Probing for New Physics in Polarized $Λ_b$ decays at the Z

Abstract

Polarized $Λ_b \to Λγ$ decays at the Z pole are shown to be well suited for probing a large variety of New Physics effects. A new observable is proposed, the angular asymmetry between the $Λ_b$ spin and photon momentum, which is sensitive to the relative strengths of the opposite chirality and Standard Model chirality $b \to s γ$ dipole operators. Combination with the $Λ$ decay polarization asymmetry and comparison with the $Λ_b$ polarization extracted from semileptonic decays allows important tests of the $V-A$ structure of the Standard Model. Modifications of the rates and angular asymmetries which arise at next-to-leading order are discussed. Measurements for $Λ_b \to Λγ$ and the CP conjugate mode, with branching ratios of a few times $10^{-5}$, are shown to be sensitive to non-standard sources of CP violation in the $Λ_b \to Λγ$ matrix element. Form factor relations for heavy-to-light baryon decays are derived in the large energy limit, which are of general interest.

18 pages, 2 figures, comments and references on 1/m effects added, conclusions unchanged, version to appear in PRD