Measurement of double polarisation asymmetries in ω -photoproduction

Abstract

The first measurements of the beam-target-helicity-asymmetries E and G in the photoproduction of ω-mesons off protons at the CBELSA/TAPS experiment are reported. E (G) was measured using circularly (linearly) polarised photons and a longitudinally polarised target. E was measured over the photon energy range from close to threshold (E γ = 1108 MeV) to E γ = 2300 MeV and G at a single energy interval of 1108 < E γ < 1300 MeV. Both measurements cover the full solid angle. The observables E and G are highly sensitive to the contribution of baryon resonances, with E acting as a helicity filter in the s-channel. The new results indicate significant s-channel resonance contributions together with contributions from t-channel exchange processes. A partial wave analysis reveals strong contributions from the partial waves with spin-parity J P = 3/2 + , 5/2 + , and 3/2 − .

Photoproduction of Mesons

“…Klein et al [42] measured photon beam asymmetries, Σ, and pion asymmetries, Σ π , for ω photoproduction close to threshold. Eberhardt et al measured the beam-target helicity asymmetries, E, G and G π from threshold [43], and Wilson et al the differential cross sections and spin-density matrix elements [44].…”

“…8, bottom). A study of ω photoproduction at CLAS [46,47], via the decay ω → π + π − π 0 was made after the paper described here by Klein et al [42], but before the paper by Eberhardt et al [43]. Strong evidence for several N * states were found, including N(1680)5/2 + , N(1700)3/2 + and N(2190)7/2 + .…”

“…The measurements of E, G and G π that were made in [43] also act to understand the photoproduction mechanism close to threshold. For Pomeron or pion exchange, E should be zero.…”

“…The measurements with unpolarised and linearly polarised photon beams [42][43][44] used an electron energy of 3.2 GeV from ELSA [31]. The electrons were incident upon a diamond radiater, 500 μm thick, and aligned to produce linearly polarised photons via the coherent bremsstrahlung process.…”

“…The electrons were incident upon a diamond radiater, 500 μm thick, and aligned to produce linearly polarised photons via the coherent bremsstrahlung process. For the measurement of E presented in [43], a longitudinally polarised electron beam with an energy of 2.4 GeV was also used to produce a circularly polarised photon beam. The electrons were incident upon a Møller radiator to produce bremsstrahlung photons and simultaneously determine the degree of polarisation (see section 2.2).…”