Proton Dissociative Rho and Elastic Phi Electroproduction at HERA

TL;DR

This study delivers the first comprehensive measurements of proton-dissociative rho electroproduction at Q^2>7 GeV^2 and elastic phi electroproduction at Q^2>6 GeV^2 with H1 at HERA. It tests the diffractive factorisation framework by comparing proton-dissociative and elastic channels, extracting t- and Q^2-dependences, and determining vector-meson polarisation. The rho results yield a proton-dissociative to elastic cross-section ratio around 0.65 and a small dissociative t-slope, with Q^2 and polarisation patterns consistent with factorisation. The phi measurements reveal a modest Q^2-driven rise in the γ^* p cross section, a relatively large t-slope, and a phi/rho cross-section ratio near 2/9 at high Q^2, indicating a transition toward perturbative dynamics and providing stringent tests of diffraction models.

Abstract

The electroproduction of rho mesons with proton diffractive dissociation for Q^2 > 7 GeV^2 and the elastic electroproduction of Phi mesons for Q^2 > 6 Gev^2 are studied in e^+ p collisions at HERA with the H1 detector, for an integrated luminosity of 2.8 pb-1. The dependence of the cross sections on P_t^2 and Q^2 is measured, and the vector meson polarisation obtained. The cross section ratio between proton dissociative and elastic production of rho mesons is measured and discussed in the framework of the factorisation hypothesis of diffractive vertices. The ratio of the elastic cross section for Phi and rho meson production is investigated as a function of Q^2.

Figures (7)

• Figure 1: Diffractive $\rho$ meson production: a) elastic scattering; b) proton dissociation.
• Figure 2: a) The invariant $\pi^+ \pi^-$ mass spectrum for the kinematical region $P_t^2 < 0.8\, \hbox{${\rm GeV}^2$}$. The solid curve represents the fit described in the text. The dashed curve shows the non-resonant background contribution. b) The efficiency corrected $P_t^2$ distribution for the proton dissociative data sample. The solid line represents the fit to the overall distribution, with the elastic (dotted line), DD and DIS (dashed line) contributions fixed (see text). Only statistical errors are presented.
• Figure 3: a) $\hbox{$Q^2$}$ distribution of the proton dissociative $\rho$ meson production $\hbox{$\gamma^* p$}$ cross section after efficiency correction. The solid curve represents a fit with the $Q^{-n}$ function, the backgrounds being taken into account in the fit procedure. b) Efficiency corrected $\cos\vartheta^*$ distribution, the backgrounds being subtracted in the fit. Only statistical errors are presented for both distributions.
• Figure 4: $M_{K^+K^-}$ invariant mass distribution for the selected events; the curve is the result of a fit to a Breit-Wigner distribution convoluted with a Gaussian, over a linear background.
• Figure 5: Cross section for $\hbox{$\gamma^* p$}\ \to \hbox{$\phi$} p$ as a function of $W$ for several values of $\hbox{$Q^2$}$ ($\hbox{${\rm GeV}^2$}$). The overall normalisation uncertainties of $25\%$ for the results of Cassel et al., $20\%$ for NMC and $32\%$ for ZEUS are not included. The H1 errors are the quadratic sums of the statistical and the systematic uncertainties.