Landau-Yang theorem

The Landau-Yang theorem , also Yang's theorem , is a proposition in quantum field theory . It determines which decay of a particle into two photons is possible or which polarization these photons have after annihilation. The most important statement of the Landau-Yang theorem is that the decay of a particle with spin 1 into two photons is forbidden. This leads to the characterization of the Landau-Yang theorem as one of the no-go theorems of quantum field theory and to a simple exclusion criterion for the spin of a particle whose decay into two photons has been observed. This was historically significant, for example, for determining the spin of the particle that was discovered in 2012 at the Large Hadron Collider and was then considered to be a candidate for the Higgs boson . The Higgs boson has now been confirmed.

The Landau-Yang theorem was independently proven in 1948 by Lew Dawidowitsch Landau and in 1949 by Chen Ning Yang .

Selection rules

The selection rules, which decays are allowed or forbidden, and with what probability the photons have a certain polarization relative to each other, depend on the spin and the parity of the decaying particle: ${\ displaystyle S}$ ${\ displaystyle P}$

Spin	parity	Polarization of the photons
of the mother particle		parallel	orthogonal
${\ displaystyle 0}$	${\ displaystyle +}$	${\ displaystyle 100 \%}$	${\ displaystyle 0 \%}$
${\ displaystyle 0}$	${\ displaystyle -}$	${\ displaystyle 0 \%}$	${\ displaystyle 100 \%}$
${\ displaystyle 1}$	${\ displaystyle +}$	forbidden transition
${\ displaystyle 1}$	${\ displaystyle -}$	forbidden transition
${\ displaystyle 2,4,6, \ dots}$	${\ displaystyle +}$	${\ displaystyle \ geq 50 \%}$	${\ displaystyle \ leq 50 \%}$
${\ displaystyle 2,4,6, \ dots}$	${\ displaystyle -}$	${\ displaystyle 0 \%}$	${\ displaystyle 100 \%}$
${\ displaystyle 3,5,7, \ dots}$	${\ displaystyle +}$	${\ displaystyle 50 \%}$	${\ displaystyle 50 \%}$
${\ displaystyle 3,5,7, \ dots}$	${\ displaystyle -}$	forbidden transition

Individual evidence

^ The CMS Collaboration: Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC . In: Physical Letters B . tape 716 , no. 30 , 2012, p. 30 - 61 (English).
^ Lew Dawidowitsch Landau: On the angular momentum of a system of two photons . In: Doklady Akademii Nauk Ser. Fiz. tape 60 , no. 2 , 1948, p. 207 - 209 (English).
↑ Chen Ning Yang: Selection Rules for the Dematerialization of a Particle into Two Photons . In: Physical Review . tape 77 , no. 2 , 1950, p. 242 - 245 (English).

[1] The CMS Collaboration: Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC . In: Physical Letters B . tape 716 , no. 30 , 2012, p. 30 - 61 (English).

[2] Lew Dawidowitsch Landau: On the angular momentum of a system of two photons . In: Doklady Akademii Nauk Ser. Fiz. tape 60 , no. 2 , 1948, p. 207 - 209 (English).

[3] Chen Ning Yang: Selection Rules for the Dematerialization of a Particle into Two Photons . In: Physical Review . tape 77 , no. 2 , 1950, p. 242 - 245 (English).