Selection rules for the decay of a particle into two identical massless particles of any spin

TL;DR

This paper generalizes the Landau-Yang theorem to include decays into two identical massless particles of any spin, providing new selection rules based on symmetry principles and explicit amplitude calculations.

Contribution

It introduces a generalized Landau-Yang theorem for particles decaying into identical massless particles of arbitrary spin, with detailed derivations and symmetry-based selection rules.

Findings

Derived Lorentz-covariant triple vertices.

Explicit decay helicity amplitudes in Jacob-Wick convention.

Established consistency of the generalized theorem.

Abstract

The well-known Landau-Yang (LY) theorem on the decay of a neutral particle into two photons is generalized for analyzing the decay of a neutral or charged particle into two identical massless particles of any spin. Selection rules categorized by discrete parity invariance and Bose/Fermi symmetry are worked out in the helicity formulation. The general form of the Lorentz-covariant triple vertices are derived and the corresponding decay helicity amplitudes are explicitly calculated in the Jacob-Wick convention. After checking the consistency of all the analytic results obtained by two complementary approaches, we extract out the key aspects of the generalized LY theorem.