ZEUS Diffractive Physics Group
12 July 2000

High-t Vector-Meson Photoproduction
- Some Phenomenological Considerations -

J.A. Crittenden
P.I. Bonn

Last year at the EPS conference in Tampere, ZEUS presented the first measurements of the differential cross section $\frac{d\sigma}{dt}$ for proton-dissociative $\rho^0$, $\phi$, and $J/\psi$ photoproduction at values for |t| between 1 and 10 $\mathrm{{\rm Ge}\kern-1.pt{\rm V}}^2$. In two weeks we will present at the ICHEP conference in Osaka an update of these results with an increase in statistics of a factor of three. These measurements are intrinsically interesting due to the fact that the |t| values exceed the mass scales in the process, reaching an asymptotic region where the t dependence yields direct information on the dynamics of the scattering process. This is the first measurement at |t| values comparable in magnitude to the Q² values attained at SLAC in 1967, when Bjorken scaling and the presence of point-like charged constituents of the proton were discovered. Recently it became clear that our data are sensitive enough to show a clear preference of a power n of 3 in a parametrization of the differential cross section of the form t^-n, appearing to exclude powers of 2 and 4 with considerable significance. This scaling law seems to indicate that we are observing the lowest-order process in a quantum field theoretical description of diffractive strong interactions.
The pQCD model of Brodsky et al based on two-gluon exchange predicts such a t dependence, but only in the case of longitudinal polarization of the vector meson, which our data exclude to an accuracy of about 10%. Ivanov et al have published work on a chiral-odd contribution to the production of transverse mesons which they claim is dominant in the t region of our measurements, but this contribution scales with t^-4. Kochelev et al have calculated the contribution of the exchange of an anomalous trajectory with the quantum numbers of the f₁ axial vector meson (``Regge-ized f₁ exchange''). Via private communication I received their prediction for the t dependence and found it to be a remarkably good fit to our data.
Unfortunately, due to time constraints at the time of this talk, I was not able to make the scaling and comparison plots with the new points for Osaka, and used instead last year's Tampere points. Since the results are consistent, I do not expect the messages of this talk to change, but it will be interesting to see these plots with the new points.


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