06 01 1 v 1 3 J un 2 00 3 1 Vector meson production and inclusive K 0 s K 0 s final state at HERA

Measurements of vector mesons at HERA allow a detailed study of diffractive and non-diffractive production mechanisms, fragmentation and decay branching ratios. Results are presented on hadronic resonance measurements. In addition, the first observation of two meson states at masses around 1500 MeV and 1700 MeV are reported using inclusive K 0 s K 0 s production in deep inelastic ep scattering in ZEUS at HERA. Production of light and heavy vector mesons (V) in exclusive and in proton dissociation processes has been studied in ep reactions at HERA in a wide range of the γp centre-of-mass energy W and photon virtuality Q 2. For very low Q 2 and W > 10 GeV, these reactions display features characteristic of a soft diffractive process. The cross sections rises weakly with the energy W , σ V (W) ∝ W δ (δ ≃ 0.22), and has a steep exponential t dependence, where t is the squared four-momentum transfer at the proton vertex. Such processes are well described within the framework of Regge phenomenology [1,2,3] and the Vector-Meson Dominance model (VDM) [4,5], where the photon is assumed to fluctuate into a vector meson before scattering from the proton via Pomeron exchange. However, this approach fails at high values of Q 2 or high vector meson masses. In the hard regime, models based on perturba-tive QCD (pQCD) [6,7] can be used to describe the vector meson production. The photon fluctuates into a q ¯ q state and the quark dipole interacts with the proton in the lowest order via two gluons exchange [8,9,10,11]. The exchange of the gluon ladder has also been calculated in the leading logarithm approximation (LLA) [8,11,12,13]. The cross section is related to the rise of the gluon density in the proton as x decreases, where x is the Bjorken scale variable, and has a strong W dependence, σ V (W) ∝ W δ (δ ≃ 0.8.) In this contribution, perturbative QCD models are compared with results from vector meson production in photoproduction and deep inelastic scattering at HERA. 1.1. Exclusive Vector Meson Production Figure 1 shows σ γ * p→J/ψp tot from ZEUS [14,15] and H1 [16] as a function of W for fixed Q 2 , and also in photoproduction. The data is fitted with W δ. The ZEUS measurements are shown in table 1. The results are compatible with results in photoproduction from H1 [17] δ = 0.83 …