Helicity Soft Dipole Pomeron Model for Vetor Meson Photoproduction by Circularly or Linearly Polarized Photons down to the Production Threshold

TL;DR

The paper addresses vector-meson photoproduction by arbitrarily polarized photons over a broad energy and momentum-transfer range, aiming to access spin dynamics and SDMEs near threshold. It develops a Helicity Soft Dipole Pomeron (HSDP) model within Regge theory, combining a dipole Pomeron with linear Regge trajectories $oldsymbol{ar{oldsymbol{ ext α}}}_{ ext E}$ and $oldsymbol{ extα}'_{ ext E}$, and uses factorized helicity amplitudes with explicit $oldsymbol{ ext Σ}_{0,1,2}$ terms and WZW-driven pseudoscalar exchanges. A 20-parameter fit to three data classes ($\sigma_{tot}$, $d(\sigma)/dt$, and linear SDMEs) from GlueX, SLAC, and HERA yields best-fit values with asymmetric uncertainties and shows improved agreement over previous models (SDPM, JPACM), including predictions for circular SDMEs. The results illuminate the soft-to-hard transition in nonperturbative QCD spin dynamics and enable potential polarimetry of GeV cosmic photons, while providing a flexible framework for extending to other vector mesons and electroproduction.

Abstract

We present a model with dipole Pomeron bassed on Regge theory framework for vetor meson photoproduction by arbitarily polaried photons. The accurate helicity amplitude is constructed with free trajectory parameters. This model consistently describes the total and momentum-transfer differential cross sections and he spin-density matrix elements in photoproduction of $ρ^{0}$, by fitting it to the data of $3$ categories simultaneously to determine its $20$ parameters. The agreements with experimental data of our model improve those of the previous models remarkably and predictions for circular SDMEs are made. The model provides key description of the process for our innovative polarimetry of cosmic photons and essential insight to explore the non-perturbative regime and the spin dynamics of strong interaction.

Figures (6)

• Figure 1: (a) The $V$ photoproduction in Regge theory, i.e., the factorized helicity amplitude in Eq.(\ref{['heliamp']}); (b) The quark-gluon picture of the process in perturbative QCD.
• Figure 2: Comparison of the $\rho^{0}$ photoproduction total cross setions $\sigma_{tot}$ of our model with the previous models and the experimental data. The sampled SDPM points with their projected uncertainies are shown in cyan. Besides the data collected in xsecmodel, new HERA data in 2020 are shown in purple.
• Figure 3: Theoretical model comparison of the normalized $d\sigma/dt$ of $\rho^{0}$ photoproduction with the experimental data after the treatment in Appendix \ref{['Appd3']}: $3$ SLAC and GlueX (right bottom), which are marked in blue. The low $-t_{L}$ and high $-t_{H}$ regions of $-t$ are shown in different horizontal and vertical scales.
• Figure 4: Comparison of the $9$ linear SDMEs $rho^{\alpha}_{ij}$ among our model, the JPAC model family and the GlueX meaurement at $E_{\gamma}=8.5 GeV$.
• Figure 5: Comparison of the natural (top) and unnatural (bottom) linear SDMEs among the models with the SLAC data at $E_{\gamma}=2.8 GeV$ and $4.7 GeV$.