Kinematic Issues of GPDs in DVCS Chueng-Ryong Ji Department of Physics, North Carolina State University, Raleigh, NC 27695-8202,U.S.A. Bernard L.G. Bakker Department of Physics and Astrophysics, Vrije Universiteit, De Boelelaan 1081, NL-1081 HV Amsterdam, The Netherlands. Abstract Generalized Parton Distributions (GPDs) in Deeply Virtual Compton Scattering (DVCS) have been widely recog- nized and used as a useful tool to explore the quark and gluon structure of the target hadrons. However, we recently pointed out treacherous kinematic issues in analyzing DVCS in terms of GPDs. We present our key findings in the simplest possible level of complete amplitude including the lepton current. We also discuss an implication on theoretical frameworks to cover the kinematic region of DVCS experiment with the 12 GeV upgrade at Jefferson Lab. Keywords: Deeply Virtual Compton Scattering (DVCS), Generalized Parton Distributions (GPDs) 1. INTRODUCTION The investigation of the kinematic issues involving GPDs in DVCS is highly relevant to the 12 GeV up- grade at Jefferson Lab (JLab). Although the value of Q 2 = -q 2 (see Fig. 1) for the virtual photon can be twice larger by doubling the energy of the electron from 6 GeV to 12 GeV at JLab, the magnitude of the Mandel- stam variable t = (q - q ′ ) 2 representing the square of the momentum transfer to the target hadron also gets large as Q 2 gets large. This is due to a practical constraint in the set up of the coincidence experiment for DVCS. The ratio of -t/Q 2 doesn’t get negligible as one might have hoped for the 12 GeV upgrade of JLab. As we pointed out in our recent review article[1], the present theoreti- cal framework based on the assumption of -t/Q 2 ≪ 1 needs to be improved in order to cover the kinematic region of virtual Compton scattering experiments. In this work, we present our benchmark calculation with the “bare bone” structure of the DVCS process and dis- cuss the outlook of future developments in the theoret- ical framework. In Section 2, we use the kinematics p l l’ p’ q q’ k+q k+q-q’ k soft part Figure 1: Complete handbag diagram for s-channel Virtual Compton scattering. A quark with momentum k is hit by a virtual photon with momentum q. The final, real photon has momentum q ′ . corresponding to realistic coincidence experiments and focus on the tree-level bare bone amplitudes of DVCS. We discuss how one can see explicit evidence that the original formulation of GPDs is limited to t = 0 when the nucleon mass is neglected. This motivates our dis- cussion in Section 3 regarding two available but not yet Available online at www.sciencedirect.com Nuclear Physics B (Proc. Suppl.) 251–252 (2014) 75–80 0920-5632/© 2014 Elsevier B.V. All rights reserved. www.elsevier.com/locate/npbps http://dx.doi.org/10.1016/j.nuclphysbps.2014.04.013