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3

The electron is the lightest lepton and the proton is the lightest baryon, so it's hard to see what reaction could occur without violating lepton number or baryon number. I suppose if proton decay (to a pion and positron) occurs then there could be a reaction to give a pion and two photons.


3

The symmetries that you're missing are conservation of baryon number $B$ and lepton number $L$. We strongly suspect that baryon number is not an exact symmetry, because the universe appears to contain very many baryons and very few antibaryons. Actually, a better metric for the baryon asymmetry of the universe is to compare the baryon density to the density ...


2

Let us start with the wiki article: The singlet state with antiparallel spins (S = 0, Ms = 0) is known as para-positronium (p-Ps) and denoted 1S0. It has a mean lifetime of 125 picoseconds and decays preferentially into two gamma quanta with energy of 511 keV each (in the center of mass frame). Detection of these photons allows for the reconstruction of ...


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The question is based on a misunderstanding. We will do the work in the center of mass frame of the incident particles, and assume a two photon final-state (in practice it also occurs to a thre-photon state). We have a total incident a four-vector of $(\gamma m_e c^2,\vec{0})$ (this simple form is why I chose to work in the CoM frame). The two photons ...


1

Positronium is a quantum mechanical entity. It is a bound state in the potential create between two elementary "particles". Even though the wiki article draws a nice picture of he electron and positron orbiting about their center of mass this is a representation as if they are classical particles . They are not. The electron and positron are not in orbits, ...


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I'm afraid the responses so far are either misleading or do not answer the question. In fact in a dense medium a positron when it forms the longer lived ortho-positronium can "pick-up" an electron from an adjacent atom and then decay into 2 x 511 keV photons long before the 3-photon ortho-positronium state would have decayed. Look up "pick-up effect".


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Well, the details are important. From the abstract in your link of the paper we see that: 1) it is a publication from 1978 2) it calculates rates for positronium annihilation in the very high magnetic fields found in astrophysical situations, 10^12 Gauss It explicitly states that the momentum contribution comes from the magnetic field. In the relevant ...



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