# Spin rotation effects in diffractive electroproduction of heavy quarkonia

## TL;DR

This paper investigates how spin rotation effects influence the diffractive electroproduction of heavy quarkonia, revealing significant impacts on cross sections that vary with photon energy and virtuality.

## Contribution

It provides the first comprehensive analysis of Melosh spin rotation effects in heavy quarkonia electroproduction within the color dipole framework.

## Key findings

- Spin rotation effects significantly increase cross sections for charmonium and excited states.
- Effects cause a 20-30% enhancement in production cross sections for certain bottomonium states.
- Spin effects diminish with increasing photon virtuality Q^2, following universal patterns.

## Abstract

In this work we present for the first time the comprehensive study of the Melosh spin rotation effects in diffractive electroproduction of S-wave heavy quarkonia off a nucleon target. Such a study has been performed within the color dipole approach using, as an example and a reference point, two popular parametrizations of the dipole cross section and two potentials describing the interaction between Q and bar{Q} and entering in the Schroedinger equation based formalism for determination of the quarkonia wave functions. We find a strong onset of spin rotation effects in 1S charmonium photoproduction which is obviously neglected in present calculations of corresponding cross sections. For photoproduction of radially excited Psi'(2S) these effects are even stronger leading to an increase of the photoproduction cross section by a factor of 2-3 depending on the photon energy. Even in production of radially excited Y'(2S) and Y"(3S) they can not be neglected and cause the 20-30% enhancement of the photoproduction cross section. Finally, we predict that the spin effects vanish gradually with photon virtuality Q^2 following universality properties in production of different heavy quarkonia as a function of Q^2 + M_V^2.