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*if* the higgs can not be found

  1. Feb 16, 2006 #1

    wolram

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    Is there a model that will permit a higgs-less model, without to much adjustment to the standard model ?
     
  2. jcsd
  3. Feb 16, 2006 #2

    wolram

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    why has this question been moved,after all it is to to the heart of astro phytsics
     
  4. Feb 16, 2006 #3

    wolram

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    Perhaps i should also ask about the super symetric particles that have
    also not been found.
     
  5. Feb 16, 2006 #4

    selfAdjoint

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    But astrophysics is not the field in which the answer to the question should be found, if it exists. This is.
     
  6. Feb 16, 2006 #5

    Physics Monkey

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    The Higgs is extremely important in the Standard Model. All the gauge bosons and most of the fermions are protected from having direct mass terms by gauge invariance. The only exception is the right handed neutrinos which are gauge sterile and hence can have a direct mass term in addition to a coupling to the Higgs. Everything else gets its mass from the Higgs vacuum expectation value. In particular, the Goldstone modes associated with the electroweak symmetry breaking are absorbed by some of the gauge bosons. We do observe massive fermions and massive gauge bosons, and we don't observe massless scalar fields. So the point is that Higgs is critical the Standard Model we all know and love. Of course, just because the Higgs isn't found at the LHC doesn't mean it doesn't exist. The theory doesn't predict the mass of the Higgs, and while many researchers have convinced themselves that the Higgs must be somewhere near, it is entirely possible that the Higgs sector could be more complicated than we suspect. I can't say exactly what the full gamut of alternatives is, but I can tell you that the Higgs is very important and so a major modification would be necessary.
     
  7. Feb 17, 2006 #6

    wolram

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    Thankyou Physics Monkey, as a pleb, i find the standard model seems to be
    a swiss cheese, the only things going for it is a contrived Hubble constant, and an untestable red shift, the SM still needs dark energy, dark matter,
    super symetric particles, if any thing the SM should be moved to Ivans
    forum.
     
  8. Feb 17, 2006 #7

    selfAdjoint

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    Umm, one problem with the thread being moved from astrophysics to particle physics is that the meaning of the phrase "Standard Model" is different. Physics Monkey isn't talking about the big bang and inflation, he's talking about the theory of the electroweak and strong forces. Quarks, gluons, and all that...

    I am a skeptic about supersymmetry myself, and experiments at Tevatron have put it under the gun for likely discoveries at LHC, but sheer speculation it isn't.
     
  9. Feb 23, 2006 #8
    A small correction to what PhysicsMonkey said - if the only fundamental particles are those we know of, plus the Higgs, then it *will* be found at the LHC. This is due to the fact that, in the SM, there is an upper bound to the Higgs mass, and that is below the highest mass Higgs the LHC can see. Now, if there's new physics such as SUSY, then there are ways for the Higgs to hide (although generally not by being too heavy, the new physics can sometimes produce enough extra background that the Higgs signal is lost).

    To answer the original question: there have been proposals for models of Electroweak symmetry breaking that do not involve the Higgs. The most famous is called Technicolor, which for many years was the main competitor to SUSY for the correct theory of physics at the TeV scale. In this model the role of the Higgs is shifted to a set of new gauge bosons and fermions that are strongly interacting. Experiments at the LEP accelerator at CERN have disfavored Technicolor models, but have not excluded the idea, and a fair bit of work is ongoing in that direction.
     
  10. Feb 23, 2006 #9

    Physics Monkey

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    Hi BenLillie,

    I feel the silly need to defend my post, so here goes. I did in fact say that most physicists expect to find the Higgs at LHC unless the Higgs sector is more complicated i.e. supersymmetry. The Standard Model, in and of itself, doesn't necessarily constrain the Higgs mass all that closely, yes? When we say that the Higgs *will* be found, what we mean is that assuming the Standard Model is valid as we know it now and using precision electroweak data we put the Higgs mass at less than 190 GeV (if memory serves, please correct me if I have the latest number wrong) with 95% CL. My only point is that this is not quite certainty especially since it based on a number of assumptions that we expect to be modified anyway.

    On a less defensive note, as someone who is interested in effective theories of QCD, I have a special place in my heart for technicolor even if it doesn't pan out. Chiral perturbation theory is just too cool.
     
  11. Feb 23, 2006 #10
    No need to worry, it's a minor point :)

    But, the LHC can, in fact, find a Standard Model Higgs up to masses of about 1 TeV. There is a bound coming from unitarity of longitudinal W scattering that requires the Higgs mass to be < 850 GeV, hence the statement that the LHC will find a SM Higgs. You're absolutely correct that the precision electroweak bounds are not airtight, but the unitarity bound is very solid.
     
  12. Feb 23, 2006 #11

    Physics Monkey

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    No problem, this isn't my area of expertise, so I'm happy to hear from someone else on the matter. Regarding the unitarity bound, perhaps you can clarify something for me. I understood that the bound you quote was perturbative, and that if you summed to all order in the Higgs coupling then you got a true bound which was a bit over a TeV so that there was still a loophole there at least as far as the LHC was concerned.
     
  13. Feb 23, 2006 #12

    Haelfix

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    Unitarity bounds are pretty strong, except that you can nearly always add extra degrees of freedom to push the bound around. So assuming the vanilla SM, we must see something Higgs like, otoh if we see extra stuff (not necessarily supersymmetry, but instead extra fields) then all bets are off, you would have to redo the computation.

    Fortunately if we see new physics, it is almost guarenteed to be more interesting and informative than if we simply see a 120 GeV Higgs scalar and nothing else.
     
  14. Feb 23, 2006 #13
    Haelfix is absolutely correct. Also, the fact that you have to introduce new degrees of freedom to evade the unitarity bound leads to one of the famous "no lose theorems" for the LHC. Basically, if the Higgs isn't there to be found, then there must be something else that couples to Ws and Zs with a mass not much above 1 TeV. Obviously, that's not an airtight argument that the LHC can discover the new states, but in every model I've seen *something* is visible at the LHC.

    I've never seen an all-orders calculation of longitudinal gauge boson scattering, and I'd be surprised if it was possible. I know the one-loop correction has been done, but I don't remember the direction of the shift, only that it didn't change the argument qualitatively. At the very high end there could be a region that's invisible, that's believeable.
     
  15. Feb 23, 2006 #14

    Physics Monkey

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    I definitely agree with both of you, and I'm reminded of the old Fermi theory of weak decays where the violation of unitarity was a signal of the new physics of the W and Z. I know Haelfix and I have talked about this before, how unitarity is a very powerful tool. Anyways, hopefully we will get much more than we bargained for, just to keep things interesting. :wink:
     
  16. Oct 21, 2011 #15
    @ Physics Monkey...

    You are saying about the full calculation of the WW scattering... But as long as we are in the perturbative region, I don't think it's necessary... See, in the tree level amplitude we have terms of order of λ, (apart from
    the constant terms, irrelevant at high energies). Now, this term cannot be modified by any λ^2 terms or the higher order terms... What we need is, the divergence will be cancelled order by order.... So, I think there should be a bound From each Order of λ... Of course, it is good to keep an eye on each order, but that will be
    a tedious work... What I mean to say is that, one may have better bounds from higher order calculations.. but
    the bound from tree level amplitude will not be modified...

    Sorry for my english... And thank you guys, for such kind of threads...
     
  17. Oct 21, 2011 #16

    Vanadium 50

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    This thread is 5 years old.
     
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