Axial anomaly effect on three-quark and five-quark singly heavy baryons

Effects of the U(1)A axial anomaly on the mass spectrum of singly heavy baryons (SHBs) is studied in terms of the chiral effective theory based on the chiral linear representation for light flavors. We consider SHBs made of both three quarks (Qqq) and five quarks (Qqqqq over bar ). For the three-quark SHBs, we prove that the inverse mass hierarchy for the negative-parity Ac and Ec is realized only when the U(1)A anomaly is present. For the five-quark SHBs, in contrast, it is found that the U(1)A anomaly does not change the mass spectrum at the leading order, and accordingly, their decay properties induced by emitting a pseudoscalar meson are not affected by the anomaly. Moreover, taking into account small mixings between the three quark and five-quark SHBs, we find that the observed Ec excited state, either Ec(2923) or Ec(2930), can be consistently regarded as a negative-parity SHB that is dominated by the five-quark component. We also predict a new negative-parity five-quark dominant Ac, whose mass is around 2700 MeV and the decay width is of order a few MeV, which provides useful information for future experiments to check our description.