Endpoint symmetries of helicity amplitudes

We investigate helicity amplitudes (HAs) of A→BC-type decays for arbitrary spin towards the kinematic endpoint. show that HAs are proportional to product Clebsch-Gordan coefficients (CGC) and velocity a non-negative power. The latter can be zero in which case HA is non-vanishing at At endpoint explicit breaking rotational symmetry, through external momenta, restored findings interpreted as special Wigner-Eckart theorem. Our useful i) checking theoretical computations ii) where there sequence decays, say B→B1B2 with pair (B1B2) not interacting (significantly) C-particle. Angular observables, ratios HAs, given by CGC briefly discuss power corrections leading order.