Helicity-dependent cross sections and double-polarization observable E in η photoproduction from quasifree protons and neutrons

Abstract

Precise helicity-dependent cross sections and the double-polarization observable E were measured for η photoproduction from quasifree protons and neutrons bound in the deuteron. The η → 2γ and η → 3π 0 → 6γ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz. A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98% of the full solid angle. The results show that the narrow structure observed earlier in the unpolarized excitation function of η photoproduction off the neutron appears only in reactions with antiparallel photon and nucleon spin (σ 1/2 ). It is absent for reactions with parallel spin orientation (σ 3/2 ) and thus very probably related to partial waves with total spin 1/2. The behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with * Also at Dipartimento di Fisica, Università di Pavia, I-27100 Pavia, Italy. Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which uses an interference of P 11 and S 11 partial waves to explain the narrow structure.

First measurement of helicity-dependent cross sections in π0η photoproduction from quasi-free nucleons

“…No significant effects were found for ππ and πη pairs. Also, for the helicity asymmetry E, as defined in this paper, no effects were found for single π 0 [37] and η production [33][34][35], although the effects on absolute cross sections for π 0 production are substantial. For πη production also shapes of invariant mass distributions of meson-meson and mesonnucleon pairs are basically unaffected [24].…”

“…However, as discussed in detail in [33,34] the target polarization was not homogeneous across the target diameter for the first three beam times. Thus, the standard NMR measurements of the polarization did not correspond to the effective polarization in the target center hit by the photon beam.…”

“…The present results are based on the same data set that was previously used to extract the polarization observable E for the production of η mesons [33,34] and for π 0 mesons [37] off quasi-free nucleons (most details are given in [34]). Therefore, we give only a short summary of the experimental details.…”

Experimental study of the γp→π0ηp reaction with the A2 setup at the Mainz Microtron

“…energy W = 1.68 GeV [33][34][35][36][37], while the γp → ηp total cross section showed a dip at the same energy [38,39]. So far this effect, which was called a "neutron anomaly," has no unique explanation in various partial-wave analyses (PWA) [28,40]. Meanwhile, the latest analysis of the available γp → ηp data with the revised ηMAID model describes the dip at W = 1.68 GeV well, without introducing any narrow state [39].…”

Effects of nucleon resonances on η photoproduction off the neutron reexamined

“…These works were motivated by the results from the chiral quark-soliton model (χQSM) [16,17]. We want to mention that recently, A2 Collaboration has carried out the measurement of the double polarization observables and the helicity-dependent cross sections for η photoproduction from the quasi-free proton and neutron [18,19]. Interestingly, the narrow bump-like structure was seen only in the spin-1/2 helicity-dependent cross section.…”