RE: A crash course on particle physics to get prepared for the steemSTEM meetup at CERN - 2 - interactions and symmetries
Interesting that the very common massless photon is really a composite of a hypercharge boson (B) and a neutral weak boson (W3). Is that right?
The photon is not composite but the physical state is an admixture of two unphysical state. This must be seen more like a rotation than as the photon being composed of smaller entities. The photon is elementary.
Question: If we have an electron and another charged particle approaching that could be an electron or a positron, the 2 particles will exchange photons. These photons need to carry the information of the charge of their source particles resulting in an attraction or a repulsion of the 2 particles. Can we conclude that the photons from electrons and the photons from positrons are different? And if they are different, how are they different? If they are not different, how do the 2 particles choose attraction or repulsion?
The photon is the same.
In order to conclude about what does the interaction, the actual calculations must be performed (relying on the sole interaction of a photon with a positron-electron pair). You can check on the slide 9 of the PDF, there is a diagram on the bottom. This diagram exists for all the cases: electron-electron, positron-positron and electron-positron. However, for the electron-positron, there is another one in which the particle pair annihilates into a (virtual) photon that gives it back. This difference explains the different observation.
I am not so sure to have explained it clearly. It is late here... But please let me know :)
Good, the photon is still elementary and not a composite. Is that kind of like how a vector can be considered mathematically to be the sum of 2 virtual orthogonal vectors forming a right triangle where the hypotenuse is the photon?
All photons are the same, except for their frequency.
It is still not clear to me how the information of the charge of the electron or positron is transmitted between particles via photons, except by the sign of the electric fields generated by the particles. Your diagram shows repulsion. Are you saying that attraction requires pair annihilation and then pair production?
Sorry for the stupid questions and thanks for your expert answers.
Not a sum but a rotation. You have a basis made of B and W3, you rotate the axis by a rotation of the electroweak mixing angle, and you end up with another basis made of the photon and the Z-boson. You can check my answer to @muphy's comment above. This can help too.
The photons are the same guys. Different frequencies means different energies, but these are still the same photons :)
It is not transmitted. Please see below where I try to explain :D
In quantum field theory, you need to get all possible diagrams for a process to compute what is going on. We have three considered processes here:
In the case of electron-positron scattering, you have one more diagram which changes everything.
I actually plan to discuss this more in details in the course #4 (for which I will draw extras stuff). I hope the information I provide here is enough. Otherwise, please shoot again on me :p