Register to reply

Photons and the weak force

by MalachiK
Tags: force, photons, weak
Share this thread:
MalachiK
#1
Sep18-13, 03:53 AM
MalachiK's Avatar
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 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
Phys.Org News Partner Physics news on Phys.org
UCI team is first to capture motion of single molecule in real time
And so they beat on, flagella against the cantilever
Tandem microwave destroys hazmat, disinfects
mfb
#2
Sep18-13, 04:31 AM
Mentor
P: 12,053
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.
MalachiK
#3
Sep18-13, 05:21 AM
MalachiK's Avatar
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.

UltrafastPED
#4
Sep18-13, 06:23 AM
Sci Advisor
Thanks
PF Gold
UltrafastPED's Avatar
P: 1,911
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.
jtbell
#5
Sep18-13, 08:10 AM
Mentor
jtbell's Avatar
P: 11,864
Quote Quote by mfb View Post
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
#6
Sep18-13, 08:42 AM
Mentor
P: 12,053
Quote Quote by jtbell View Post
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
#7
Sep18-13, 09:06 AM
Sci Advisor
Thanks
PF Gold
UltrafastPED's Avatar
P: 1,911
Quote Quote by mfb View Post
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
#8
Sep18-13, 09:14 AM
Sci Advisor
Thanks
Bill_K's Avatar
P: 4,160
Quote Quote by mfb View Post
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
#9
Sep18-13, 11:27 AM
Sci Advisor
HW Helper
PF Gold
P: 2,602
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.
dauto
#10
Sep18-13, 01:34 PM
Thanks
P: 1,948
Photons cannot interact through weak interaction because they have vanishing weak isospin and no weak hypercharge either.
MalachiK
#11
Sep18-13, 03:23 PM
MalachiK's Avatar
P: 112
Quote Quote by fzero View Post
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.
RGevo
#12
Sep18-13, 04:05 PM
P: 87
And what about the terms in the lagrangian;
A Z W+ W-?
fzero
#13
Sep18-13, 04:18 PM
Sci Advisor
HW Helper
PF Gold
P: 2,602
Quote Quote by MalachiK View Post
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.
Attached Thumbnails
photon4ptfn-pole.png  


Register to reply

Related Discussions
Is electro-weak force intermediate for strong force happening? High Energy, Nuclear, Particle Physics 4
A massive force carrying particle? The weak nuclear force, the W boson has mass? High Energy, Nuclear, Particle Physics 5
Are photons affected by the strong force? The weak force? Quantum Physics 19
Exchange of photons/virtual photons and electrostatic or EM force Quantum Physics 19
Question: Electromagnetism, Weak-force, Gravity, Strong Nuclear force ? General Physics 11