Angular Observables in the Λb→ Λνν̅ Decay
Abstract
We study the rare decay Λb → Λ(→ pπ-)νν̅ of polarized Λb baryons produced at the Z pole. We find that the differential decay distribution is characterized by five independent angular observables, four of which require initial Λb polarization. While standard observables in mesonic b → s νν̅ decays (B → K(*) νν̅, Bs → ϕνν̅) probe only two combinations of short-distance Wilson coefficients, |CL + CR| and |CL − CR|, the angular observables in Λb → Λ(→ pπ-)νν̅ provide access to the orthogonal combinations |CR|2 − |CL|2 and Im(CL CR*). In particular, we identify a T-odd CP asymmetry, A5, that directly probes CP-violating phases in the short-distance effective Hamiltonian. We present Standard Model predictions for the branching ratio and all angular observables using lattice QCD form factors, and evaluate the projected new physics sensitivity at the FCC-ee. Our results demonstrate that baryonic angular observables are essential for resolving parameter degeneracies and for completely characterizing the chiral and CP structure of possible new physics in b → s νν̅ transitions.
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BibTeX
@misc{altmannshofer2026angular,
title = {{Angular Observables in the $Λ_b\to Λν\barν$ Decay}},
author = {Wolfgang Altmannshofer and Shibasis Roy},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.04309},
eprint = {2610.04309},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.04309},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Angular Observables in the Λ_b→ Λνν̅ Decay
AU - Wolfgang Altmannshofer
AU - Shibasis Roy
PY - 2026
DA - 2026/10/03
PB - arXiv
DO - 10.48550/arxiv.2610.04309
UR - https://arxiv.org/abs/2610.04309
AB - We study the rare decay Λ_b → Λ(→ pπ^-)νν̅ of polarized Λ_b baryons produced at the Z pole. We find that the differential decay distribution is characterized by five independent angular observables, four of which require initial Λ_b polarization. While standard observables in mesonic b → s νν̅ decays (B → K^(*) νν̅, B_s → ϕνν̅) probe only two combinations of short-distance Wilson coefficients, |C_L + C_R| and |C_L − C_R|, the angular observables in Λ_b → Λ(→ pπ^-)νν̅ provide access to the orthogonal combinations |C_R|^2 − |C_L|^2 and Im(C_L C_R^*). In particular, we identify a T-odd CP asymmetry, A_5, that directly probes CP-violating phases in the short-distance effective Hamiltonian. We present Standard Model predictions for the branching ratio and all angular observables using lattice QCD form factors, and evaluate the projected new physics sensitivity at the FCC-ee. Our results demonstrate that baryonic angular observables are essential for resolving parameter degeneracies and for completely characterizing the chiral and CP structure of possible new physics in b → s νν̅ transitions.
ER -