Vector meson production and inclusive Ks0Ks0 final state at HERA

Abstract

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. Vector Meson Production at HERAProduction 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 ,, and has a steep exponential t dependence, where t is the squared fourmomentum transfer at the proton vertex. Such processes are well described within the framework of Regge phenomenology [1,2,3] and the VectorMeson 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 perturbative QCD (pQCD) [6,7] can be used to describe the vector meson production. The photon fluctuates into a qq 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. . The results are sensitive to the PDF used and have large theoretical uncertainties, but they qualitatively describe the strong W-dependence of the cross section. They also indicate that at high vector meson masses M V , a hard regime already exist at very low values of Q 2 , and that M 2 V may set a hard scale. The Q 2 -dependence of the δ parameter is shown in Fig. 2 Exclusive Vector Meson Production