# Parity of Pi0 and Its Photon Decays

• Joe1
In summary, the decay of a neutral pion to two photons follows from the axial anomaly in the presence of a static electric field. This anomaly arises because the pion's spin is aligned along the field vector, causing its decay to be very fast.

#### Joe1

Hi,

I thought pions were assigned parity of -1, but how then can the Pi0 decay into two photons with parity -1?? What am I missing?

Thanks,
Joe

intresting, violation of parity??
But this decay as far as I know is due to the electromagnetic force, there should be no parity violation..
can someone explain this?

Neutral pions decay quickly by electromagnetic means. You can think of it as a single quark having two electromagnetic interactions that send off gamma rays, and then goes backwards in time (and is the antiquark).

The total parity for the two photons is still negative. Each photon contributes -1, and their wave function gives a third -1. (-1)^3 = -1.

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The pi0--> two photon decay was actually used by Yang to determine the parity of the pi0 to be negative.
If the parity is positive, the linear polarization of the two photons would be in the same direction, corresponding to the scalar E.E.
For negative parity, the polarizations would be perpendicular, corresponding to the pseudoscalar E.B.
Yang actually did it in terms of the corresponding circular polarization correlations, relating them to the zero spin of the pi0.
It is also of interest to note that the experiment is an example of EPR, since measuring one photon polarization determines the other. How could such a simple experiment confuse Einstein is the real EPR paradox.

Meir Achuz said:
The pi0--> two photon decay was actually used by Yang to determine the parity of the pi0 to be negative.
If the parity is positive, the linear polarization of the two photons would be in the same direction, corresponding to the scalar E.E.
For negative parity, the polarizations would be perpendicular, corresponding to the pseudoscalar E.B.
Yang actually did it in terms of the corresponding circular polarization correlations, relating them to the zero spin of the pi0.
It is also of interest to note that the experiment is an example of EPR, since measuring one photon polarization determines the other. How could such a simple experiment confuse Einstein is the real EPR paradox.

It should also be noted that the pion decay to two photons cannot be treated in the standard VMD method, but instead follows from the axial anomaly because of the orientation of polarizations. Hence, the only calculation that gives the proper decay width of the pion is derived solely from the axial anomaly.

Thanks everyone!