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We investigate helicity amplitudes (HAs) of $A \to B C$-type decays forarbitrary spin towards the kinematic endpoint. We show that they areproportional to product of Clebsch-Gordan coefficients (CGC) and the velocityto some positive power. The latter can be zero in which case the HA isnon-vanishing at the endpoint. In essence the spatial rotational symmetry,broken by the relative spatial momenta of the particles, is restored at thekinematic endpoint. Therefore SO(3) and SU(2), for bosons and fermion in thedecay, act like a global internal symmetry groups. Some of our results can beunderstood in terms of the Wigner- Eckart theorem. The findings are useful fori) checking theoretical computations and ii) the case where there is a sequenceof decays, say $B \to B_1B_2$ with the pair $(B_1B_2)$ not interacting(significantly) with the $C$-particle. An example is $H \to Z Z^* \to 4\ell$where our findings might be of use for experimentally determining the Higgsquantum numbers. Angular observables, which are essentially ratios of HAs, aregiven by ratios of CGC at the endpoint. We briefly discuss power corrections inthe velocity to the leading order.

Endpoint Symmetries of Helicity Amplitudes