Landau–Yang theorem explained

In quantum mechanics, the Landau–Yang theorem is a selection rule for particles that decay into two on-shell photons. The theorem states that a massive particle with spin 1 cannot decay into two photons.^{[1] [2]}

Assumptions

A photon here is any particle with spin 1, without mass and without internal degrees of freedom. The photon is the only known particle with these properties.

Consequences

The theorem has several consequences in particle physics. For example:

• The meson ρ cannot decay into two photons, differently from the neutral pion, that almost always decays into this final state (98.8% of times).^[3]
• The boson Z cannot decay into two photons.
• The Higgs boson, whose spin was not measured before 2013, but whose decay into two photons was observed in 2012^{[4] [5]} cannot have spin 1 in models that assume the Landau–Yang theorem.

Notes and References

1. Chen Ning . Yang . Chen Ning Yang . Selection Rules for the Dematerialization of a Particle into Two Photons . . 77 . 2 . 242–245 . 1950 . 10.1103/PhysRev.77.242. 1950PhRv...77..242Y.
2. Lev Davidovich. Landau . Lev Landau . The moment of a 2-photon system . Dokl. Akad. Nauk SSSR . 60 . 207–209 . 1948.
3. Web site: Particle Data Group . Light Unflavored Mesons . 4 August 2012.
4. Web site: ATLAS collaboration . Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC . Phys. Lett. B . 4 August 2012.
5. Web site: CMS collaboration . Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC . Phys. Lett. B . 4 August 2012.