# Photons and the weak force

## Main Question or Discussion Point

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 electro-weak 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

## Answers and Replies

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mfb
Mentor
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.

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.

UltrafastPED
Gold Member
But look at this closed thread: "Does the Z boson couple to photons?"

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.

jtbell
Mentor
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".
But isn't the coupling in this case nevertheless electromagnetic?

mfb
Mentor
But isn't the coupling in this case nevertheless electromagnetic?
I guess that is just a matter of definition. Both views are possible, and if we call it "electroweak interaction" the difference vanishes completely.

UltrafastPED
Gold Member
if we call it "electroweak interaction" the difference vanishes completely.
That is the name of the unified force ... but at "everyday" energies they are independent.

Bill_K
I guess that is just a matter of definition. Both views are possible, and if we call it "electroweak interaction" the difference vanishes completely.
No,mfb, I don't agree with this. There is no ambiguity - the photon has no other interaction. It couples to the electromagnetic current and that is all.

fzero
Homework Helper
Gold Member
Of course the original question is a bad one for an expert, since we are forced to assume that it refers to tree-level 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.

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

Of course the original question is a bad one for an expert,
Right. The text book is for a very low level introductory course that's mainly concerned with a qualitative description of the elementary features of the standard model. My own background with these topics is from a introductory particle physics course I took in the dim and distant past at university. A little knowledge is a dangerous thing and I often find myself having to reason conclusions based on my sketchy, qualitative understanding of the underlying theory.

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.

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

fzero
Homework Helper
Gold Member
Right. The text book is for a very low level introductory course that's mainly concerned with a qualitative description of the elementary features of the standard model. My own background with these topics is from a introductory particle physics course I took in the dim and distant past at university. A little knowledge is a dangerous thing and I often find myself having to reason conclusions based on my sketchy, qualitative understanding of the underlying theory.

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.
I think the book is looking for the answer No, because as other people have said, the photon does not couple directly to anything but the electromagnetic current. The expert's problem with this is that there are indirect couplings to the other currents once we consider quantum corrections. These corrections tend to get more important at higher energies, but remain a small contribution at lower energies.

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 two-loop 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 "tree-level" 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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