The Constituent Counting Rule in Meson-Baryon Photoproduction*

The Constituent Counting Rule (CCR), based on perturbative Quantum Chromodynamics (pQCD) framework, predicts the scaling behavior of the differential cross-sections of scattering processes at high energies: dσ/dt=f(cosθ)/s^N.

Where s and t are Mandelstan variables, θ  is the scattering angle and N=n-2, in which n is the number of constituents fields in the reaction. On meson-baryon photoproduction, N is expected to be 7. Nonetheless, various studies have shown that it does not hold true for all reactions at all kinematic regimes.

We present a systematic study of pseudoscalar mesons-π, η, η'-photoproduction reactions in the 3-8 GeV² s-range. We also explore the evolution of N with respect to the transverse momentum (P_⊥²) of the produced particles.

We investigate the CCR applicability at different angles for these reactions, mainly by analyzing previously published data from Jefferson Lab's CLAS experiment. These different behaviors could allow us to understand the various contributing production mechanisms. We are collaborating with theorists to better understand the photoproduction of the pseudoscalar-mesons.