Baryon– Antibaryon Photoproduction off the Proton

 The GlueX Experiment at Jefferson Lab uses linearly polarized photon beam to study the photoproduction of light mesons and baryons off a proton target with unprecedented precision. We have observed pp¯ photoproduction and, for the first time, ΛΛ¯ photoproduction from thresholds up to Eγ = 11.6 GeV. 

We reproduce the observed kinematic distributions with a phenomenological model that includes t-channel Regge exchange and double Regge exchange. The model uses few parameters to match all the kinematic spectra via an optimization algorithm developed specifically for this purpose. 

We report the total cross sections of the two reactions, as well as differen?tial cross sections with respect to various momentum transfers and invariant mass spectra of pp¯, ΛΛ¯, pΛ¯ systems which show enhancements right above pro?duction thresholds. Furthermore, the beam asymmetries of the pp¯ system are extracted from the GlueX data to further probe the nature of the t-channel Regge exchanges. We also report preliminary measurements of the spin polarizations of the individual hyperons.

 No prominent resonance structure was found in these reaction channels, but the angular distributions and the correlation of longitudinal momenta indicate the existence of multiple production mechanisms. Besides a dominant forward peak in the polar angle of the baryon-antibaryon pair in t-channel production, we see wide-angle antibaryon distributions different from that of the baryon. In the hyperon channel, we see a clear separation between the photoproduction of the ΛΛ¯, pΛ¯ systems