Can a SINGLE photon produce an electron-positron pair?

[rwooduk]

A homework question is asking for a diagram illustrating a photon becoming a electron-positron pair (please don't post an image as it's homework).

Looking through the internet there seems to be some disagreement as to whether a single photon can create an electron-positron pair:

"Show that a single photon cannot produce an electron-positron pair, but needs additional matter or light quanta."

"For example you've got a 5MeV photon, so you think that there is plenty of energy to make e − e + pair. Now you make a boost along the direction of the photon momentum with v=0.99c and you get a 0.35MeV photon. That is not enough even for one electron. "

so a photon would have to have a massive amount of energy, but is it even possible?

if not why is this diagram on wiki?

http://upload.wikimedia.org/wikipedia/commons/a/a5/Pair_Production.png

i could just draw that diagram but if it's not a realistic possibility then would like to put in a comment as such.

Thanks for any comments / ideas on this.

 

[vela]

A homework question is asking for a diagram illustrating a photon becoming a electron-positron pair (please don't post an image as it's homework).

Looking through the internet there seems to be some disagreement as to whether a single photon can create an electron-positron pair:

"Show that a single photon cannot produce an electron-positron pair, but needs additional matter or light quanta."

"For example you've got a 5MeV photon, so you think that there is plenty of energy to make e − e + pair. Now you make a boost along the direction of the photon momentum with v=0.99c and you get a 0.35MeV photon. That is not enough even for one electron. "

so a photon would have to have a massive amount of energy, but is it even possible?

You seemed to have missed the point of the example. Say the photon did have even more energy in your frame. Can't you always boost to a frame where the energy of the photon is too low for pair production?

 

[rwooduk]

You seemed to have missed the point of the example. Say the photon did have even more energy in your frame. Can't you always boost to a frame where the energy of the photon is too low for pair production?

Thanks for the reply.

are you saying you could be in a frame where the energy required would be LESS to produce a positron electron pair? therefore it is possible?

 

[vela]

How did you get that interpretation from the phrase "too low for pair production"?

Try writing down the equations using conservation of momentum and conservation of energy for a photon decaying into an electron-position pair. You should be able to show there's no solution to those equations. Think about what happens in the center-of-mass frame of the electron and positron.

 

[rwooduk]

How did you get that interpretation from the phrase "too low for pair production"?

hmm not sure, went a little abstract.

Try writing down the equations using conservation of momentum and conservation of energy for a photon decaying into an electron-position pair. You should be able to show there's no solution to those equations. Think about what happens in the center-of-mass frame of the electron and positron.

Ahh, thanks I see now!

Also asked my tutor who says there would have to be nuclear recoil, i.e. it would have to interact with something to produce a electron positron pair, but that it is often ignored in a simple diagram.

Problem solved, thanks again!