New strategies for new physics search with Λb→Λνν¯ decays

Abstract

We examine the new physics sensitivity of the rare decay Λb→Λνν¯, which can be accessible at future Z-pole machines like Future Circular Collider and Circular Electron Positron Collider. We find that the longitudinal polarization of Λb baryons produced in Z decays introduces a novel observable, the forward-backward asymmetry AFB↑ in the angle between the outgoing Λ momentum and the Λb spin. We provide Standard Model predictions for the Λb→Λνν¯ branching ratio and AFB↑, and show that future precision measurements of these observables are complementary and probe new physics scales comparable to other b→sνν¯ and b→sℓ+ℓ− processes. We also show that the zero crossing of the forward-backward asymmetry offers a robust test of form factor calculations independent of new physics. Published by the American Physical Society 2025