Multichannel parametrization ofπNscattering amplitudes and extraction of resonance parameters

Abstract

We present results of a new multichannel partial-wave analysis for πN scattering in the c.m. energy range 1080 to 2100 MeV. This work explicitly includes ηN and KΛ channels and the single pion photoproduction channel. Resonance parameters were extracted by fitting partial-wave amplitudes from all considered channels using a multichannel parametrization that is consistent with S-matrix unitarity. The resonance parameters so obtained are compared to predictions of quark models.

Scrutinizing the evidence for N(1685)

“…In 2012, the observations reported in [1,3,6] were introduced into the Review of Particle Properties (RPP) under the heading of a new one-star nucleon resonance N (1685) [11] but was removed from the listings in the most recent issue of RPP [12]. The interpretation of the structure as narrow resonance was supported by further studies [13][14][15][16], the results reported in [17] were ambiguous. In a study of the helicity asymmetry of the reaction γp → ηp, no evidence for N (1685) was found [18].…”

Production of η and η′ mesons on nucleons and nuclei

“…20b is the original scattering-length fit [137] to the older results. This clearly fails to reproduce the new data close to threshold but, on the other hand, a parameterization of the S 11 (1535) resonance [81] describes well the forward dip in |f | 2 . However, other masses and widths for the S 11 resonance give an equally good description so that these data are not precise enough to tie down the resonance parameters accurately.…”

“…Various scenarios have been suggested in the literature for the narrow structure in the neutron excitation function. They range from different coupled-channel effects of known nucleon resonances [70,76], interference effects in the S 11 partial wave [77,46], effects from the opening of strangeness thresholds [78], to intrinsically narrow states [73,77,79,80,81]. The available data are still insufficient for an unambiguous analysis.…”

“…A negative sign was also preferred by quark models (e.g., Capstick [85]: −35; Burkert et al [86]: −31±3). However, the most recent analyses of the Bonn-Gatchina group [46] (25±20) and Shresta and Manley [81] (11±2) find positive values corresponding to a constructive interference. This solution might be appealing, in so far as it could explain the structures in the proton and neutron excitation function with the same mechanism.…”

“…The solid (black) line represents the contribution from the N * (1535) isobar with parameters taken from Ref. [81]. The (blue) dashed line is an effective-range fit [137] to the older data.…”