I'm sorry I wasn't around a couple of weeks ago for this interesting discussion. I'm writing my thesis on topics related to technicolor, so perhaps I can add something even after the fact.
First of all, on the subject of "supersymmetric technicolor", I would recommend those interested check out section 3.7 of the
big technicolor review by Hill and Simmons, and the references therein. They begin with the Witten paper already mentioned by humanino, and also include at least one reference specifically on combining topcolor with susy.
Some brief comments on the previous discussions:
ensabah6 said:
Is there a reason the vast majority of physicist prefer Higgs + SUSY over dynamical technicolor models?
What I consider the main reason doesn't seem to have been mentioned: technicolor is a strong interaction, analytically incalculable! If you want to perform some calculation in technicolor, you basically only have three options:
* try to extrapolate heuristic expectations from the single "data point" of QCD;
* try to develop some duality relating the model of interest to some weakly-coupled system in which calculations can actually be performed;
* try to use lattice gauge theory to construct improved heuristics or extract some nonperturbative inputs.
The first option isn't really reliable, and the others aren't really feasible yet, though a lot of effort is being put into them. I'm tempted to rant for a while, but I promised brief comments.
Of course, this isn't the only difficulty of technicolor models. Humanino mentioned that simple scaled-up QCD is experimentally ruled out (about which more below). And even without susy in the mix, ETC model-building quickly becomes very elaborate, with too many possibilities that can't yet be experimentally constrained.
humanino said:
The first "natural" technicolor models had been ruled out by precision electroweak models, so many people quit working on them. By now maybe a few dozens of people came up with various "improvements" (see extended technicolor or ETC, and walking technicolor) but one can argue that they are less "natural" and certainly more difficult.
ETC is not really an "improvement" of technicolor. Rather, it addresses a distinct issue from a similar perspective: While technicolor deals (only) with electroweak symmetry breaking (generating masses for the W and Z), ETC attempts to solve the "flavor problem" (generating fermion masses, mixings and CP violation).
The flavor problem is much more difficult than EWSB, and it pays to keep the two concepts distinct. My recollection is that one or the other (or maybe both) of these articles by Chris Quigg makes this point rather well:
http://arxiv.org/abs/0905.3187
http://arxiv.org/abs/0704.2232
It also comes up less explicitly in Ken Lane's "Two Lectures on Technicolor",
http://arxiv.org/abs/hep-ph/0202255
I would not describe walking theories as any more or less natural (or more or less difficult) than the earliest models of technicolor-as-scaled-up-QCD. Even though QCD is a confining theory, infrared fixed points are generic in gauge theories (cf. http://dx.doi.org/10.1016/0550-3213(82)90035-9 ), and it's reasonable to expect walking behavior around the strongly-coupled ends of "conformal windows". Of course, since these are strongly-interacting theories (as above), no one has yet rigorously demonstrated that walking can even occur, much less produce the desired effects.
Perhaps this is just a semantic issue depending on what we each mean by "natural".
blechman said:
from a "practical" point of view: susy was proposed to solve the "hierarchy problem" and "fine tuning problem" of the standard model. So was technicolor. Thus if one of these proposals is discovered, it would sort of take the wind out of the motivation for the other. that's why theorists typically don't combine them.
Personally, I find susy to be better motivated by the fact that it is the unique extension of Poincare invariance allowed by the Coleman--Mandula theorem. (I believe this is the historical motivation for susy as well, with applications to particle physics coming only after a few years of more abstract theoretical consideration.)
From a practical point of view, we would like susy and technicolor to "cure" each others' "problems". Sketchily, introducing susy-protected fundamental scalars (and their Yukawa couplings) could provide more leeway for ETC, making it easier to avoid experimental constraints from flavor-changing neutral currents. And on the other hand, if technicolor takes care of electroweak symmetry breaking, constraints on susy and susy-breaking phenomenology (which with I am less familiar) could also be relaxed.
This is also the general idea behind topcolor-assisted technicolor: topcolor takes care of part of the flavor problem (specifically, the large top quark mass) while (extended) technicolor deals with EWSB and less-problematic aspects of flavor. I'm told this hasn't really worked out, but I've never looked into topcolor much myself.
As Haelfix mentioned, the resulting constructions (like plain ETC models) are generally elaborate, Rube-Goldbergesque, and difficult to relate to experiment -- overall, probably not useful model-building exercises. But the general philosophy of "decoupling" the flavor problem from dynamical EWSB strikes me as a promising way to proceed. One interesting (if slightly tangential) approach to this that I read recently is the "monopole condensation" proposal by Csaki, Shirman and Terning:
http://arxiv.org/abs/1003.1718
