Analysis of the data on differential cross sections and spin density matrix elements for $\gamma p \to \rho^0 p$

TL;DR

This study systematically analyzes $ ho^0$ photoproduction data using an effective Lagrangian model, revealing the dominant background contributions and the importance of nucleon resonances for understanding reaction mechanisms.

Contribution

It introduces a comprehensive effective Lagrangian approach with Regge propagators and resonance contributions to describe $ ho^0$ photoproduction data.

Findings

Background dominated by $f_2$-trajectory exchange explains high-energy data.

Including specific nucleon resonances improves low-energy angular distributions.

Predictions for asymmetries provide guidance for future experiments.

Abstract

The newly published data on spin density matrix elements from the GlueX Collaboration, along with the previously released differential cross-section data from the CLAS Collaboration and the other two experiments for the $\gamma p \to \rho^0 p$ reaction, are systematically investigated using an effective Lagrangian approach within the tree-level Born approximation. The model combines contributions from $t$-channel meson exchanges ($\pi$, $\eta$, and $f_2$), $s$-channel nucleon ($N$) and nucleon resonance ($N^\ast$) exchanges, $u$-channel $N$ exchange, and a generalized contact term to construct the scattering amplitudes. Regge propagators are employed for $t$-channel amplitudes to incorporate the contributions from mesons with various spins lying on the same trajectories. The analysis shows that the background contributions, dominated by the $f_2$-trajectory exchange, provide a satisfactory description of the data in the high-energy and forward-angle regions. The inclusion of specific nucleon resonances, such as $N(2100)1/2^{+}$, $N(2060)5/2^{-}$, or $\Delta(2000)5/2^{+}$, significantly improves the description of the differential cross-section data at near-perpendicular scattering angles in the low-energy region. Predictions of photon beam and target nucleon asymmetries are provided, offering valuable insights to discriminate reaction mechanisms when corresponding data become available in the future.