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

We give a general parameterization of the Λb→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→sγ dipole coupling in Λb→Λ(1520)γ→pKγ decays. Furthermore we revisit Λb-polarization at hadron colliders and update the prediction from heavy-quark effective theory. Opportunities associated with b→dγ afforded by high-statistics Λb samples are briefly discussed in the general context of CP and flavour violation.