Evidence for a negative-parity spin-doublet of nucleon resonances at 1.88\,GeV

TL;DR

This paper presents evidence for a pair of nearly degenerate negative-parity nucleon resonances around 1.88 GeV, supporting models predicting chiral symmetry restoration and parity doublets in high-mass nucleon excitations.

Contribution

It provides experimental evidence for a negative-parity spin-doublet of nucleon resonances near 1.88 GeV, supporting theoretical models of chiral symmetry restoration.

Findings

Identification of two nearly degenerate resonances with J^P=1/2^- and 3/2^-

Observation of parity doublets in high-mass nucleon excitations

Implications for the missing resonance problem

Abstract

Evidence is reported for two nucleon resonances with spin-parity $J^P=1/2^-$ and $J^P=3/2^-$ at a mass just below 1.9\,GeV. The evidence is derived from a coupled-channel analysis of a large number of pion and photo-produced reactions. The two resonances are nearly degenerate in mass with two resonances of the same spin but positive parity. Such parity doublets are predicted in models claiming restoration of chiral symmetry in high-mass excitations of the nucleon. Further examples of spin parity doublets are found in addition. Alternatively, the spin doublet can be interpreted as member of the 56-plet expected in the third excitation band of the nucleon. Implications for the problem of the {\it missing resonances} are discussed.