Reconciling gauge invariance and Reggeization of pion exchange

The Reggeized pion is expected to provide the main contribution to the forward cross section in light meson photoproduction reactions with charge exchange at high energies. Up until now, the pion exchange mechanism has been studied using phenomenological approaches. Given new and forthcoming high-precision data from JLab for a variety of photo- and electroproduction reactions in which the Reggeized pion plays a pivotal role, including the photoproduction of light exotic hybrid mesons, the JPAC collaboration found it essential to revisit the pion exchange mechanism in a more fundamental approach. This work, led by Gloria Montana (JLab), Daniel Winney (Bonn U., HISKP), and Adam Szczepaniak (Indiana U. & JLab, PI), along with Lukasz Bibrzycki (AGH Krakow), Cesar Fernandez-Ramirez (UNED), Giorgio Foti (Messina U. graduate student), Nadine Hammoud (Barcelona U.), Vincent Mathieu (Barcelona U., co-PI), Robert Perry (Barcelona U.), Alessandro Pilloni (Messina U., co-PI), Arkaitz Rodas (ODU & JLab, co-PI), Vanamali Shastry (Indiana U., ExoHad postdoc), and Wyatt Smith (GWU, ExoHad postdoc), discusses the Reggeization of pion exchange in charged pion photoproduction with an emphasis on ensuring consistency with current conservation. The study shows that the gauge-invariant amplitude for the exchange of a particle with generic even spin $J \ge 2$ in the $t$-channel is analytic at $J = 0$, and that it can be interpreted in terms of the nucleon electric current. This enables the authors to reconcile the dynamics in the $s$- and $u$-channel, which involve nucleon exchanges, with the amplitude expressed in terms of $t$-channel partial waves, as required by Regge theory.

Papers: Phys. Rev. D 110 (2024), 114012