Landau–Yang theorem

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

Assumptions

A photon here is any particle with spin 1, without mass and without internal degrees of freedom. However, 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).^[1]
• The boson Z cannot decay into two photons.
• The Higgs boson, whose spin was never measured, but whose decay into two photons was observed recently ^[2][3] cannot have spin 1 in prevailing models that assume the truth of the Landau-Yang theorem. However, since the Higgs is known to have spin 0, the new particle cannot rightly be called a Higgs until its spin is measured and the result of that measurement shows spin 0.

Original references

1. Yang, Chen Ning (1950). "Selection Rules for the Dematerialization of a Particle into Two Photons". Physical Review. 77: 242–245. doi:10.1103/PhysRev.77.242.
2. Landau, Lev Davidovich (1948). "The moment of a 2-photon system". Dokl. Akad. Nauk. 60: 207–209.

Additional references

1. Particle Data Group. "Light Unflavored Mesons" (PDF). Retrieved 4 August 2012.
2. ATLAS collaboration. "Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC". Submitted to Phys. Lett. B. Retrieved 4 August 2012.
3. CMS collaboration. "Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC". Submitted to Phys. Lett. B. Retrieved 4 August 2012.