Alternative experimental evidence for chiral restoration in excited baryons

Abstract

Given existing empirical spectral patterns of excited hadrons it has been suggested that chiral symmetry is approximately restored in excited hadrons at zero temperature/density (effective symmetry restoration). If correct, this implies that mass generation mechanisms and physics in excited hadrons is very different as compared to the lowest states. One needs an alternative and independent experimental information to confirm this conjecture. Using very general chiral symmetry arguments it is shown that strict chiral restoration in a given excited nucleon forbids its decay into the N πchannel. Hence those excited nucleons which are assumed from the spectroscopic patterns to be in approximate chiral multiplets must only "weakly" decay into the N πchannel, (f_{N^*Nπ}/f_{NNπ})^2 << 1. However, those baryons which have no chiral partner must decay strongly with a decay constant comparable with f_{NNπ}. Decay constants can be extracted from the existing decay widths and branching ratios. It turnes out that for all those well established excited nucleons which can be classified into chiral doublets N_+(1440) - N_-(1535), N_+(1710) - N_-(1650), N_+(1720) - N_-(1700), N_+(1680) - N_-(1675), N_+(2220) - N_-(2250), N_+(?) - N_-(2190), N_+(?) - N_-(2600), the ratio is (f_{N^*Nπ}/f_{NNπ})^2 ~ 0.1 or much smaller for the high-spin states. In contrast, the only well established excited nucleon for which the chiral partner cannot be identified from the spectroscopic data, N(1520), has a decay constant into the Nπchannel that is comparable with f_{NNπ}. This gives an independent experimental verification of the chiral symmetry restoration scenario.