Speaker
Description
Generalized parton distributions (GPDs) encoding multidimensional information of hadron partonic structure appear as the building blocks in a factorized description of hard exclusive reactions. The nucleon GPDs have been accessed by deeply virtual Compton scattering and deeply virtual meson production with lepton beam. A complementary probe with hadron beam is the exclusive pion-induced Drell-Yan process, $\pi^- p \to \mu^+ \mu^- n$. We discuss recent theoretical advances on describing this process in terms of QCD factorization as the partonic subprocess convoluted with the nucleon GPDs and the pion distribution amplitudes [1-3]. Based on this result, we mention the feasibility study for measuring the exclusive pion-induced Drell-Yan process via a spectrometer at the High Momentum Beamline being constructed at J-PARC [3].
We also point out that there could be the corrections beyond the QCD factorization framework due to ``soft mechanisms’’ caused by the treatment of the pion pole contribution arising in the relevant GPDs in the ERBL region, the parton transverse momentum to regularize the endpoint singularities, the so-called soft-overlap mechanism, etc. Those soft mechanisms could give important contributions at J-PARC kinematics [2], and we present a new estimate of the soft mechanisms making use of dispersion relations and quark-hadron duality [4]. We note that our approach allows us to calculate the soft mechanisms as well as the factorizable mechanisms using the GPDs; thus, realization of the measurement of the exclusive pion-induced Drell-Yan process at J-PARC will provide a new test of QCD descriptions of a novel class of hard exclusive reactions. It will also offer the possibility of experimentally accessing nucleon GPDs at large timelike virtuality.
This talk is based on my recent publication [3] in collaboration with several theorists and experimentalists, and my new result [4] in preparation.
References:
[1] E.R. Berger, M. Diehl, and B. Pire, Phys. Lett. B523 (2001) 265.
[2] S. V. Goloskokov and P. Kroll, Phys.Lett. B748 (2015) 323.
[3] T. Sawada, W. C. Chang, S. Kumano, J. C. Peng, S. Sawada and K. Tanaka, Phys.
Rev. D 93, 114034 (2016) [arXiv:1605.00364 [nucl-ex]].
[4] K. Tanaka, in preparation.
