Lepton polarization and sensitivity to the form factors in Λ→ p ℓ^-ν̅_ℓ with lattice QCD

@misc{salam2026lepton,
  title = {{Lepton polarization and sensitivity to the form factors in $Λ\to p\,\ell^-\barν_\ell$ with lattice QCD}},
  author = {Qazi Maaz Us Salam and Ishtiaq Ahmed},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.35717},
  eprint = {2610.35717},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.35717},
}

TY  - UNPB
TI  - Lepton polarization and sensitivity to the form factors in Λ→ p ℓ^-ν̅_ℓ with lattice QCD
AU  - Qazi Maaz Us Salam
AU  - Ishtiaq Ahmed
PY  - 2026
DA  - 2026/10/26
PB  - arXiv
DO  - 10.48550/arxiv.2610.35717
UR  - https://arxiv.org/abs/2610.35717
AB  - We investigate the polarization of the charged lepton in the semileptonic hyperon decay Λ→ p ℓ^-ν̅_ℓ (ℓ=e,μ) within the Standard Model, using the form factors obtained from lattice QCD. We compute the longitudinal, transverse and normal polarization of the lepton, the forward-backward asymmetry of each lepton spin state, and the lepton flavor universality ratio R_μe. We show that the longitudinal polarization determines the ratio g_1/f_1 of the axial-vector to vector couplings, and that the forward-backward asymmetries of the electron and of negative-helicity muons determine the weak magnetism. The asymmetry of positive-helicity muons is fixed by kinematics up to a small dependence on the second-class form factors, whereas R_μe is nearly independent of g_1/f_1 and the weak magnetism. For comparison, we also use the QCD sum-rule form factors, whose g_1/f_1 and weak magnetism are opposite in sign to experiment and lattice QCD, and find very different values for the observables that depend on them. Moreover, the q^2 dependence of R_μe fixes the longitudinal polarization of the muon, which can thus be measured from the decay rates, without measuring the muon spin. The normal polarization is generated by the electromagnetic final-state interaction and provides the Standard Model reference for searches for time-reversal violation. The electron forward-backward asymmetry and R_μe can be measured at BESIII and the proposed Super Tau-Charm Facility, and R_μe also at LHCb. These measurements can test the Standard Model and the lattice-QCD form factors.
ER  -