Landau–Yang theorem

The Landau–Yang theorem forbids a spin-1 particle from decaying into two identical massless spin-1 particles, such as photons.

Angular momentum: A conserved physical quantity associated with rotational motion and spatial symmetry. Rotational symmetry restricts the total angular momentum available to a two-photon state.
Two-photon decay: A process in which an unstable particle decays into two photons. This is the decay channel directly constrained by the theorem.
Lev Landau: A Soviet theoretical physicist whose work shaped quantum theory, condensed matter physics, and particle physics. Landau independently established the restriction on spin-one two-photon states.
Spin-0 particle: A particle with zero intrinsic angular momentum. Spin-zero particles can decay into two photons, as the Higgs boson does.
Bose–Einstein statistics: The quantum statistics governing identical particles with integer spin, which occupy symmetric states under exchange. Photons are identical bosons, so exchanging them constrains their combined state.
Higgs boson: The spin-0 elementary particle associated with the Higgs field and electroweak symmetry breaking. Its observed decay to two photons illustrates a channel forbidden to spin one.
Chen-Ning Yang: A Chinese-American theoretical physicist known for work on symmetries and fundamental interactions. Yang independently derived the theorem governing a particle's decay into two photons.
Spin-2 particle: A particle with intrinsic angular momentum quantum number two. Spin-two states can couple to photon pairs, unlike spin-one states.
Photon: The massless spin-1 quantum of the electromagnetic field. Two photons are the canonical pair to which the theorem applies.
Diphoton resonance: A particle-like excess in the invariant-mass distribution of photon pairs. A genuine diphoton resonance cannot have spin one under the theorem's assumptions.

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Furry's theoremCompared with: It also forbids amplitudes, but uses angular momentum and photon identity rather than loop-orientation cancellation.