Probing for New Physics in Polarized ${\Lambda }_b$ decays at the Z

Abstract

Polarized ${\Lambda }_b\to \Lambda \gamma$ 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 ${\Lambda }_b$ spin and photon momentum, which is sensitive to the relative strengths of the opposite chirality and Standard Model chirality $b\to s\gamma$ dipole operators. Combination with the $\Lambda$ decay polarization asymmetry and comparison with the ${\Lambda }_b$ 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 ${\Lambda }_b\to \Lambda \gamma$ 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 ${\Lambda }_b\to \Lambda \gamma$ matrix element. Form factor relations for heavy-to-light baryon decays are derived in the large energy limit, which are of general interest.

arXiv comment18 pages, 2 figures, comments and references on 1/m effects added, conclusions unchanged, version to appear in PRD

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