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 Schrödinger 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 2div 3 depending on the photon energy. Even in production of radially excited Upsilon '(2S) and Upsilon ''(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.
Highlights
QCD, minimizing the uncertainties coming from the nonperturbative interactions.This expectation can be verified within the color dipole approach [5]
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 Qand entering in the Schrödinger equation based formalism for determination of the quarkonia wave functions
Summary
QCD (pQCD), minimizing the uncertainties coming from the nonperturbative interactions (for a detailed review on quarkonia physics, see e.g. Refs. [2,3,4]). The above statement is such that elastic electroproduction of heavy quarkonia at Q2 0 probes mainly the phenomena at hard scale, when one should rely on pQCD calculations, while semihard-scale physics is of a nonperturbative origin and can be probed only at small Q2 (see Eq (1.2)) Given this statement, in the current paper we present for the first time a detailed analysis of effects of Melosh spin rotation [17] within the color dipole formalism [5,7,8,9,18,19]. We demonstrate a strong onset of spin rotation effects, which are usually neglected in present calculations, in production of radially excited 2S and 3S states They lead to an enhancement of the ratio of ψ (2S) to J/ψ(1S) photoproduction cross sections by a factor of 2 ÷ 3 in a reasonable agreement with the data. We discuss how manifestations and magnitudes of spin rotation effects are correlated with production of different states of heavy quarkonia
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