
#1
Feb1710, 09:08 PM

P: 886

I realize we haven't been able to see the signal for a Higgs boson directly yet, but I have heard there is plenty of indirect evidence for the Higgs (in that it is needed in the model to match experimental results). My question is essentially: are all forms of this indirect evidence essentially only showing something is playing the role of the Higgs mechanism or is there actually indirect evidence of the Higgs boson itself (ie. extra particle content to what we've observed directly)?
Related, and probably more specific to allow answering, I've seen some high energy physicists mention in talks about the possibility of top quark loops playing the role of the higgs. Does this mean it is possible for the LHC to find no Higgs, and yet have it turn out that the standard model absent the higgs already explains everything, its just that there is a nonperturbative solution involving a top condensate? Or do these "top condensate" or "composite higgs" models generically require something else (either interactions or particle content) in the standard model? If it doesn't require anything new to be added to the standard model, then what would experimentalists (or alternatively theorists if it is merely a mathematical issue) have to see to convince everyone one way or the other on the top condensate or composite higgs models? Can the LHC definitively answer questions on top condensate if a higgs boson is never found? Or whether the higgs is composite if it is found (ie. are decay properties enough, or would yet higher energies be needed)? 



#2
Feb1810, 04:28 PM

Sci Advisor
P: 779





#3
Feb1810, 09:13 PM

Emeritus
Sci Advisor
PF Gold
P: 5,500

This may be useful: http://en.wikipedia.org/wiki/Higgsless_model
Can the LHC distinguish these models from one another, and from the Higgs? 



#4
Feb1810, 09:19 PM

Sci Advisor
P: 779

"Higgsless" Standard modelI guess it depends on your tastes. Yes, they're pretty finetuned. Then again, so is the SM higgs! There are many papers out there that try to come up with observables that can distinguish one model from another. But it is typically hard at the LHC (although not impossible). I cannot be more specific than that without reference to any particular model. But the usual argument is that the future "International Linear Collider" (ILC) will be able to nail down WHICH model the LHC discovers. IF there is an ILC... 



#5
Feb1910, 10:21 AM

Emeritus
Sci Advisor
PF Gold
P: 5,500





#6
Feb1910, 10:32 AM

Sci Advisor
P: 779

Higgs mass naively wants to be as heavy as it can be (that means Planck mass!). This cannot be right  it should be around 100 GeV. That's one hell of a finetuning!!! 



#7
Feb1910, 03:43 PM

P: 886

Thanks for the great responses.
http://www.worldscinet.com/mpla/04/0...389001210.html but thought I heard people referring to "top condensates" without extra dimensions. I probably just misunderstood. Are there any broad overview type review journal articles you can recommend on higgless/compositehiggs approaches? 



#8
Feb1910, 05:07 PM

Sci Advisor
P: 779

that's an old paper, from the days before the top quark was discovered. I don't think they work anymore. But I never studied them carefully so I don't know for sure. The more modern versions (2005 and later) involve things like stronglycoupled CFT, or if you believe in the "Maldecena conjecture", warped extra dimensions.
there are similar (but different) ideas to this paper where you use a FOURTH generation of quarks. Look up the recent papers by Bob Holdom for that. That has no extra dimensions, but it's not the top quark that's condensing. You can check out TASI lectures, but they're pretty advanced. 



#9
Feb1910, 05:36 PM

P: 886

Since we're discussing terminology, does "spontaneously broken" mean something specific such that the phrase "dynamically broken" is not considered part of that? I feel I am missing connotations when reading through abstracts for interesting papers. 



#10
Feb1910, 05:50 PM

Sci Advisor
P: 779

as you rightfully point out, the universe is not conformal! We assume that there is a hidden sector that is. This is not so crazy: it would happen if, for example, the universe underwent a second order phase transition at some high temperature (higher than the weak scale). The conformal symmetry is not spontaneously broken. Such theories are a mess! We just say that there are a bunch of operators that form a CFT, and that they only interact with the rest of the universe (quarks, leptons, etc) through "irrelevant operators", that is, operators whose effects go to zero in the longwavelength (low energy) limit. "Dynamical symmetry breaking" usually means that the symmetry is actually broken by "dynamical" effects. I know that sounds sort of redundant! The famous example is anomalies. Sometimes you start with a symmetry at the lowest order of perturbation theory, but you find that when you do higher order calculations, the symmetry is broken. If you heard of people talk about "anomalous symmetries," for example, that's what they're talking about. Hope that helps! 


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