Abstract
Following our earlier work we establish kinematic endpoint relations for baryon decays using the Wigner-Eckart theorem and apply them to frac{1}{2}to frac{1}{2} and frac{1}{2}to frac{3}{2} baryon transitions. We provide angular distributions at the kinematic endpoint which hold for the generic d = 6 model-independent effective Hamiltonian and comment on the behaviour in the vicinity of the endpoint. Moreover, we verify the endpoint relations, using an explicit form factor parametrisation, and clarify constraints on helicity-based form factors to evidence endpoint relations. Our results provide guidance for phenomenological parameterisations, consistency checks for theory computations and experiment. Results are applicable to ongoing and future new physics searches at LHCb, BES III and Belle II with rare semileptonic-, dineutrino-and charged-modes, which include Λb → Λ(*)ℓℓ, Λb → Λ(*)νν, Ωb → Ωℓℓ, Λc → pℓℓ, Σ → pℓℓ and Λb → {Lambda}_c^{left(ast right)} ℓν.
Highlights
The kinematic endpoint of a decay is characterised by the velocities, of some of its decaying particles, approaching zero
We provide angular distributions at the kinematic endpoint which hold for the generic d = 6 model-independent effective Hamiltonian and comment on the behaviour in the vicinity of the endpoint
This results in the restoration of spherical symmetry and leads to enhanced symmetries in the helicity amplitudes [1], which crucially extends to effective field theories such as c → u + − or b → s + − [2]
Summary
The kinematic endpoint of a decay is characterised by the velocities, of some of its decaying particles, approaching zero This results in the restoration of spherical symmetry and leads to enhanced symmetries in the helicity amplitudes [1], which crucially extends to effective field theories such as c → u + − or b → s + − [2].1. Observables induced by FCNC b → s + − transitions have been reported that hint at new physics and new phenomenon, violating lepton flavour universality [6, 7] This includes studies with Λb-decays [8], consistent with the flavour anomalies evidenced in B-decays [9], with currently larger uncertainties. Appendices A and B contain conventions and a form factor derivation respectively
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