Measurement of theΣbeam asymmetry for theωphotoproduction off the proton and the neutron at the GRAAL experiment

Abstract

We report on new measurements of the beam asymmetry for ω photo-production on proton and neutron in Hydrogen and Deuterium targets from the GRAAL collaboration. The beam asymmetry values are extracted from the reaction threshold (Eγ = 1.1 GeV in the free nucleon kinematics) up to 1.5 GeV of incoming photon energy. For the first time both the radiative and the threepion decay channels are simultaneously investigated on the free proton. Results from the two decay channels are in agreement and provide important constraints for the determination of resonant state contributions to the ω production mechanism. First experimental results on the deuteron allow the extraction of the Σ beam asymmetry on quasi-free nucleons. Comparison of the results for free and quasi-free kinematics on the proton shows a generally reasonable agreement, similar to the findings in pseudo-scalar meson photo-production reactions. For the first time measurements on quasi-free neutrons are available, showing that both the strength and the angular distributions of the beam asymmetry are sensibly different from the results on the proton target.

Pomeron, nucleon-resonance, and (0+,0−,1+) -meson contributions in ϕ -meson photoproduction

“…Consequently, as shown in the present work, our rigorous theoretical analyses on the presently available highstatistics and wide-angle-coverage experimental data for the various physical observables will be valuable for a profound understanding for the light-flavor (ρ and ω) and hidden-flavor (J/ψ) vector meson photoproductions [64][65][66][67][68][69][70][71]. Related works will appear elsewhere.…”

Measurement of the helicity asymmetry E in ω→π+π−π0 photoproduction

“…The photon-beam asymmetry Σ was first measured by the GRAAL Collaboration in 2006 from the decay modes ω → π 0 γ and ω → π + π − π 0 , and was presented in four energy bins that cover an energy range from threshold up to a photon energy of 1.5 GeV [12]. A second measurement based on both decay modes was published in 2015 and showed improved angular coverage [17]. The photon-beam asymmetry Σ was also measured by the CBELSA/TAPS Collaboration for a maximum energy of E γ = 1.5 GeV [18].…”

Nucleon resonances in γp→ωp reaction

“…Many data were reported in the year 2015 in particular: new measurements of differential cross sections and SDMEs and the first measurements of the double polarization asymmetries, the beam-target-helicity asymmetries E and G, were reported by the CBELSA/TAPS Collaboration [33,34]; a new measurement of the differential cross section with full production-angle coverage was reported by A2 Collaboration at MAMI [35]. Also, the beam asymmetry Σ has been measured by the GRAAL Collaboration [36]. More recently, the CLAS Collaboration reported the newest data of the photon beam asymmetry and a comparison with previous results were made [37].…”

“…The numbers in the bracket denote the photon incident energy (left number) and the center-of-mass energy of the system (right number), in MeV. Data are taken from CLAS Collaboration [40] (star), [37] (circle), CBELSA/TAPS Collaboration [54] (square), GRAAL Collaboration [36] (triangle). Only the data from Ref.…”