Why do electron-positron pairs move backward from photon pair production?

[steve0606]

I am learing the particle physics bit of my physics course and don't understand how momentum is conserved when a photon produces an electron/positron pair.
In the bubble chamber chamber picture here:

http://www.sciencephoto.com/image/1...ctron-positron_pair_matter_antimatter-SPL.jpg

I don't understand why the particles produced (that is the electron and positron) appear to initially move backwards. Surely if the photon is traveling in a more or less straight line before it decays the reultant particles would move initially perpendicular to the direction of the photon in order for the resultant momentum to stay zero?

Also why is one spiral bigger than the other?

Thanks

 

[AdrianTheRock]

Both events in this image are surely surely the result of gamma rays coming downwards from the top.

From the top event, there are two tracks that curve towards the right (when viewed from the top downwards) but only one curving towards the left. These correspond to oppositely charged particles, which on the face of it doesn't make sense unless the left-curving one has a double charge as the charges otherwise wouldn't add up to zero. I would guess, therefore, that one of the particles created by the initial event has collided almost immediately with something in the cloud - probably an electron that leaves behind a positively charged ion which you don't see because it doesn't really go anywhere. This would also explain the difference in spiral sizes as that particle would have lost energy through the collision.

 

[mathman]

Pair production by a single gamma ray cannot happen by itself, since momentum conservation can't hold. Usually it occurs when the photon passes near a nucleus, and the nucleus moves to conserve momentum.