The Constituent Counting Rule in Meson-Baryon Photoproduction*

In 1975, S.J. Brodsky and G.R. Farrar proposed the dimensional scaling laws as an approach to understand the energy dependence of high energy scattering processes at a fixed center-of-momentum angle. 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 introduce a systematic study of pseudoscalar mesons-π,η,η^'-photoproduction reactions in the 3-8 GeV² s-range. At the same time, we study the evolution of N with respect to the transverse momentum (P_⊥²) of the produced particles.

We investigate the energy dependence of pseudoscalar-meson photoproduction by analyzing previously published data from Jefferson Lab's CLAS experiment. We also investigate why some reactions do not have the expected N=7. A reason we have considered is the different production mechanism of the reactions and the energy range of the data. We are collaborating with theorists to better understand the photoproduction of the pseudoscalar-mesons.