Abstract
We report measurements of π+ and π0 meson photoproduction from longitudinally spin-polarised protons by an energy tagged (0.73-2.3 GeV) and linearly polarised photon beam. A close to complete solid angle coverage for the reaction products was provided by the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The double-polarisation observable G is extracted from Maximum Likelihood fits to the data, enabling the first accurate determination for the reaction γ→p→→π+n, while also significantly extending the kinematic coverage for γ→p→→π0p. This large data set provides an important constraint on the properties and spectrum of excited nucleon states decaying to Nπ in the mass range from 1.4 to 2.2 GeV, as well as for background (non-resonant) photoproduction processes. The considerable improvement achieved in the description of the observable G within the SAID and Bonn-Gatchina approaches after implementation of our data, illustrates that the partial-wave analyses now significantly extend the knowledge on Nπ photoproduction amplitudes at W>1.8 GeV. A partial-wave analysis using the new high-precision data set has a large impact on the extracted properties of high-spin nucleon excited states.
Highlights
Hadrons are composite strongly-bound systems, whose fundamental properties derive from the internal dynamics between their constituents, quarks and gluons
The considerable improvement achieved in the description of the observable G within the SAID and Bonn-Gatchina approaches after implementation of our data, illustrates that the partial-wave analyses significantly extend the knowledge on Nπ photoproduction amplitudes at W > 1.8 GeV
The results for the double-polarisation observable G as a function of the pion polar production angle in the c.m. frame for selected W bins for both reactions are shown in Fig. 3
Summary
Hadrons are composite strongly-bound systems, whose fundamental properties derive from the internal dynamics between their constituents, quarks and gluons. Phenomenological constituent quark models and, more recently, lattice QCD calculations [1,2] predict a rich spectrum of nucleon resonances, in contrast to the more limited spectrum of states that have (currently) been established experimentally [3]. The cause of this so-called “missing resonance” problem [4] has been a major focus for contemporary experimental hadron physics and on the theory side has led to alternative interpretations of nucleon structure [5,6,7,8] that reduce the number of predicted states. Meson photoproduction offers the possibility of measurement of an extensive set of polarisation observables, information that is mandatory for partial-wave analysis to disentangle the underlying spectrum
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