Polarization in $Λ\to pμ^-\barν_μ$ as a probe of CP-violating new physics

We investigate the normal $μ$ polarization in the hyperon decay $Λ\to pμ^-\barν_μ$ as a probe of new physics that violates CP, since this polarization vanishes at tree level in the Standard Model. Using an effective Lagrangian and lattice QCD form factors, we compute its sensitivity to the imaginary parts of the couplings. With the present bounds from kaon decays and the Large Hadron Collider, one coupling at a time, this sensitivity gives an effect below the Standard Model contribution from the electromagnetic final-state interaction. Only a pseudoscalar coupling with a particular phase can exceed this contribution, by several times. The final-state interaction gives the same polarization in $Λ$ and $\barΛ$ decays, so the difference between the two decays isolates the new physics effect.