Observation of the pη′ Cusp in the New Precise Beam Asymmetry Σ Data for γp→

Abstract

Data on the beam asymmetry Σ in the photoproduction of η mesons off protons are reported for tagged photon energies from 1130 to 1790 MeV (mass range from W = 1748 MeV to W = 2045 MeV). The data cover the full solid angle that allows for a precise moment analysis. For the first time, a strong cusp effect in a polarization observable has been observed that is an effect of a branch-point singularity at the pη threshold [Eγ = 1447 MeV (W = 1896 MeV)]. The latest BnGa partial wave analysis includes the new beam asymmetry data and yields a strong indication for the N (1895) 1 2 − nucleon resonance, demonstrating the importance of including all singularities for a correct determination of partial waves and resonance parameters.

Light Baryon Spectroscopy

“…This cusp-like structure gives evidence for a third S-wave resonance, the N (1895) 1 2 − (S 11 ), as the cusp is caused by the opening of the pη decay channel of this resonance. This claim is further supported by the recently published beam asymmetry Σ data of the CBELSA/TAPS collaboration [106]. Here, the pη cusp was observed as well in a moment analysis of the beam asymmetry data.…”

“…The multipole-series achieves a crucial separation of the dependencies on energy W and angle θ at the level of amplitudes. When one tries to accomplish the same at the level of observables, the formalism of (Legendre-) moment analysis arises, which recently has received attention as a useful analysis-tool for single-meson photoproduction [105,72,106]. Truncating the multipole-series (eqs.…”

New graphical criterion for the selection of complete sets of polarization observables and its application to single-meson photoproduction as well as electroproduction

“…This convention will turn out to be important for the discussion from section III onwards. Consistency-relations such as (7) are crucial for the derivation of fully complete sets. The real-and imaginary parts of the bilinear amplitude-products corresponding to the solid and dashed lines in the considered graph lead, via equations ( 4) and (6), to a set of 2 N potentially ambiguous solutions.…”

“…There is another sign which is important: this has to do with the index-structure of the relative-phases, as they appear in our standard-convention for the consistency relation (7) (or ( 8)). For instance, in case φ ij appears in this equation with reversed placement φ ji , there appears yet another sign one has to carefully keep track of.…”

“…For the spectroscopy of baryons [1,2] in particular, most experimental activities in the recent years have taken place at facilities capable of measuring reactions induced by electromagnetic probes. Well-known experiments, all capable of measuring the photoproduction of one or several pseudoscalar mesons (as well as vector mesons), are the CBELSA/TAPS experiment at Bonn [3][4][5][6][7][8], CLAS at JLab (Newport News) [9][10][11][12][13][14][15], A2 at MAMI (Mainz) [16][17][18][19][20][21][22][23][24], and LEPS at SPring-8 (Hyōgo Prefecture) [25,26]. The GlueX Collaboration has started exploring completely new kinematic regimes for single-meson photoproduction recently [27][28][29].…”