The target asymmetry in hard vector-meson electroproduction and parton angular momenta

Abstract

The target asymmetry for electroproduction of vector mesons is investigated within the handbag approach. While the generalized parton distribution (GPD) H is taken from a previous analysis of the elctroproduction cross section, we here construct the GPD E from double distributions and constrain it by the Pauli form factors of the nucleon, positivity bounds and sum rules. Predictions for the target asymmetry are given for various vector mesons and discussed how experimental data on the asymmetry will further constrain E and what we may learn about the angular momenta the partons carry. 1

Transverse target spin asymmetries in exclusiveρ0muoproduction

“…Also, the small gluon and sea contributions evaluated in Ref. [12] cancel here to a large extent. The asymmetries A sin S UT and A cos S LT represent imaginary and real part, respectively, of the same difference of two products M * M of two helicity amplitudes, where the first term of this difference represents a combination of GPDs H T and H , and the second a combination of E T and E. As can be seen in the left panel of Fig.…”

“…(5), the first two terms represent each a combination of chiral-even GPDs H and E. The inclusion of chiral-odd GPDs by the third term has negligible impact on the behaviour of A sin(− S ) UT , as can be seen when comparing calculations of Refs. [12] and [6]. The asymmetry A sin(− S ) UT itself may still be of small magnitude, because for GPDs E in 0 production the valence quark contribution is expected to be not large.…”

Generalized parton distributions from neutrino experiments: twist-three effects

“…In the asymptotic Bjorken limit the cross-section is dominated by the first angular independent term d L /dt dx B dQ 2 d which was studied in our previous paper [17] and is a straightforward extension of the electroproduction of pions studied in [19][20][21][22][23][24][25]. As we will see below the twist-3 corrections are small, for this reason it is convenient to normalize all the cross-sections in (1) to this term,…”

“…For example, the angle-integrated cross-section d /d ln x B dt dQ 2 is not sensitive to those harmonics at all. For numerical estimates we used the Kroll-Goloskokov parametrization of GPDs [21][22][23]27]. …”

The GPD program at COMPASS

“…A contribution of chiral-odd GPDs is negligible here, as one can see from comparison of calculations of Refs. [3] and [6]. The A sin φ s UT asymmetry represents an imaginary part of two bilinear products of helicity amplitudes.…”