Narrow Structure in the Excitation Function ofηPhotoproduction off the Neutron

Abstract

The photoproduction of η-mesons off nucleons bound in 2 H and 3 He has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the η → 2γ and η → 3π 0 decays and the recoil nucleons were detected with an almost 4π electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of γn → nη. The results from the two measurements are consistent taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are W = (1670±5) MeV and Γ = (30±15) MeV. For the first time precise results for the angular dependence of this structure have been extracted.

Partial wave analysis

“…Fig. 17 shows the total cross section for γp → ηp, for γn → ηn and -as insert -the ratio σ n /σ p from the most recent experiment [150]. The new data exceed the earlier data both in quality and in statistics.…”

“…In contrast to Θ + (1540), the peak at about 1685 MeV became increasingly significant with better experiments at ELSA and MAMI [148][149][150], its existence is beyond doubt. But the interpretation as a resonance is unclear.…”

“…Figure 17. Left: The total cross section for γn → ηn (multiplied by 3/2), γp → ηp, and their ratio (as inset) [150] . The solid curves represent our fit [88] folded with the experimental resolution (thick ηn, thin ηp), the dashed curves the contributions from the S 11 waves.…”

Spectroscopy of baryon resonances

“…In 2012, the structure was listed in the RPP as one-star resonance. At Mainz, the structure was studied with high statistics and again confirmed [128,129]. The peak is discussed controversially in the literature, and different interpretations were given [130][131][132][133][134][135][136].…”

“…The peak is discussed controversially in the literature, and different interpretations were given [130][131][132][133][134][135][136]. So far, the most recent and most precise data [128,129] were only fitted in [136]. The latter fit gave a nearly perfect description of the data assuming that the structure emerges from the interference of N(1535)1/2 − and N(1650)1/2 − within the 1/2 − -partial wave.…”