garrett said:
Coin:
Distler's use of chirality is nonstandard -- it usually refers to how the weak force interacts with only left chiral fermions -- so I prefer to speak of complex and non-complex representations, which I think is what Distler is calling chiral and non-chiral. But, as I've explained above, it is possible to find a "chiral" representation space as a subspace of a "non-chiral one." The standard model algebra I'm working with is the usual algebra of the gravitational so(1,3) and standard model s(u(2)xu(3)) acting on the 64 dimensional representation space of one generation of fermions. This is the algebra that I find embedded in E8.
Okay, so... let me see if I follow all this.
Assuming we're only trying for a single-gen SM, what is bothering D&G here
really is that they want to be able to fit both a generation and its "antigeneration" (i.e. the antiparticles of that generation?) in. They assert the way you go from a generation to the antigeneration is to take the complex conjugate of the gauge group (for the 1-generation standard model group). This I take it is their reason for demanding the SM group not be self-conjugate, because if the conjugate is not unique then of course there are no antiparticles, and thus no "left and right handed fermions" (because the right handed fermions are supposed to be the antiparticles of the left handed ones?). This potential failure to distinguish between left and right handed fermions is what they mean by the word "nonchiral".
What you're saying however is that you can somehow take a "nonchiral" (self-conjugate?) group, and split it into a pair of "chiral" groups. So (for example) if the group you start with is SO(6) then you break SO(6) in two, you get two SO(3) and you identify one SO(3) as being the "antigeneration" of the other. (Meaning one SO(3) is the complex conjugate of the other, and also meaning one will wind up being the "left" and the other the "right"?) Am I correct so far?
So this makes sense intuitively, but I'm still not sure how you get around D&G's proof. The problem is that their proof appears to be inductive-- they talk about
subgroups, so if (for example) SO(6) is a subgroup of E8 and their proposition applies to SO(6) then their propostion will also apply to SO(3) since SO(3) is a subgroup of SO(6) and so also a subgroup of the gauge group. You depict their argument as being a claim that chirality is impossible so long as "the conjugate is there too" inside of E8. But this is not how they depict their own argument. They claim they can show all of the groups
self-conjugate.
Now, maybe what you're saying is this: SO(3) is self-conjugate. But you have
two SO(3)s. You say one gets to be the "mirror" or conjugate or antigeneration of the other. So when you say "both the group and its conjugate are present in E8" and D&G say "the group is self-conjugate" you are really describing the same situation, it is just that D&G are unwilling to treat two "equivalent" groups as being antigenerations of one another whereas you are. Is this where the disagreement over chirality ultimately lies? If so, can you point to anything that might guide those of us without gauge theory knowledge as to how this particular problem has been handled in theory in the past? For example, you claim you can use two copies of a single self-conjugate group as the left and right handed fermions for a single generation-- is this really standard practice in gauge theory? D&G meanwhile claim that the groups for a fermion generation and its antigeneration must be inequivalent-- is this a condition that existing GUTs, like I don't know the Georgi-Glashow/SU(5) theory, actually follow?
Thanks for the patient explanations, and please excuse me if I am garbling any concepts here...
(One particular caveat that may be confusing what I write above, I notice that D&G consistently speak of subgroups of the gauge group whereas you consistently speak of subspaces of the representation. I'm mixing the two freely. Are these things actually equivalent, or are the two different terminologies in some important way masking two different sets of rules?)