A measurement of the t dependence of the helicity structure of diffractive rho meson electroproduction at HERA

TL;DR

This work extends the measurement of the helicity structure in diffractive ρ electroproduction to large t', extracting spin-density-matrix elements from angular distributions and establishing that $r^{04}_{00}$ is t'-independent while SCHC-violating combinations grow with t'. The results are interpreted within a two-gluon-exchange perturbative QCD framework (Ivanov–Kirschner), yielding a good description of the data with an effective gluon anomalous dimension $\gamma=0.60\pm0.09$ and an effective $q\bar{q}$ mass $m=0.58\pm0.04$ GeV, indicating asymmetric momentum sharing in the photon fluctuation. Backgrounds from $\omega$, $\phi$, and $\rho'$ are carefully modeled and subtracted, including a novel $\zeta$-based method to constrain the $\rho'$ contribution. Overall, the findings provide strong evidence for a diffractive mechanism governed by two-gluon exchange and offer quantitative constraints on gluon dynamics in the proton at relevant scales.

Abstract

The helicity structure of the diffractive electroproduction of rho mesons, e + p -> e + rho + Y, is studied in a previously unexplored region of large four-momentum transfer squared at the proton vertex, t: 0 < t' < 3 GeV^2, where t' = |t| - |t|_min. The data used are collected with the H1 detector at HERA in the kinematic domain 2.5 < Q^2 < 60 GeV^2, 40 < W < 120 GeV. No t dependence of the r^04_00 spin density matrix element is found. A significant t dependent helicity non-conservation from the virtual photon to the rho meson is observed for the spin density matrix element combinations r^5_00+2r^5_11 and r^1_00+2r^1_11. These t dependences are consistently described by a perturbative QCD model based on the exchange of two gluons.

Figures (4)

• Figure 1: Uncorrected $\pi^+\pi^-$ mass distributions for the selected events with $0.3 < M_{\pi \pi} <~1.3$ GeV. The shaded areas describe the $\omega$ and $\phi$ backgrounds and the hatched areas the $\rho^\prime$ background. a) and c) show tag events with $t^\prime$$<$ 0.5 $\mathrm{GeV}^2$ and 0.5 $<$$t^\prime$$< 3.0 \ {\rm GeV^{2}}$, respectively; b) and d) show notag events.
• Figure 2: Distributions of a-b) the $\zeta$ and c-d) the $\Phi$ variables of the selected events in the mass range (\ref{['eq:rho_mass']}) with $0.5 < \hbox{$t^\prime$} < 3 \ {\rm GeV^2}$, in the tag (a and c) and in the notag (b and d) sample. The shaded areas describe the $\omega$ and $\phi$ background as obtained from the simulations. The hatched areas correspond to the $\rho^\prime$ background and the open areas to the $\rho$ contribution, as determined using the iterative fitting procedure described in section \ref{['sect:rhoprim_bg']}.
• Figure 3: Normalised cross sections for $\rho$ electroproduction in five bins in $t^\prime$. The superimposed curves show the results of fits of a) relation (\ref{['eq:Phi']}) and b) relation (\ref{['eq:cost']}). The error bars represent the statistical errors.
• Figure 4: Measurement of a) $r^5_{00} + 2 r^5_{11}$ , b) $r^1_{00} + 2 r^1_{11}$ , c) $r^{04}_{00}$ as a function of $t^\prime$, for the present analysis and for refs. h1-rho (labeled "H1 elastic") and zeus (labeled "ZEUS $M_Y < 4$ GeV"). ⁠ The inner error bars represent the statistical errors, the full error bars include the systematic errors added in quadrature. The full curves correspond to the predictions of the model ivanov with parameters extracted from the fit described in section \ref{['sect:global']}. The dashed lines in a) and b) correspond to SCHC.