Weak Hyperon-Nucleon Interaction in a Quark Model and Application to thepn→ΛpScattering

Abstract

The standard theory of elementary particles has achieved remarkable success in describing weak-electromagnetic properties and reactions of leptons. It is successful for hadrons although a care must be taken for corrections due to strong interaction. At high energy perturbative calculations are reliable thanks to the asymptotic free nature of the strong interaction, while low energy phenomena can be studied with the help of low energy effective theories based on the low energy symmetries, such as gauge invariance and chiral symmetry. Also the renormalizability guaranteed by the gauge invariance allows us to relate phenomena at different energy-momentum scale with each other. For instance, employing the renormalization group equation we study energy-scale dependence of physical matrix elements. Yet, a few unsolved problems remain for the hadronic weak interactions. One such problem is the mechanism for nonleptonic weak interactions of hadrons. For instance, the ∆I =1 /2 dominance in the flavor changing weak decays has not convincingly been explained. Chiral perturbation theory for the weak decays of hyperons cannot even fit experimental data of the decay amplitudes. One of the reasons of the difficulty seems that nonperturbative corrections of the strong interaction are fully intermingled with the weak interaction. There is no current conservation which protects the weak vertex from strong renormalization contrary to leptonic processes, where the vector and the axial vector currents are (almost) conserved. This has been a challenging problem in the hadron physics. Recent experimental studies of weak decays of hypernuclei have added a new difficulty to the nonleptonic weak interaction of hadrons. In hypernuclei, the Λ decays from the ground state into nonstrange hadrons. The standard decay modes of Λ are Λ → pπ − and Λ → nπ 0 , but these modes are known to be suppressed in nuclear medium as the momentum of the final nucleon is not large enough to go above the Fermi surface. Instead, the main decay modes of medium-to heavy