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# Photon Collison

cowhitmyface
When 2 photons collide and create a matter-antimatter particle (ie electron and positron) how is mass conserved seeing as photons are mass less?

Surely the theoretical higgs boson would have no mass to exchange as the other exchange particles would?
kelseymh
 cowhitmyface wrote: When 2 photons collide and create a matter-antimatter particle (ie electron and positron) how is mass conserved seeing as photons are mass less? Surely the theoretical higgs boson would have no mass to exchange as the other exchange particles would?

You have picked up a lot of jargon from reading popular science articles, but it seems that you're not so clear on what the terms mean.

Photons don't collide directly. Put another way, there is no photon self-interaction term in the Lagrangian.

If you want a two-gamma interaction to produce a particle anti-particle pair (e.g., gamma gamma -> e+ e-), the main contribution is from the two photons exchanging a virtual particle (http://en.wikipedia.org/wiki/File:Direct.jpg). Since you have two four-vectors in the initial state, and two four-vectors in the final state, you can always conserve four-momentum.

The photons are the quanta which are exchanged in electromagnetic interactions. The "Higgs" has nothing to do with it: for example, the Higgs is not involved in helping a magnet stick to your refrigerator. That's all photons.
_AVG_
Yeah, the Higgs is totally unrelated. And in general, mass is not conserved for such collisions. What is conserved is 4-momentum. Or another way to put it is that energy is conserved:
each photon has a certain amount of energy which partially converts to mass with the corresponding formula E = mc^2 . The remaining energy is the kinetic energy of the particle antiparticle pair (so as to conserve linear momentum - unless the photons are traveling in opposite directions). The pair must be a particle antiparticle pair because charge needs to be conserved.
kelseymh
 _AVG_ wrote: Yeah, the Higgs is totally unrelated. And in general, mass is not conserved for such collisions. What is conserved is 4-momentum. Or another way to put it is that energy is conserved: each photon has a certain amount of energy which partially converts to mass with the corresponding formula E = mc^2 . The remaining energy is the kinetic energy of the particle antiparticle pair (so as to conserve linear momentum - unless the photons are traveling in opposite directions). The pair must be a particle antiparticle pair because charge needs to be conserved.

You are quite right.

Energy and momentum must be conserved (i.e., four-momentum). If the photons are in opposite directions, then the particles come out in opposite directions; if the photons collide at an angle, then the particles will have some forward motion as well.

In addition to conserving charge, the particle-antiparticle requirement occurs because photons are their own antiparticles, so all quantum numbers in the final state must add up to zero (isospin, quark flavor, weak hypercharge, etc.).
_AVG_
kelseymh wrote:
 _AVG_ wrote: Yeah, the Higgs is totally unrelated. And in general, mass is not conserved for such collisions. What is conserved is 4-momentum. Or another way to put it is that energy is conserved: each photon has a certain amount of energy which partially converts to mass with the corresponding formula E = mc^2 . The remaining energy is the kinetic energy of the particle antiparticle pair (so as to conserve linear momentum - unless the photons are traveling in opposite directions). The pair must be a particle antiparticle pair because charge needs to be conserved.

You are quite right.

Energy and momentum must be conserved (i.e., four-momentum). If the photons are in opposite directions, then the particles come out in opposite directions; if the photons collide at an angle, then the particles will have some forward motion as well.

In addition to conserving charge, the particle-antiparticle requirement occurs because photons are their own antiparticles, so all quantum numbers in the final state must add up to zero (isospin, quark flavor, weak hypercharge, etc.).

Yeah . I forgot how many conservation laws need to be satisfied in particle collisions lol
cowhitmyface
kelseymh wrote:
 cowhitmyface wrote: When 2 photons collide and create a matter-antimatter particle (ie electron and positron) how is mass conserved seeing as photons are mass less? Surely the theoretical higgs boson would have no mass to exchange as the other exchange particles would?

You have picked up a lot of jargon from reading popular science articles, but it seems that you're not so clear on what the terms mean.

Photons don't collide directly. Put another way, there is no photon self-interaction term in the Lagrangian.

If you want a two-gamma interaction to produce a particle anti-particle pair (e.g., gamma gamma -> e+ e-), the main contribution is from the two photons exchanging a virtual particle (http://en.wikipedia.org/wiki/File:Direct.jpg). Since you have two four-vectors in the initial state, and two four-vectors in the final state, you can always conserve four-momentum.

The photons are the quanta which are exchanged in electromagnetic interactions. The "Higgs" has nothing to do with it: for example, the Higgs is not involved in helping a magnet stick to your refrigerator. That's all photons.

I don't read many science aricles...

However thanks for clearing it up avg and kelsey

I knew about energy and momentum conservation however I was unaware that the mass was converted from the energy

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