Can parallel spin antiproton annihilate?

[snorkack]

Positronium annihilation opportunities are strongly restricted by the requirement to conserve spin. The spin of a photon is 1; and for some reasons two photons are completely unable to have spins in different directions. They can only have parallel spins (total 2) or antiparallel spins (total 0) - never 1, and this prevents a positron with spin parallel to an electron from annihilating to 2 photons. Only 3 photons are possible (also 5 or larger odd numbers) which is very much slower process.

Now how about antiproton?
The problem here is that antiprotons are said to annihilate normally into pions - but pion spin is 0.

No matter how many pions are produced, whether 3, 4, 5 or more, no combination of pions can possibly handle the combined spin of proton and antiproton with parallel spins.

So is such annihilation possible?

 

[Bill_K]

In addition to conservation of angular momentum, the analysis involves conservation of C and P. It gets rather complicated. But quoting the answer,

"The singlet ground state ¹S₀ of para-protonium can decay only into three pions. Whereas the ortho-protonium ³S₁ can decay either into two or three pions, but never into a system of uncharged pions only. (The 3π decay of ortho-protonium is much less probable than the 2π decay. Not only because the phase space is much smaller, but also because already the simplest 3π state has a high centrifugal barrier.)"