Diffractive electroproduction of ρ and ϕ mesons at HERA

Abstract

Diffractive electroproduction of ρ and φ mesons is measured at HERA with the H1 detector in the elastic and proton dissociative channels. The data correspond to an integrated luminosity of 51 pb −1 . About 10500 ρ and 2000 φ events are analysed in the kinematic range of squared photon virtuality 2.5 ≤ Q 2 ≤ 60 GeV 2 , photon-proton centre of mass energy 35 ≤ W ≤ 180 GeV and squared four-momentum transfer to the proton |t| ≤ 3 GeV 2 . The total, longitudinal and transverse cross sections are measured as a function of Q 2 , W and |t|. The measurements show a transition to a dominantly "hard" behaviour, typical of high gluon densities and small qq dipoles, for Q 2 larger than 10 to 20 GeV 2 . They support flavour independence of the diffractive exchange, expressed in terms of the scaling variable (Q 2 +M 2 V )/4, and proton vertex factorisation. The spin density matrix elements are measured as a function of kinematic variables. The ratio of the longitudinal to transverse cross sections, the ratio of the helicity amplitudes and their relative phases are extracted. Several of these measurements have not been performed before and bring new information on the dynamics of diffraction in a QCD framework. The measurements are discussed in the context of models using generalised parton distributions or universal dipole cross sections.

Constraining proton geometric fluctuations with incoherent diffractive vector meson production

“…1. The results obtained using two different parametrizations for the proton structure: one with large event-by-event fluctuations (having B qc = 3.3 GeV are compared with H1 data [20]. We find that the both of the two parametrizations give relatively good description fo the coherent HERA data, but the more smooth parametrization underestimates the incoherent cross section by several orders of magnitude.…”

Exclusive vector meson production and small-x evolution

“…Cross section σ(Q 2 , W 2 ) for the vector meson production plotted as a function of (Q 2 + M 2 V ) for ρ elastic production. The experimental data are from [1,3,7].…”

“…diffractive electroproduction of vector mesons, where the cross section has been integrated over the momentum transfer t. The experimental data from H1 and ZEUS for ρ [1,3,7], φ [3,6] and J/Ψ [2, 5] were used.…”

“…For the data on ρ production from [1] the range in t is |t| < 0.5 GeV 2 and |t| < 1.0 GeV 2 for data from [7]; |t| < 3 GeV data from [3]. For the data on φ production from [6] the range is |t| < 0.6 GeV 2 and |t| < 3 GeV 2 for data from [3]. Finally for J/Ψ the range of |t| < 1.2 GeV 2 in [2] and |t| < 1.0 GeV 2 for data from [5].…”

“…Both H1 [1][2][3] and ZEUS [4][5][6][7] collaborations performed detailed measurements of this process in a wide kinematic range. In this process the final state consists of the elastically scattered proton accompanied by a vector meson.…”