
#1
Sep1813, 03:53 AM

P: 112

Hi guys,
Could someone with a better understanding of the standard model answer a question about the weak force? In a class I'm teaching, a question in the textbook asks if photons feel the weak force. My first thought was that it's fermions that feel the weak force and therefore the answer is no. Then I thought about electroweak unification and it occured to me that it might maker sense if photons were involved in weak interactions at higher energies. Is this correct, or have I got the wrong idea? Thanks 



#2
Sep1813, 04:31 AM

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P: 10,864

Photons interact with charged particles, and the W bosons are charged. I think you can call this "feel the weak force", but I prefer "weak interaction". The weak interaction is not like a classical force.




#3
Sep1813, 05:21 AM

P: 112

Yep, that makes sense. I'm not sure why I didn't think of the charge on the W boson. Probably because the question asked to select particles that are affected by the weak force from a list. The photon was the only boson on the list (the rest were fermions), so I kind of assumed that since it was the odd one out then that would be the one that wasn't involved in weak interactions. But that didn't seem quite right somehow.
Thanks for the help. 



#4
Sep1813, 06:23 AM

Thanks
P: 1,364

Photons and the weak force
But look at this closed thread: "Does the Z boson couple to photons?"
http://www.physicsforums.com/showthread.php?t=395739 I was taught that photons only interact with charged particles; this is an electromagnetic interaction, and is independent of the weak force. Thus I would say that photons do not interact via the weak force. 



#5
Sep1813, 08:10 AM

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#6
Sep1813, 08:42 AM

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P: 10,864





#7
Sep1813, 09:06 AM

Thanks
P: 1,364





#8
Sep1813, 09:14 AM

Sci Advisor
Thanks
P: 3,864





#9
Sep1813, 11:27 AM

Sci Advisor
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PF Gold
P: 2,606

Of course the original question is a bad one for an expert, since we are forced to assume that it refers to treelevel interactions. It isn't much of a stretch to define "feel the weak force" with "two photons in, two photons out, W or Z in the intermediate state", but without really knowing the context of the textbook, we probably shouldn't even assume that.




#10
Sep1813, 01:34 PM

P: 1,289

Photons cannot interact through weak interaction because they have vanishing weak isospin and no weak hypercharge either.




#11
Sep1813, 03:23 PM

P: 112

From reading back though this thread and the links I think I've got a better idea of a reasonable answer. Even though photons can interact electromagnetically with charged W bosons, they are not affected by the weak force at energies below those required for el. weak unification. And in that case, it's the unified force and not 'just' the weak force that's getting things done. Please correct me if I've got this wrong, and thanks for your help. 



#12
Sep1813, 04:05 PM

P: 81

And what about the terms in the lagrangian;
A Z W+ W? 



#13
Sep1813, 04:18 PM

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PF Gold
P: 2,606

Therefore we have processes such as that in the first figure at this page, where two photons interact through a loop of charged particles, which could be ##W^\pm## bosons. This diagram doesn't really represent a weak interaction, since only the electromagnetic current appears at each vertex. However, at twoloop order, we can have a diagram like where the solid lines can be quarks and the dashed line can be a Z boson. It is through interactions like this that we would say that the photon "feels" the weak force. In an introductory course we have a dilemma where the students really don't know what "treelevel" and "quantum corrections" mean, so we are forced to use language like "feel" that doesn't capture very much of the true physics. 


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