Meson photoproduction at Jefferson Lab: from two-meson final states to meson-baryon spin-density matrices

Abstract

Recent progress in the understanding of meson photoproduction, obtained from the joint effort of the Joint Physics Analysis Center (JPAC) and the GlueX and CLAS experiments at Jefferson Lab, is reviewed. The photoproduction of the $\eta \pi$ system is now well understood, from the $a_2(1320)$ resonance region to the high-mass region dominated by double Regge exchanges. Since the lightest hybrid meson, the ${\pi }_1(1600)$, is expected to decay predominantly into $b_1\pi$, reactions with a recoiling ${\Delta }^{++}$ are promising channels for exotic searches. As a first step towards $\gamma p\to b_1\pi {\Delta }^{++}$, the complete angular distribution of $\gamma p\to {\rho }^{-}{\Delta }^{++}$ with a linearly polarized photon beam is derived. It involves 56 double-index spin-density matrix elements, which describe the correlations between the $\rho$ and the $\Delta$ decays. When the production amplitudes factorize, as is the case for the exchange of a single Regge pole, the intensity reduces to the product of the standard $\rho$ and $\Delta$ decay distributions and depends on only 14 parameters.

arXiv commentProceedings of a plenary talk at the MESON conference (Kraków, June 2026)