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Fermions are massless

  1. Apr 5, 2007 #1
    I read that fermions really should be massless when you write down the Lagrangian, as it violates the gauge symmetry. It's the Higgs coupled to them that give them their masses. I was so shocked.
    I have only learned QED and abit of QCD Lagrangian, and the fermions did have masses in the Lagrangian...
    Also, how many parameters in particle physics are determined by this Higgs thing? Why is everyone still so confident in the Higgs? The LEP accelerator has found no evidence of Higgs.
  2. jcsd
  3. Apr 7, 2007 #2
    It's true in the full theory that fermions are massless. I Think of QED and QCD as useful approximations where the ugly stuff with the Higgs is stored in the "masses" of the fermions.
  4. Apr 7, 2007 #3


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    firstly, the Higgs mechanism does not predict fermion masses, they are given by the Yukawa couplings of which their values are put it by hand to fit the experimental data. There are higher dimensional theories that attempt to reduce the number of free parameters in the model: eg. 5D split fermion models.

    anyway, ppl are still hopeful that the Higgs or some Higgs like object or something new will be found at the LHC later this year or early next year.
  5. Apr 8, 2007 #4
    Although it is true that the mass of fermions is somewhat put by hand via the Yukawa couplings, the way vector mesons acquire mass through the Higgs mechanism is quite elegant and more subtle.

    Truth is far from being that simple. As evidences for a light Higgs was beginning to accumulate, some people claim that it was decided to shut down LEP before the Higgs was discovered, because the motivation for LHC would have been greatly reduced. This is way more shoking, and quite controversial in fact. :smile: But in any case, there are many things to be discovered at LHC.

    Not everybody relies blindly on the Higgs mechanism. Alternatives exist. Composite Higgs model for instance. Also, more complicated scenari.

    It is very possible that the standard model is a low-energy effective model, in fact most physicists would consider it in this manner I guess. This does not prevent a scalar Higgs-like particle to be discovered at LHC, even though the true mechanism might be much more complicated, or better, much simpler but belonging to another, yet to be found and/or established as correct, formalism :smile:
  6. Apr 8, 2007 #5
    Could you explain more on this? Why do you single out vector mesons?

    Sounds to me like "politics".

    We have never found a scalar particle. Why should we have one like Higgs? Can some other non-scalar particles take the place of a scalar Higgs?
    Last edited: Apr 8, 2007
  7. Apr 9, 2007 #6
    Because the mechanism of spontaneous breaking of electroweak symmetry involves the identification of the right degrees of freedom. The massless Higgs modes or Goldstone's bosons get eaten by the longitudinal component of the vector bosons. This is way less trivial than just postulating a Yukawa coupling for fermions.
    No they could not. I was just saying that the Higgs particle might not be fundamental. However, this is very controversial. Technicolor models have almost certainly been ruled out already. The reason I keep thinking that the Higgs might be just an effective degree of freedom is because it appears almost miraculously within the framework of Connes' noncommutative geometry.
  8. Apr 11, 2007 #7
    Maybe. I think alot of people find the higher-energy experiments to be more "sexy", as a friend of mine put it, than the lower-energy nuclear physics experiments, and so do governments. The bigger the accelerator, the bigger the status symbol it becomes. So this is really no surprise to me at all. If anything, I was more surprised when the SSC was cancelled in the mid-1990's than when this allegation came down the pipeline. It is entirely possible that the LEP shutdown in favor of the LHC was politically motivated. And, of course, the higher energy is "sexy"...

    Man, I wish nuclear physics and low-energy collider physics was still "sexy"...
  9. Apr 11, 2007 #8
    I think there are many reasons for this state of affair. Possibly, a key ingredient is the fact that those very high energy experiment reach a critical mass where they become visible to the large public. They are for sure more able to communicate to the large public than small collaboration at low or intermediate energies.

    One think in particular strikes me with amazement. If you ask a randomly chosen person what the Higgs boson is all about, you might get something sensical, that is the person might know that it is supposed to explain the origin of mass. It appears to me however that this well-known argument has a major flaw : it does not explain the origin of mass. Mass around us is hadronic, and the mass-gap problem (in general Yang-Mills theories) is also very important, possibly more important than the existence (or not) of the Higgs boson. However, people working in the hadronic community, performing direct experiments on quark confinement and/or mass spectra, do not seem to attract one thousandth of the attention the large public gives to Higgs boson researches. If we do not take into account non-commutative geometry, the Higgs boson is a very phenomelogical object to the theoretician and/or mathematician eye. But the mass-gap is a puzzle ranging from pure algebraic geometry all the way to accelerator labs.
  10. Apr 11, 2007 #9
    This is so true! It is as if the low-energy realm has been forgotten by the public almost entirely because it just isn't interesting or exciting to them. Personally, as someone who is involved entirely in low-energy meson spectroscopy, I think this is a shame because alot of worldwide medical and energy solutions are more closely related to the low-energy realm than to the high-energy realm. The high-energy realm experiments are, in my own opinion, related mostly to curiosity, and are highly speculative.
    Last edited: Apr 12, 2007
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