A Simple Estimate of the Mass of the Positron

Fig.1 A positive track curling to the left seems to change its sign (red). What is happening is that a positron collides head-on a stationary electron and loses all its momentum. this shows that the positron has the same mass as the electron.

    The curves measurements performed at CERN  give a magnitude of 54 15MeV/c for the incident positron and 54 13MeV/c for the leaving electron ( Systematic errors are negligible in comparison with the large statistical errors, forced upon us by the short lengths of track that can sensibly be used for measurements, typically 3 cm.)

Momentum and energy conservation in the head-on collision of a positron and an electron give:

     (1)

           (2)

Eliminating the final momentum of the positron, , and solving eq. (1) and (2), one can calculate the fraction of the positron’s momentum that is imparted to the electron:

 (3)

For equal masses, the target (electron) picks up all the projectile’s momentum, so the ration  .

If the positron is stopped dead in the collision, it would then be annihilate with an electron to produce two 0.511 -MeV photons, back to back. Such low-energy photons cannot materialize into electro-positron pairs (this would violate energy conservation). However, our measurements errors allow the positron to keep » 10MeV in the bubble chamber, which could be passed on the annihilation photons.

electron-positron annihilation