Polarization in diffractive electroproduction of light vector mesons

TL;DR

The paper develops a perturbative QCD description of polarization in diffractive γ* p → ρ p production, emphasizing the role of gluon density scaling in shaping helicity amplitudes. Through an impact-parameter formalism with light-cone wave functions, it derives SCHC and spin-flip amplitudes, linking the dominant nonflip amplitude to xG(x, Q^2) and showing that the largest SCHC-violating term M_(+1,0) scales with √|t|/Q. A detailed analysis of the vector meson decay angular distribution at HERA kinematics reveals observable interference effects, notably a cosΦ modulation driven by M_(+1,0), while higher-spin flips are suppressed. The results provide concrete predictions for amplitude hierarchies and angular observables, with sensitivity to the meson wave function and potential extensions to other light vector mesons.

Abstract

We study in perturbative QCD the helicity ampltiudes of the process $γ^{*} p \to ρp $ at large virtualities $Q$ of the photon $γ^{*}$. We estimate all spin flip amplitudes taking into account an important effect of the scale behaviour of the gluon density. The transition of a transverse virtual photon to a longitudinal vector meson is not small at typical HERA conditions. This helicity non-conserving amplitude leads by interference to a measurable effect in the distribution of the angle between the electron scattering and the meson production planes.

Figures (2)

• Figure 1: Diffractive electroproduction of vector meson on a proton target.
• Figure 2: The $\gamma^* \to M$ impact factor. Only one contribution of the interaction of the $q\bar{q}$ dipole with the two exchanged gluons (dashed lines) is shown.