The Constituent Counting Rule and Omega Photoproduction

Abstract

The constituent counting ruling (CCR) has been found to hold for numerous hard, exclusive processes. It predicts the differential cross section at high energies and fixed $\cos {\theta }_{c.m.}$ should follow $\frac{d\sigma }{dt}\sim \frac{1}{s^{n-2}}$, where $n$ is the minimal number of constituents involved in the reaction. Here we provide an in-depth analysis of the reaction $\gamma p\to \omega p$ at ${\theta }_{c.m.}\sim 90^{\circ }$ using CLAS data with an energy range of $s=5-8$ GeV${}^2$, where the CCR has been shown to work in other reactions. We argue for a stringent method to select data to test the CCR and utilize a Taylor-series expansion to take advantage of data from nearby angle bins in our analysis. Naïvely, this reaction would have $n=9$ (or $n=10$ if the photon is in a $q\bar{q}$ state) and we would expect a scaling of $\sim s^{-7}$ ($s^{-8}$). Instead, a scaling of $s^{-(9.08\pm 0.11)}$ was observed. Explanations for this apparent failure of the naïve CCR assumptions are examined.

Comments: 6 pages, 8 figures