Photoproduction of straight phi(1020) mesons on the proton at large momentum transfer

Spectral-fluctuations test of the quark-model baryon spectrum

We study the low-lying baryon spectrum (up to 2.2 GeV) provided by experiments and different quark models using statistical tools which allow us to postulate the existence of missing levels in spectra. We confirm that the experimental spectrum is compatible with random matrix theory, the paradigmatic model of quantum chaos, and we find that the quark models are more similar to a Poisson distribution, which is not compatible with what should be expected in a correlated spectrum. From our analysis it stems that the spectral fluctuation properties of quark-model spectra are incompatible with experimental data. This result can be used to enlighten the problem of missing resonances.

Near-threshold diffractive psi-meson photoproduction from the proton

Photoproduction of a phi meson on protons was studied by means of linearly polarized photons at forward angles in the low-energy region from threshold to Egamma = 2.37 GeV. The differential cross sections at t = -|t|min do not increase smoothly as Egamma increases but show a local maximum at around 2.0 GeV. The angular distributions demonstrate that phi mesons are photoproduced predominantly by helicity-conserving processes, and the local maximum is not likely due to unnatural-parity processes.

Observation of an exotic S = +1 baryon in exclusive photoproduction from the deuteron

In an exclusive measurement of the reaction gammad-->K(+)K(-)pn, a narrow peak that can be attributed to an exotic baryon with strangeness S=+1 is seen in the K(+)n invariant mass spectrum. The peak is at 1.542+/-0.005 GeV/c(2) with a measured width of 0.021 GeV/c(2) FWHM, which is largely determined by experimental mass resolution. The statistical significance of the peak is (5.2+/-0.6)sigma. The mass and width of the observed peak are consistent with recent reports of a narrow S=+1 baryon by other experimental groups.

Locality of quark-hadron duality and deviations from quark counting rules above the resonance region

We show how deviations from the dimensional scaling laws for exclusive processes may be related to a breakdown in the locality of quark-hadron duality. The essential principles are illustrated in a pedagogic model of a composite system with two spinless charged constituents, for which a dual picture for the low-energy resonance phenomena and high-energy scaling behavior can be established. We introduce the concept of "restricted locality" of quark-hadron duality and show how this results in deviations from the perturbative quantum chromodynamics quark counting rules above the resonance region. In particular, it can be a possible source for oscillations about the smooth quark counting rule, as seen, e.g., in the 90-degree differential cross sections for gammap-->pi(+)n.

Photoproduction of the omega meson on the proton at large momentum transfer

The differential cross section, dsigma/dt, for omega meson exclusive photoproduction on the proton above the resonance region (2.6<W<2.9 GeV) was measured up to a momentum transfer -t=5 GeV2 using the CLAS detector at Jefferson Laboratory. The omega channel was identified by detecting a proton and pi(+) in the final state and using the missing mass technique. While the low momentum transfer region shows the typical diffractive pattern expected from Pomeron and Reggeon exchange, at large -t the differential cross section has a flat behavior. This feature can be explained by introducing quark interchange processes in addition to the QCD-inspired two-gluon exchange.

Photoproduction of the rho(0) meson on the proton at large momentum transfer

The differential cross section, d sigma/dt, for rho(0) meson photoproduction on the proton above the resonance region was measured up to a momentum transfer -t = 5 GeV2 using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The rho(0) channel was extracted from the measured two charged-pion cross sections by fitting the pi(+)pi(-) and p pi(+) invariant masses. The low momentum transfer region shows the typical diffractive pattern expected from Reggeon exchange. The flatter behavior at large -t cannot be explained solely in terms of QCD-inspired two-gluon exchange models. The data indicate that other processes, like quark interchange, are important to fully describe rho photoproduction.