Transversity GPD in photo- and electroproduction of two vector mesons

The chiral-odd generalized parton distribution (GPD), or transversity GPD, of the nucleon can be accessed experimentally through the photo- or electroproduction of two vector mesons on a polarized nucleon target, \gamma^(*) N --> \rho_1 \rho_2 N', where \rho_1 is produced at large transverse momentum, \rho_2 is transversely polarized, and the mesons are separated by a large rapidity gap. We predict the cross section for this process for both transverse and longitudinal \rho_{2} production. To this end we propose a model for the transversity GPD H_T(x,\xi,t), and give an estimate of the relative sizes of the transverse and longitudinal \rho_{2} cross sections. We show that a dedicated experiment at high energy should be able to measure the transversity content of the proton.Comment: 15 pages, 9 figures. Extended discussion of experimental prospects in the conclusions. Version to appear in EPJ

Surfaces Meeting Porous Sets in Positive Measure

Let n>2 and X be a Banach space of dimension strictly greater than n. We show there exists a directionally porous set P in X for which the set of C^1 surfaces of dimension n meeting P in positive measure is not meager. If X is separable this leads to a decomposition of X into a countable union of directionally porous sets and a set which is null on residually many C^1 surfaces of dimension n. This is of interest in the study of certain classes of null sets used to investigate differentiability of Lipschitz functions on Banach spaces

Off-Forward Parton Distributions

Recently, there have been some interesting developments involving off-forward parton distributions of the nucleon, deeply virtual Compton scattering, and hard diffractive vector-meson production. These developments are triggered by the realization that the off-forward distributions contain information about the internal spin structure of the nucleon and that diffractive electroproduction of vector mesons depends on these unconventional distributions. This paper gives a brief overview of the recent developments

Twist-3 contribution to the γ∗γ→ππ\gamma^*\gamma\to \pi\pi amplitude in the Wandzura-Wilczek approximation

We have calculated the Wandzura-Wilczek contribution to the twist-3 part of $\gamma^*\gamma\to 2\pi$ amplitude. It describes interaction of the longitudinally polarized virtual photon with the real one, and it is suppressed by 1/Q, where $Q^2$ is the virtuality of the $\gamma^*$, as compared to the twist-2 contribution. We have found that, in the Wandzura-Wilczek approximation, factorization applies to the twist-3 amplitude.Comment: 13 pages, 4 figure

Angular distributions in hard exclusive production of pion pairs

Using the leading order amplitudes of hard exclusive electroproduction of pion pairs we have analyzed the angular distribution of the two produced particles. At leading twist a pion pair can be produced only in an isovector or an isoscalar state. We show that certain components of the angular distribution only get contributions from the interference of the I=1 and the (much smaller) I=0 amplitude. Therefore our predictions prove to be a good probe of isospin zero pion pair production. We predict effects of a measurable size that could be observed at experiments like HERMES. We also discuss how hard exclusive pion pair production can provide us with new information on the effective chiral Lagrangian.Comment: 17 pages, version to appear in Phys. Rev.

Scheme Independence of g1p(x,Q2)g_1^p (x, Q^2)

We work with two general factorization schemes in order to explore the consequences of imposing scheme independence on $g_1^p (x, Q^2)$. We see that although the light quark sector is indifferent to the choice of a particular scheme, the extension of the calculations to the heavy quark sector indicates that a scheme like the $\bar{MS}$ is preferable.Comment: 11 pages, 2 figures. To appear in the Brief Reports of Phys. Rev.

An attempt to understand exclusive pi+ electroproduction

Hard exclusive pi+ electroproduction is investigated within the handbag approach. The prominent role of the pion-pole contribution is demonstrated. It is also shown that the experimental data require a twist-3 effect which ensues from the helicity-flip generalized parton distribution H_T and the twist-3 pion wave function. The results calculated from this handbag approach are compared in detail with the experimental data on cross sections and spin asymmetries measured with a polarized target. It is also commented on consequences of this approach for exclusive \pi^0 and vector-meson electroproduction.Comment: 35 pages, 12 figures, using Latex, a number of additional comments have been included in the text, e.g. in paragraph above (3) or at end of sect.