Pomeron in diffractive processes $\gamma^*(Q^2)p\to\rho^0 p$ and $\gamma^*(Q^2)p\to\gamma^*(Q^2) p$ at large Q^2: the onset of pQCD

TL;DR

This paper investigates diffractive processes involving virtual photons and protons at high Q^2, demonstrating that the perturbative QCD regime, characterized by small quark-antiquark pairs, only begins at very high momentum transfers.

Contribution

It provides a detailed calculation of the amplitudes for diffractive processes using the BFKL pomeron, highlighting the delayed onset of perturbative QCD effects in these reactions.

Findings

Pure perturbative regime begins at Q^2 ~ 100 GeV^2 for rho production.

Pure perturbative regime begins at Q^2 ~ 50 GeV^2 for gamma* production.

Small interquark distances dominate only at very high Q^2 values.

Abstract

We study the reactions $\gamma^*(Q^2)p\to\rho^0 p$ and $\gamma^*(Q^2)p\to\gamma^*(Q^2) p$ at large Q^2 and $W^2/Q^2$ and small momentum transfer, $\kappa^2_\perp$, to the nucleon where the pomeron exchange dominates. At large Q^2 the virtual photon selects a hard $q\bar q$ pair, thus selecting the hard pomeron component (the BFKL pomeron). The amplitudes for both transverse and longitudinal polarizations of the initial photon and outgoing $\rho$-meson (photon) are calculated in the framework of the BFKL pomeron exchange. Our calculations show that one cannot expect the early onset of the pure perturbative regime in the discussed diffractive processes: the small interquark distances, $\rho_{q\bar q} <0.2$ fm, start to dominate not earlier than at $Q^2 \simeq 100 GeV^2, W^2/Q^2 \simeq 10^7$ in $\gamma^*(Q^2)p\to\rho^0 p$ and $Q^2 \simeq 50 GeV^2, W^2/Q^2 \simeq 10^6$ in $\gamma^*(Q^2)p\to\gamma^*(Q^2) p$.