Heavy-flavor-conserving hadronic weak decays of charmed and bottom baryons

Three decades ago, heavy-flavor-conserving (HFC) weak decays of heavy baryons such as Xi(Q) -> Lambda(Q)pi and Omega(Q) -> Xi(Q)pi for Q = c, b had been studied within the framework that incorporates both heavy quark and chiral symmetries. It was pointed out that if the heavy quark in the HFC process behaves as a spectator, then the P-wave amplitude of B-(3) over bar -> B-(3) over bar + pi, with B-(3) over bar being an antitriplet heavy baryon, will vanish in the heavy quark limit. Indeed, this is the case for Xi(b) -> Lambda(b)pi decays. For Xi(c) -> Lambda(c)pi decays, they receive additional nonspectator contributions arising from the W-exchange diagrams through the cs -> dc transition. Spectator and nonspectator W-exchange contributions to the S-wave amplitude of Xi(c) -> Lambda(c)pi are destructive, rendering the S-wave contribution even smaller. However, the nonspectator effect on the P-wave amplitude was overlooked in all the previous model calculations until a very recent investigation within the framework of a constituent quark model in which the parity-conserving pole terms were found to be dominant in Xi(c) -> Lambda(c)pi decays. Since the pion produced in the HFC process is soft, we apply current algebra to study both S- and P-wave amplitudes and employ the bag and diquark models to estimate the matrix elements of four-quark operators. We confirm that Xi(c) -> Lambda(c)pi decays are indeed dominated by the parity-conserving transition induced from nonspectator W-exchange and that they receive largest contributions from the intermediate Sigma(c) pole terms. We also show that the S-wave of Omega(b) -> Xi(b)pi decays vanishes in the heavy quark limit, while Omega(c) -> Xi(c)pi receive additional W-exchange contributions via cs -> dc transition. The P-wave contribution to Omega(c) -> Xi(c)pi is enhanced by the Xi'(c) pole, though it is not so dramatic as in the case of Xi(c) -> Lambda(c)pi. The asymmetry parameter alpha is found to be positive, of order 0.70 and 0.74 for Xi(0)(c) -> Lambda(+)(-)(c)(pi) and Xi(+)(c) -> Lambda(+)(c)pi(0), respectively. The predicted branching fraction is of order 5 x 10(-4) for Omega(0)(c) -> Xi(+)(c)pi(-) and 3 x 10(-4) for Omega(0)(c) -> Xi(0)(c)pi(0) both with the asymmetry parameter close to -1.

Automated summary

Three decades ago, the heavy-flavor-conserving weak decays of heavy baryons were studied. Recent investigations have revealed the dominant role of nonspectator effects on the P-wave amplitude. The S-wave amplitude of Omega(b) -> Xi(b)pi decays vanishes in the heavy quark limit, while additional contributions from W-exchange are observed in Omega(c) -> Xi(c)pi decays. The asymmetry parameter alpha is positive, and the predicted branching fractions are in the order of 10(-4).