Elastic Photoproduction of J/Psi and Upsilon Mesons at HERA

The H1 experiment at the HERA ep collider has measured the differential photoproduction cross sections gamma p--> J/Psi p in positron proton collisions for an extended range of gamma p center of mass energies W. This follows earlier measurements of the total photoproduction cross section (Nucl.Phys. B472 (1996) 3 ). The J/Psi, a bound charm-anticharm state, which carries the same quantum numbers as the photon, is produced in the chromodynamic field of the proton, which recoils intact.

The peripheral nature of production, namely with small momentum transfer (-t) to/from the proton, is characteristic of a diffractive mechanism. The purely charm-anticharm content of the J/Psi ensures that neither of the quark structures of the proton (uud) and of the J/Psi (ccbar) can be exchanged in the production mechanism, so that it is purely diffractive. Hence, the t-channel process is dominated only by an exchange carrying the vacuum quantum numbers, generically termed pomeron exchange, and there can be no additional contributions from meson exchange, which are characteristic for hadron-hadron collisions.

The charmonium nature of the J/Psi also provides an inherent hard scale in the form of the charm quark mass squared mc2. Thus the process is the cleanest way of studying hard diffraction, and as such it provides a unique window on its interpretation in the framework of perturbative Quantum Chromodynamics (pQCD)..

The measurements (see figure) show the W dependences at different t of the differential cross section dsig/dt in the peripheral production. The cross sections are observed to rise steeply with increasing W even out to large, less peripheral, values of -t. This rise is everywhere greater than that observed in soft diffractive vector meson photoproduction, such as in gamma p --> phi p. Such an observation has been anticipated to a lesser or greater extent in theoretical calculations based on perturbative QCD. The quantitative outcome of these measurements now becomes a challenge to these attempts in QCD to understand diffraction and a QCD based procedure of calculating the way hadrons interact, as is clear from the comparison of different calculations with the results (compare NLO BFKL).

It would be interesting to investigate the same behaviour for the even heavier Upsilon vector meson, a bound state of beauty and anti-beauty quarks. Being 10 times as heavy as the proton the mass should provide the natural scale for pQCD calculations and lead to results with even less theoretical ambiguity. Because of its large mass the production cross section in gamma p interactions is severely suppressed and the data are not yet of sufficient statistical accuracy to disentangle the t-dependence. The H1 collaboration has however managed to present a measurement of the total cross section for Upsilon production, which is found to agree with the models based on pQCD within the large uncertainties.

Last Update Mar 3, 2000, E. Elsen