Higgsless Standard model

[JustinLevy]

"Higgsless" Standard model

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 non-perturbative 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)?

 

[blechman]

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)?

All the precision EW data is currently consistent with a STANDARD MODEL HIGGS model (a single 0++ scalar field with a mass around 115 GeV). All other models of "Higgs mechanism sans Higgs boson" that we know of are not as good a fit (unfortunately!). Although there is still wiggle room...

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 non-perturbative 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?

No, these models have other things built into them, such as extra dimensions, or a conformal hidden sector. The ordinary SM top quark cannot play the role of the Higgs.

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)?

See my above comments. If it was top composite, then there would usually be a tower of states above the "Higgs boson" that we would ultimately see either directly or indirectly, for example.

 

[bcrowell]

This may be useful: http://en.wikipedia.org/wiki/Higgsless_model

No, these models have other things built into them, such as extra dimensions, or a conformal hidden sector. The ordinary SM top quark cannot play the role of the Higgs.

Is there any reason to dislike these models? Do they require fine tuning? Or because people prefer not to invoke extra dimensions if they don't have to?

Can the LHC distinguish these models from one another, and from the Higgs?

 

[blechman]

This may be useful: http://en.wikipedia.org/wiki/Higgsless_model



Is there any reason to dislike these models? Do they require fine tuning? Or because people prefer not to invoke extra dimensions if they don't have to?

Can the LHC distinguish these models from one another, and from the Higgs?

Well, as I wrote a paper on one of them, I hope not everyone dislikes them!

I guess it depends on your tastes. Yes, they're pretty fine-tuned. 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...

 

[bcrowell]

Yes, they're pretty fine-tuned. Then again, so is the SM higgs!

What's fine-tuned about the standard Higgs?

 

[blechman]

What's fine-tuned about the standard Higgs?

This is the famous problem, the motivation for SUSY, etc.

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 fine-tuning!

 

[JustinLevy]

Thanks for the great responses.

No, these models have other things built into them, such as extra dimensions, or a conformal hidden sector. The ordinary SM top quark cannot play the role of the Higgs.

Ah okay. I've heard of this before:
http://www.worldscinet.com/mpla/04/0411/S0217732389001210.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/composite-higgs approaches?

 

[blechman]

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 strongly-coupled 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.

 

[JustinLevy]

The more modern versions (2005 and later) involve things like strongly-coupled CFT, or if you believe in the "Maldecena conjecture", warped extra dimensions.

Since the physics we see clearly isn't scale invariant (does conformal mean more than that?) then does any conformal field theory have to be in a hidden sector? Or do people sometimes refer to things as conformal field theories that only have that symmetry in the lagrangian, but the symmetry is spontaneously broken in our universe "somehow"? (I'm not even sure that makes sense)

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.

 

[blechman]

Since the physics we see clearly isn't scale invariant (does conformal mean more than that?) then does any conformal field theory have to be in a hidden sector? Or do people sometimes refer to things as conformal field theories that only have that symmetry in the lagrangian, but the symmetry is spontaneously broken in our universe "somehow"? (I'm not even sure that makes sense)

"conformal" is technically a little stronger than "scale invariant", but they are more or less the same thing...

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 long-wavelength (low energy) limit.

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.

Usually, "spontaneous symmetry breaking" refers to when a symmetry is manifest in the action, but the vacuum of the universe breaks the symmetry. For a simple example: think of a ferromagnet: at high temperatures (paramagnetic state) every direction is the same as every other, so there is a symmetry, but at low temperatures (ferromagnetic state) the spins of the material line up to create a macroscopic magnetic field, and there IS a preferred direction, and the symmetry is broken. But both cases are described by the same action, and it is still symmetric, even though the ground state is not.

"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!