Photon polarization from helicity suppression in radiative decays of polarized Λb to spin-3/2 baryons

Abstract We give a general parameterization of the Λ b → Λ ( 1520 ) γ decay amplitude, applicable to any strange isosinglet spin-3/2 baryon, and calculate the branching fraction and helicity amplitudes. Large-energy form factor relations are worked out, and it is shown that the helicity-3/2 amplitudes vanish at lowest order in soft-collinear effective theory (SCET). The suppression can be tested experimentally at the LHC and elsewhere, thus providing a benchmark for SCET. We apply the results to assess the experimental reach for a possible wrong-helicity b → s γ dipole coupling in Λ b → Λ ( 1520 ) γ → p K γ decays. Furthermore we revisit Λ b -polarization at hadron colliders and update the prediction from heavy-quark effective theory. Opportunities associated with b → d γ afforded by high-statistics Λ b samples are briefly discussed in the general context of CP and flavour violation.