E. Barriga

The experiment at Jefferson Lab has observed p p ¯ and, for the first time, Λ Λ ¯ and p Λ ¯ photoproduction from a proton target at photon energies up to 11.6 GeV. The angular distributions are forward peaked for all produced pairs, consistent with Regge-like t -channel exchange. Asymmetric wide-angle antibaryon distributions show the presence of additional processes. In a phenomenological model, we find consistency with a double- t -channel exchange process where antibaryons are created only at the middle vertex. The model matches all observed distributions with a small number of free parameters. In the hyperon channels, we observe a clear distinction between photoproduction of the Λ Λ ¯ and p Λ ¯ systems but general similarity to the p p ¯ system. We report both total cross sections and cross sections differential with respect to momentum transfer and the invariant masses of the created particle pairs. No narrow resonant structures were found in these reaction channels. The suppression of s s ¯ quark pairs relative to d d ¯ quark pairs is similar to what has been seen in other reactions.

We measure the spin-density matrix elements (SDMEs) for the photoproduction of ϕ ( 1020 ) off of the proton in its decay to K S 0 K L 0 , using 105 pb − 1 of data collected with a linearly polarized photon beam using the GlueX experiment. The SDMEs are measured in nine bins of the squared four-momentum transfer t in the range − t = 0.15 − 1.0 GeV 2 , providing the first measurement of their t dependence for photon beam energies of E γ = 8.2 − 8.8 GeV. We confirm the dominance of Pomeron exchange in this region and put constraints on the contribution of other Regge exchanges. We also find that helicity amplitudes where the helicity of the photon and ϕ ( 1020 ) differ by two units are negligible.

Applying the technique of partial-wave analysis, there are cases where more than one set of underlying complex-valued amplitudes can describe the measured observables. These ambiguities can sometimes be resolved using additional information, but assumptions are often required. It is known that the partial-wave analysis of two-pseudoscalar meson systems produced in photoproduction with a linearly polarized photon beam is free from discrete ambiguities stemming from the Barrelet zeros when the nucleon spin is ignored. In this article, we show that continuous ambiguities are possible for certain wave sets, even though the discrete ambiguities do not appear. We also explore ways to resolve these ambiguities and determine the maximal amount of information that can be obtained from analyses that suffer from these continuous ambiguities.

Applying the technique of partial-wave analysis, there are cases where more than one set of underlying complex-valued amplitudes can describe the measured observables. These ambiguities can sometimes be resolved using additional information, but assumptions are often required. It is known that the partial-wave analysis of two-pseudoscalar meson systems produced in photoproduction with a linearly polarized photon beam is free from discrete ambiguities stemming from the Barrelet zeros when the nucleon spin is ignored. In this article, we show that continuous ambiguities are possible for certain wave sets, even though the discrete ambiguities do not appear. We also explore ways to resolve these ambiguities and determine the maximal amount of information that can be obtained from analyses that suffer from these continuous ambiguities.

The spin-exotic hybrid meson π_{1}(1600) is predicted to have a large decay rate to the ωππ final state. Using 76.6 pb^{-1} of data collected with the GlueX detector, we measure the cross sections for the reactions γp→ωπ^{+}π^{-}p, γp→ωπ^{0}π^{0}p, and γp→ωπ^{-}π^{0}Δ^{++} in the range E_{γ}=8-10 GeV. Using isospin conservation, we set the first upper limits on the photoproduction cross sections of the π_{1}^{0}(1600) and π_{1}^{-}(1600). We combine these limits with lattice calculations of decay widths and find that photoproduction of η^{'}π is the most sensitive two-body system to search for the π_{1}(1600).