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.
تحميل البحث