arXiv subjects

C. F. Berger

Publications and source records attributed to C. F. Berger.

21 records · Page 2Linked to original sources

arXiv1999

Hard exclusive photoproduction of $Φ$ mesons

We calculate the differential cross section and single-polarization observables for the reaction $γp \to Φp$ within perturbative QCD, treating the proton as a quark-diquark system. The phenomenological couplings of gauge bosons to (spatially extended) diquarks and the quark-diquark distribution amplitude of the proton are adopted from previous investigations of baryon form factors and two-photon processes. Going beyond leading order, we take into account hadron-mass effects by means of a systematic expansion in the small parameter (hadron mass/ photon energy). With the $Φ$-meson distribution amplitude taken from the literature our predictions for the differential cross section at $| t | \agt 4 \text{GeV}^2$ seem to provide a reasonable extrapolation of the low-t data and are also comparable in magnitude with the results of a two-gluon exchange model in which the gluons are considered as a remnant of the pomeron. For momentum transfers of a few GeV hadron-mass effects appear still to be sizeable.

hep-ph↗

arXiv1999

Hard exclusive photoproduction of $Φ$ and $J/Ψ$ mesons

We consider the reaction $γp \to M p$, with M being either a $Φ$ or a $J/Ψ$ meson, within perturbative QCD treating the proton as a quark-diquark system. The phenomenological couplings of gauge bosons to (spatially extended) diquarks and the quark-diquark distribution amplitude of the proton are adopted from previous investigations of baryon form factors and two-photon processes. Going beyond leading order, we take into account hadron-mass effects by a systematic expansion in the small parameter (hadron mass/ photon energy). With the meson distribution amplitudes taken from the literature our predictions for the differential cross section at $| t | \agt 3 \hbox{GeV}^2$ seem to provide a reasonable extrapolation of the low-$t$ data in case of the $Φ$. They fail, however, completely for the $J/Ψ$. A closer inspection reveals that `` hadron-mass corrections\rq\rq become dominant for t values of only a few GeV in case of the $J/Ψ$. This indicates that one has to go to much larger energies and momentum transfers in order to achieve a reliable description of $J/Ψ$ production within the hard-scattering approach.

hep-ph↗

arXiv1999

Diquarks as effective particles in hard exclusive scattering

In the context of hard hadronic reactions diquarks are a useful phenomenological device to model non-perturbative effects still observable in the kinematic range accessible by present-day experiments. In the following we present diquark-model predictions for $γγ\to p \bar{p}$ and $Λ\barΛ$. We also sketch how the (pure quark) hard-scattering formalism for exclusive reactions involving baryons can be reformulated in terms of quarks and diquarks. As an application of these considerations we analyze the magnetic proton form factor with regard to its quark-diquark content.

hep-ph↗