Extraction of reaction amplitudes from complete photo-production experiments

Abstract

Abstract.A new generation of complete experiments in pseudoscalar meson photo-production is being pursued at several laboratories. Here the designation of complete refers to measurements of most if not all of the possible reaction observables, of which there are 16 involving spins of the beam, target and recoil baryon. Hyperon production to ! or " + final states affords attractive opportunities, since their weak decays provide an efficient self-analysis of their spin. When the beam and target are also polarized, the resulting triple polarization measurements determine the full suite of observables with a single target orientation. This has been a focus at Jefferson Lab in the recently completed g9/FROST and g14/HDice experiments now under analysis. Multipole analyses of #p $ % + ! have been carried out with recently published (incomplete) polarization data, and the uniqueness of the extracted amplitudes has been studied. Experiments with realistically achievable uncertainties require a significantly larger number of spin asymmetries than the in-principle minimum needed for a mathematical solution of the amplitude.Keywords: meson photo-production, baryon resonances, multipole analysis, polarization. PACS: 13.60Le, 13.75.Gx, 13.75.Jz, 13.88.+e, 14.20.Gk; THE N* PROBLEM AND MEASUREMENTS OF EVERYTHINGThe low-energy structure of QCD lies encoded in the excited-state spectrum of the nucleon, which is a complicated overlap of many resonances. Recently, Lattice QCD calculations [1] have confirmed the long standing Quark Model predictions [2] of a large number of excited nucleon states, only a fraction of which have been identified. Only the lowest predicted levels have been reported and these come mainly from analyses of &N scattering. This may be a reflection of the &N partial widths, which are predicted to diminish with energy while higher lying states are expected to have larger branches to KY and &&N final states. Such final states are excited in photo-production and the spin of the photon provides additional probes of the amplitude. The outstanding challenge is to unravel the N* spectrum in as model-independent a way as possible. The goal of a recent series of experiments at Jefferson Lab is a measurement of all possible spin-observables in order to determine the production amplitude as a curve in the complex plane. This would serve as the ultimate starting point in a search for poles by analytic continuation. Since the electromagnetic interaction does not conserve isospin, this requires data on both proton and neutron targets. The experiments with polarized protons were the focus of the g9/FROST runs, while the g14/HDice experiment focused on polarized neutron reactions.