Measurement of the differential cross section for the reaction gamman-->pi- p from deuterium.

Strong interactions exhibit two distinct behaviors: confinement and asymptotic freedom. Although the quantum chromodynamics (QCD) in terms of quarks and gluonsis the fundamental theory of strong interactions, at lower energies, the hadronic processes are usually described by meson-exchange models. With two distinct pictures of strong interaction: quark-gluon and nucleon-meson, it has been very challenging to understand the transition between them. Many exclusive processes such as pion photoproductions have been used to study the transition region. In recent experiment of charged pion photoproduction [1,2] a broad enhancement has been seen around√s= 2.1 GeV in the scaled differential cross section followed by the entry into the scaling region at center-of-mass (c.m.) angleθc.m.= 90◦. With very limited data points, further confirmation of these findings requires fine energy and angular scans of the differential cross section at photon energies around a few GeV. The JLabg10 data are ideal for this purpose. We present a measurement of the differential cross section using the g10 data fortheγn→π−pprocess from the CLAS detector at JLab in Hall B for photon energiesbetween 1.0 and 3.5 GeV andθc.m.between 50◦and 115◦. We confirm a previous indication of a broad enhancement in the scaled differential cross section s7dσdt and a rapid fall-off in the c.m. energy region of about 400 MeV following the enhancement. Our data show an angular dependence of this enhancement for the first time as the suggested scaling region is approached for θc.m.from 70◦to 105◦