New fits to the reaction $\gamma p\to K^+\Lambda$

TL;DR

This paper develops a comprehensive theoretical model to analyze the $\gamma p o K^+\Lambda$ reaction, fitting recent experimental data to extract resonance parameters and improve understanding of baryon resonances.

Contribution

It introduces a tree-level effective Lagrangian model including well-established and higher-mass nucleon resonances, with two fits exploring SU(3) symmetry constraints, to better describe experimental reaction data.

Findings

Model fits well to recent differential cross section data

Inclusion of higher-mass resonances improves reaction description

SU(3) symmetry constraints have minimal impact on fit quality

Abstract

The reaction $\gamma p\to K^+\Lambda$ has been investigated over the center-of-momentum energy, $W$, range from threshold up to 2.2 GeV in a tree-level effective Lagrangian model that incorporates most of the well-established baryon resonances with spins equal to or below 5/2. Four less well-established nucleon resonances of higher mass are also included. The fitted parameters consist, for each resonance included, of the products of the coupling strengths at the electromagnetic and strong interaction vertices and, for the less-established nucleon resonances, the total decay width. For the well-established nucleon resonances, the energy and momentum dependence of the widths is treated within a dynamical model that is normalized to give the empirical decay branching ratios on the resonance mass shells. For the less-established resonances, the total decay width is treated as a single parameter independent of the reaction kinematics. The model is used to fit recent data for the unpolarized differential cross section (CLAS), the induced hyperon polarization asymmetry, $P$ (CLAS, GRAAL, and SAPHIR), the beam spin asymmetry, $\Sigma$ (LEPS), and the double polarization observables $C_x$ and $C_z$ (CLAS). Two different fits were obtained: one that incorporates SU(3) symmetry constraints on the Born contributions to the reaction amplitude and one in which these constraints are relaxed. Explicit numerical results are given only for the first fit since the two fits gave nearly identical results for the observables and the $\chi^2$ per degree of freedom obtained with the second fit was only marginally better than that of the first fit ($<1%$ better). Results are presented for the fitted observables at several different energies and center-of-momentum (c.m.) frame kaon angles.