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I Photons, Higgs -- Mass question

  1. May 18, 2016 #1
    I am really not sure. I watch Susskind lecture


    But I am still confused. He sad electron in motion emits photons. Quarks in motion emited gluons. Is that correct? Could quark be in motion?

    Also why all matter is made on fermions? And does Higgs gave mass to all particles? Why photon after all do not have mass?
     
  2. jcsd
  3. May 18, 2016 #2

    mfb

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    Motion is meaningless (in which frame?), acceleration is the point.
    We just happen to live in a universe where this is the case. At least current physics cannot give a more fundamental answer.
    All apart from photons, gluons and maybe neutrinos.

    There is a deeper reason why the Higgs field cannot give mass to photons and gluons, but I explaining that properly would need quantum field theory.
     
  4. May 18, 2016 #3

    mathman

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    Although Higgs gives mass to quarks, most of the mass of nucleons comes from internal energy - gluons, etc.
     
  5. May 19, 2016 #4
    Ok. But how is possible to know that? Some measurment?

    Ok Higgs gives mass to quarks, gluons?, electrons? I do not understand why all that particles need to get mass from Higgs? What about photons?
     
  6. May 19, 2016 #5
    Science works as follows. Theories are formulated which try to logically explain, and predict experimental results. Theories whose predictions differ from observed results are discarded or modified.

    Theories whose predictions match experiments are accepted as "what we know". Some are pretty much 100% solid (such as existence of atoms). Some match experiments *so far* - there are no guarantees they will not be shown to be erroneous by future experiments.

    The Standard Model is one of such theories. It says that Higgs gives mass to particles (and explains how that happens). And so far SM's predictions match experiment.

    Saying "we know that Higgs gives mass to particles" is a simplification of the above extended explanation.
     
    Last edited: May 19, 2016
  7. May 19, 2016 #6

    mfb

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    Not to gluons, see my previous post. And not to photons either. They do not "need" to get a mass, but we observe that they do have a mass (if they would be massless, we would not exist), and the Higgs mechanism is our best explanation for that.
    Not just some measurement, all the measurements - they all have to agree with our theories, otherwise the theory is wrong and gets discarded (or used as approximation where it is accurate enough).
     
  8. May 19, 2016 #7

    ChrisVer

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    How is it possible to know what? The last time I looked into the world, particles had mass...
    The Higgs Mechanism was one of the best ways we had to describe how particles that were in theory massless could gain the mass we observe... and things became better for it eversince the (Standard Model-like) Higgs Boson was discovered.
    Rougly speaking: The point with the quarks (that get their masses from the Higgs field) and protons is that the valance quarks of the protons are the uud, and all 3 of them would have a mass bellow a few MeV.... the proton however has a mass ~1GeV....all the extra mass has to come from something else [thus the pointing to internal energy by mathman]...

    Higgs gives the mass to all massive elementary particles of the SM (except maybe neutrinos as it's already pointed out). Those are the electrons, muons, taus, the 6 quarks and the massive bosons (Z,W).
    The massless particles [like gluons and photons] remain massless after the process of the Standard Model's Electroweak Symmetry Breaking; if you are OK with particle physics with Lagrangians you can try out a Higgs Mechanism review...
     
  9. May 19, 2016 #8

    ohwilleke

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    The Standard Model started out as a theory in which all fundamental particles were massless. And, the proto-Standard Model was great, except for the not ascribing mass to particles that clearly behaved as if they had masses which had been experimentally measured before the Standard Model was formulated. So, somebody sat down and tried to figure out the easiest way to impart mass to the massive fundamental particles in the Standard Model without screwing up the rest of the equations in it and the rest of its theoretical structure. Higgs and a few other people figured out that one way to impart mass to the massive fundamental particles in the Standard Model was to give those particles Yukawa interaction terms with a constant Higgs field with a vacuum expectation value of 246 GeV. But, for this solution to work, the theory implied that you needed to add one more particle to the Standard Model, a Higgs boson with a mass of roughly the same order of magnitude at the Higgs vacuum expectation value. All of the properties of this Higgs boson, except its mass, could be determined directly from the theory (spin-0, even parity, massive, neutral electric charge, no color charge, certain decay patterns as a function of its mass, etc.). So, then, scientists spent another 40 years seeing if anything fitting that description could be detected experimentally, and forty years later at the LHC, they happened to find it and therefore believed that the theory that predicted its existence was probably true and the Standard Model was probably complete and correct (except for some questions about neutrino oscillation and the way the SM fits with quantum gravity).

    Particles need to get mass from the Higgs because that is the part of the theory that give fundamental particles mass and there are only a handful of mathematically consistent ways to achieve this outcome without screwing up the correct predictions of the rest of the Standard Model. There are a couple of alternative ways that particles could get mass without the Higgs (e.g. technicolor theory). But, once the Higgs was discovered, the need to consider these alternative possibilities evaporated.
     
  10. May 20, 2016 #9

    ChrisVer

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    I would say they get changed to fit the new scheme...
    For example Higgs doesn't come problem-free in the theory, as it's an elementary scalar....For example it arises questions of naturalness, fine-tuning of its mass radiative corrections etc....
    I think for example that the Technicolor has been modified to fit the results of the Higgs Boson... but I may be outdated on this.
    (http://journals.aps.org/prd/abstract/10.1103/PhysRevD.90.035012 or https://arxiv.org/abs/1309.2097)
     
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