 February 2000     

Elastic and proton-dissociative \rho^0 photoproduction at HERA

Elastic and proton-dissociative photoproduction (, , respectively, with ) has been studied in interactions at HERA for photon-proton centre-of-mass energies in the range GeV and for GeV, where is the square of the four-momentum transfer at the proton vertex; the results on the proton-dissociative reaction are presented for masses of the dissociated proton system in the range . For the elastic process, the invariant mass spectrum has been investigated as a function of . As in fixed target experiments, the resonance shape is asymmetric; this asymmetry decreases with increasing , as expected in models in which the asymmetry is ascribed to the interference of resonant and non-resonant production. The cross section has been studied as a function of ; a fit to the resonant part with the form gives . The resonant part of the cross section is b at GeV. The dependence of the cross section can be described by a function of the type with (stat.) (syst.) GeV and (stat.) (syst.) GeV. The dependence has also been studied as a function of and a value of the slope of the pomeron trajectory GeV has been deduced. The spin density matrix elements , and have been measured and found to be consistent with expectations based on -channel helicity conservation. For proton-dissociative photoproduction in the mass range, the distributions of the two-pion invariant mass, and the polar and azimuthal angles of the pions in the helicity frame are the same within errors as those for the elastic process. The distribution has been fitted to an exponential function with a slope parameter GeV. The ratio of the elastic to proton-dissociative photoproduction cross section is . Received: 3 December 1997 / Online publication: February 26, 1998     

Exclusive leptoproduction of \rho^0 mesons from hydrogen at intermediate virtual photon energies

Measurements of the cross section for exclusive virtual-photoproduction of mesons from hydrogen are reported. The data were collected by the HERMES experiment using 27.5 GeV positrons incident on a hydrogen gas target in the HERA storage ring. The invariant mass W of the photon-nucleon system ranges from 4.0 to 6.0 GeV, while the negative squared four-momentum of the virtual photon varies from 0.7 to 5.0 GeV. The present data together with most of the previous data in the intermediate W-domain are well described by a model that infers the W-dependence of the cross section from the dependence on the Bjorken scaling variable x of the unpolarized structure function for deep-inelastic scattering. In addition, a model calculation based on Off-Forward Parton Distributions gives a fairly good account of the longitudinal component of the production cross section for GeV. Received: 25 April 2000 / Revised version: 3 July 2000 / Published online: 25 September 2000     

Validity of double scaling analysis in semi-inclusive processes –J/\psi production at HERA

In this paper we check the validity of the ideas of double scaling as given by Ball and Forte in a semi inclusive process like production at HERA, in different kinematical regions, for low values of the Bjorken variable . In particular, we study production in the inelastic and diffractive (elastic) regimes using the double scaling form of the gluon distribution functions. We compare these predictions with data (wherever available) and with other standard parameterisations. We find that double scaling holds in the inelastic regime over a larger kinematic region than that given by the analysis of the proton structure function . However, in the diffractive region, double scaling seems to suggest an admixture of hard pomeron boundary conditions for the gluon distribution, while predicting a steeper rise in the cross section than suggested by present data. Received: 17 December 1997 / Published online: 10 March 1998     

J/\psi +{\mathrm{jet}} diffractive production in the direct photon process at HERA

We present a study of the diffractive production in direct photon processes at HERA based on the factorization theorem for lepton-induced hard diffractive scattering and the factorization formalism of the non-relativistic QCD (NRQCD) for quarkonia production. Using the diffractive gluon distribution function extracted from HERA data on diffractive deep inelastic scattering and diffractive dijet photon production, we demonstrate that this process is measurable at DESY HERA. The experimental study of this process can give valuable insight in the diffractive production mechanism and test the color-octet mechanism for heavy quarkonia production in a new environment. Received: 13 September 1999 / Revised version: 31 December 1999 / Published online: 6 April 2000     

Color dipole phenomenology of diffractive electroproduction of light vector mesons at HERA

We develop the color dipole phenomenology of diffractive photo- and electroproduction γ* N → V(V′)N of light vector mesons (V(1S) = ?⁰, ω⁰, ρ⁰) and their radial excitations (V′(2S) ?′, ω′, ρ′). The node of the radial wave function of the 2S states in conjunction with the energy dependence of the color dipole cross section is shown to lead to a strikingly different Q ² and ν dependence of diffractive production of the V(1S) and V′(2S) vector mesons. We discuss the restoration of flavor symmetry and universality properties of production of different vector mesons as a function of Q ² + m _V ² . The color dipole model predictions for the ρ ⁰ and ? ⁰ production are in good agreement with the experimental data from the EMC, NMC, ZEUS and H1 collaborations. We present the first direct evaluation of the dipole cross section from these data.     

J/\psi production in Indium-Indium collisions

The study of dilepton production in heavy-ion collisions at the CERN SPS provided some of the most interesting observations done so far in the search for the quark gluon plasma. However, several aspects of the understanding of these measurements need to be clarified. For example, the study of the J/ production in different colliding systems should help to disentangle which is the variable driving the J/ suppression. The NA60 experiment, with a new radiation tolerant Silicon pixel detector, studied Indium-Indium interactions in the year 2003. In this paper, we present the J/ / DY ratio, integrated over all the centralities of the collisions, together with a first study of the J/ transverse momentum and polarization. Arrival of the final proofs: 1 July 2005 PACS: 25.75.Dw, 25.75.Nq, 13.20.Gd     

Investigation of the inelastic electroproduction of J/ψ mesons at the HERA collider within the semihard QCD approach

The results of theoretical calculations within the semihard (k _T-factorization) approach are compared with the latest data obtained in the ZEUS experiment at the HERA collider for the inelastic electroproduction of J/ψ mesons in the kinematical region specified by the inequalities 2 GeV² < Q ² < 80 GeV², 50 GeV < W < 250 GeV, 0.2 < z < 0.9, and ?1.6 < y _ψlab < 1.3. The dependence of these theoretical results on the choice of parametrization among three widely used parametrizations of unintegrated gluon distributions is studied.     

The rare annihilation decays $\bar{B}^0_{s,d} \to J/\psi\gamma$

We investigate the physics potential of the annihilation decays in the standard model and beyond. In a naive factorization approach, the branching ratios are estimated to be and . In the framework of QCD factorization, we compute the non-factorizable corrections and get , . Future measurements of these decays would be useful for testing the factorization frameworks. The smallness of these decays in the SM makes them sensitive probes of new physics. As an example, we will consider the possible admixture of the (V + A) charge current to the standard (V-A) current. This admixture will give a significant contribution to the decays.Received: 29 August 2003, Revised: 17 January 2004, Published online: 19 March 2004Corresponding author: Y.D. Yang     

The J/\psi normal nuclear absorption

We present a new determination of the ratio of cross-sections as expected for nucleus-nucleus reactions if would only be normally absorbed by nuclear matter. This anticipated behaviour is based on proton-nucleus data exclusively, and compared, as a function of centrality, with updated S-U results from experiment NA38 and with the most recent Pb-Pb results from experiment NA50. Arrival of the final proofs: 1 July 2005 PACS: 25.75.-q, 25.75.Nq, 13.20.Gd