η,η′photoproduction and electroproduction off nucleons

Abstract

The photoproduction and electroproduction of the ,Ј mesons on nucleons are investigated within a relativistic chiral unitary approach based on coupled channels. The s-wave potentials for electroproduction and meson-baryon scattering are derived from a chiral effective Lagrangian, which includes Ј as an explicit degree of freedom and incorporates important features of the underlying QCD Lagrangian such as the axial U͑1͒ anomaly. The effective potentials are iterated in a Bethe-Salpeter equation and the cross sections for ,Ј photoproduction and electroproduction from nucleons are obtained. The results for the Ј photoproduction cross section on protons reproduce the appearance of an S 11 resonance around 1.9 GeV observed at ELSA. The inclusion of electromagnetic form factors increases the predicted electroproduction cross sections on the proton, providing a qualitative explanation for the hard form factor of the photocoupling amplitude observed at CLAS.

Constraints on the S=-1 meson-baryon interaction at NLO

“…As we see, the WT term depends only on one parameter -the pion decay constant f , which is well known experimentally, f π = 92.4 MeV. However in UχPT calculations this parameter is usually taken to be larger than the experimental value, ranging from f = 1.15 f π to f = 1.36 f π [7][8][9][10][11][12][13][14][15][16][18][19][20][21], meaning to be a sort of average over the decay constants of the mesons involved in the various coupled channels. We will leave it as a free parameter of our fits.…”

“…From the studies of the meson-baryon interaction in s-wave approximation [8][9][10][11][12][13][14][15][16][18][19][20][21], it can be concluded that the significant term that allows one to get a good agreement with the experimental data is the Weinberg-Tomozawa (WT) one, which is of order O(p 2 ), and the addition of other terms such as the direct and crossed Born terms (O(p 2 )) as well as the NLO terms (O(p 4 )) only play a finetuning role. The corresponding diagrams are represented in figure 1.…”

Nuclear reactions at intermediate energies

“…The effective methods that are used to describe the dynamics of meson production reactions, include explicitly the baryon resonance states whose properties are extracted by comparing the predictions of the theory with the experimental data [9][10][11][13][14][15][16][17]. However, for a reliable realization of this goal, a model is required that can analyze different reactions over the entire energy range using a single Lagrangian density, which generates all non-resonance contributions from Born, u-and t-channel contributions without introducing new parameters.…”

Photoproduction of ηʹ-mesons from the Proton

“…Moreover, because of an anomalous gluon component, the structure function of the η ′ meson is different from the one for mesons which are built up from quark-antiquark configurations. In a meson exchange model the intrinsic structure of the η ′ meson enters effectively through the form factors at the ρη ′ γ and ωη ′ γ vertices, which do not necessarily have to be of the standard monopole or dipole type [40][41][42][43][44]. In principle, the form factor at large |t| is given by the meson structure function at short distances and in case of η ′ meson might be defined through its gluon component.…”

“…Currently experimental and theoretical [40][41][42][43][44][45] studies of the reaction γp→η ′ p are motivated by the possibility to investigate excited baryons coupled to the η ′ meson. Assuming that the resonance production is the dominant contribution, an isobar analysis [37] of the SAPHIR data shows that the η ′ -photoproduction at photon energies 0.9≤E γ ≤2.6 GeV can be described by the coherent excitation of two resonances, S 11 (1897) and P 11 (1986).…”