New Strategies for New Physics Search with $\Lambda_b \to \Lambda \nu \bar\nu$ Decays

TL;DR

This paper explores the potential of future Z-pole experiments to detect new physics through the rare decay $oldsymbol{ ext{Lambda}_b o ext{Lambda} u ar{ u}}$, introducing a novel polarization-based asymmetry observable and providing Standard Model benchmarks.

Contribution

It introduces a new forward-backward asymmetry observable in $ ext{Lambda}_b$ decays, offering a novel probe for new physics at future colliders.

Findings

Predicted Standard Model branching ratio and asymmetry values.

Future measurements can probe new physics scales comparable to other rare decays.

Zero crossing of asymmetry provides a form factor test.

Abstract

We examine the new physics sensitivity of the rare decay $\Lambda_b \to \Lambda \nu\bar\nu$ which can be accessible at future $Z$-pole machines like FCC-ee and CEPC. We find that the longitudinal polarization of $\Lambda_b$ baryons produced in $Z$ decays introduces a novel observable, the forward-backward asymmetry $A_\text{FB}^\uparrow$ in the angle between the outgoing $\Lambda$ momentum and the $\Lambda_b$ spin. We provide Standard Model predictions for the $\Lambda_b \to \Lambda \nu\bar\nu$ branching ratio and $A_\text{FB}^\uparrow$, and show that future precision measurements of these observables are complementary and probe new physics scales comparable to other $b \to s \nu\bar\nu$ and $b \to s \ell^+ \ell^-$ 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.