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What's the Standard Model of Particle Physics

  1. Jul 4, 2009 #1
    I'm trying to figure out what the standard model of particle physics is, what is included, who decides what's included, and how one would know when a new concept/theory is included or possibly rejected. It would be great to have a list of recent additions or experimental confirmations. Must everything in the standard model be experimentally verified before inclusion? Is there a standard reference site on Physicsforums?

    Comment: (I'm not loooking for explicit replies on this laudry list.)
    I skimmed Wikipedia "Standard Model", and the "mathematical formulation" as well, but I did not see anything explicit about string theory(s) (there are many) , Planck length, Heinseberg uncertainty, Holographic principle, Schrodinger wave equation, Feynman path integrals, nor spin networks to name a few. And I think general relativity and all of gravity is also omitted....unsure about special relativity.....Some sources list QCD others QCD and QED....are these relativistic?/ Many who do not know the fine points of the mathematics, like me, will not be able to tell which of the items listed above, for example, are implicitly included. And I'm guessing there are many other "quantum theories" not included.

    Thank you.
  2. jcsd
  3. Jul 4, 2009 #2


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    String theory and Gravity are not parts of the SM. This thus includes the Plank scale which is linked to the gravitational constant. The Standard Model describes the gauge particles (quarks, leptons) and gauge carriers (photon, weak boson, gluon) of the three fundamental forces Electromagnetic, Weak and Strong. It is a field theory and so Feynman path integrals are applicable but not spin networks or teh Holographic principle.

    It may help to understand the historical sequence of the Standard Model.
    Start with Quantum Mechanics and QFT
    Then we have QED describing the interaction of photons with charged quanta.
    Then there's the Salam-Weinberg electro-weak unification with spontaneous symmetry breaking.
    Finally there is the quark model of the hadrons and QCD describing the strong force.
    There ends the Standard Model though you find much speculation beyond it

    There are ongoing attempts to acheive Grand Unification which tries to get weak, strong and electromagnetism all in the same setting.

    There is the hunt for the Higgs particle to see if the Higgs mechanism is supported empirically. The Higgs mechanism seeks to explain the symmetry breaking of the electro-weak gauge symmetry which is as of now "put in by hand" within the standard model. Or more precisely to explain how the distinct weakly charged particles obtain their different masses.

    Finally there is of course the attempt at a quantum theory of gravity along with a gravito-electro-weak-color "Ultimate" unification. String/Brane theory is one approach and the one which has had the most funding and work done over the past decades. It is I fear lost in the beautiful mathematics and I don't expect much from string theory other than more mathematics papers published by physicists. (I could of course be dead wrong on this though and tomorrow awake to find someone has found a full blown string/brane theory of everything.)

    [EDIT: I summarized this from memory and may have misstated some details. Please anyone post corrections if they see a mistake.]
  4. Jul 4, 2009 #3
    Cathryn Carson has made podcasts available from her course:

    History 181B Modern Physics: From The Atom to Big Science


    (also available on iTunesU). This has an excellent (non-math) description of much of the last 100 years of physics, including QFT and the standard model.
  5. Jul 6, 2009 #4


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    The standard model is composed of the Weinberg-Salam electroweak interactions which gain their masses from a Spontaneous symmetry breaking mechanism of the Higgs Field, and the "strong" colour force. The standard model is a Relativistic Quantum Field theory with many symmetries, the symmetries which makes up the forces are the gauge symmetries of the U(1)xSU(2)XSU(3) group (rotations in abstract spaces, e.g. the SU(3) group is rotations in a 8 dimensional "colour" space)

    Now the Schrödinger equation is an EQUATION of MOTION i.e. how these particles wavefunctions "move". But Schrödinger equation is a non-relativistic equation.. so what I just said is wrong, the elementary particles of the Standard Model has Klein-Gordon, Dirac- and Proca equations of motion.

    Feynman path integrals are mathematical techniques to derive properties of the particles involved, such as decay rates and cross sections.

    The standard model physically a LAGRANGIAN, i.e. kinetic minus potential energy (i.e. "motion" and "interactions") of the particles and fields involved. By analogy, the Lagrangian of a non-relativistic spring is:
    L = (1/2)mv^2 + (1/2) omega * k * x^2

    (1/2)mv^2 is the kinetic energy, -(1/2) omega * k * x^2 is the potential energy :-)
  6. Jul 7, 2009 #5

    Right, yet the Higgs bosons have not yet been observed, so why do you think either spontaneous symmetry breaking (just after the big bang and maybe never "observable" ) and the Higgs field (supposedly here right now) ARE part of the model when neither have experimental confirmation? Note, I'm not complaining one way or the other, but other than historical precedent, I can't figure out why, for example, the Holographic principle (I believe also unproven experimentally) is out and the above two are in. What's the criteria?

    jambaugh posts:
    Don't think I realized that it was an "add on".....I am aware of a number of weaknesses of the standard model, several of which you posted, but this is a piece of additional "fine tuning" seems another. And aren't there multiple Higgs fields....who picked the one(s) for the standard model?

    Regarding the mathematics of the standard model, does anybody know if exact solutions are required for inclusion in the standatd model...or if solutions are only avialble via perturbations, is that acceptable? That still remains a big problem in string theory I believe.
  7. Jul 7, 2009 #6


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    The standard Model is NOT about what is found experimental or not, it is a model of how we think elementary particles should behave subject to the three forces weak,strong and EM. The Higgs mechanicsm is part of the model since it is needed to be first of all self consistent and consistent with OTHER experimental facts (like the one that particles do have mass for instance). Holographic principle is gravity, and gravity is not included in the SM, everything which includes gravity is PER DEFINITION "beyond" the standard model.

    In the Standard Model we only need one Higggs Field, in supersymmetric extensions we need more than one.

    No, no exact solutions in SM, only perturbation calculations.

    My advise to you is to study elementary particle physics from an historical point of view, you are posing several things like "why didn't they included that" etc.
  8. Jul 7, 2009 #7
    that seems like a good way to think about it....perhaps that's all there is to my question

    ....but who made THAT decision, who renders the definition??

    What does this mean?

    I may be mistaken, but I believe the holographic principle is valid for ANY region of space, gravity present or not.

    But the AdS/CFT correspondence, the Maldacena duality, does involve vaccum gravity so maybe that's why some would make a statement like yours.

    I spent parts of two days seaching on line for insights before posting this... nor could I get any good hints nor in any of the books I have at home...

    I suspect what history would reveal is that particle physicsts decide what's in the standard model and that string theorists and many other disciplines do not. Seems like an ad hoc list of favorite theories within a limited framework and things that don't yet fit are excluded. which is ok my me, I have no horse in the race....

    Thanks for the insights....
  9. Jul 7, 2009 #8
    To be just a little provocative : Nature.

    The reason we could not fit gravitation in the standard model might be due to the fact that we have basically no experimental data about quantum gravity, that gravity is so weak, or the proton so light (which may all be related to the very idea of gauge theory of course...). The standard model is usually defined to stop with the Higgs boson, because by the same token, with have little to no data at the electroweak breaking scale. So the model was defined up to this point, it was an immense success, and the rest is (as of today) historical speculation.
  10. Jul 7, 2009 #9


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    Let me instead put it this way:

    We have theoretical frameworks, call them "theories" and model frameworks, call them "models". Examples of theories are:

    i) Newtonian Mechanics (CM), the dynamics (equation of motion) are governed by Newtons three laws (see wikipedia etc), and the force is the negative gradient of a potential.

    ii) Quantum mechanics (non relativistic) (= QM), where we have Shcördinger Equation, operators, wavefunctions etc.

    in theory (i) we might have a model of gravity, let the Force then be "gravity" and say that it should be proportional to masses and inversely proportional to distance-squared etc, and from this force we can derive the potential, the potential energy, work done, equations of motion, systems bound by gravity (planatery orbtits) and so on, etc..

    in theory (ii) we can take one force from classical mechanics, derive the potential and then "quantize" it, putting it into the Schrödinger equation and derive the wavefunctions, energylevels and so on and so forth. One famous example of this is the Hydrogen atom.

    The essential things about these models is that they should be self consistent and comparable with experiments and observations.

    Notice now a couple of things. Gravitational systems are a subset of ALL possible systems we can have in theory (i), similar for the hydrogen atom in theory (ii).

    Now let us turn to Quantum Field theory (QFT) and the standard model (SM).

    QFT is "a mixture" of Classical Field Theory, non-relativistic Quantum Mechanics and theory of special relativity. QFT is what we call a "theory". QFT's ingredients are quantum fields with lorentz structure, and we have, just as we have in ordinary QM and CM, rules and theorems on how we calculate properties of quantum fields, like cross sections and decay rates etc.

    Now the model, which has QFT as theory, which describes the physics of EM, weak, and strong interactions are called the standard model, and is composed with many different quantum fields, linked togehter, with certain symmetrical properties.

    This is how science is done:
    we first try to find a model that can describe all known things whithin a certain sub-set of events (e.g. the theory of strong interactions, the SU(3) gauge field theory should describe all know features of the strong force) but when a model is proposed, it should also make new predictions! (e.g. the strong force SU(3) proposed that it should be asymptomatic free, which later was discovered).

    One more example of this is the electroweak unification; theoretically it merged together the EM and the weak force, and it gave a connection between the Z and W boson masses, and gave a mechanism for mass generation via the Higgs Mechanism. The features it explained when it was proposed was why there are weak "doublets" and "singlets" (i.e. why did only left handed particles and right handed anti-particles participating in the weak interactions), the electroweak unification theory (by Weinberg and Salam) did answer that question and together with the Higgs Mechanism, we now got an explanation how particles acquire mass and also why Z and W boson has different masses! But, as a good theory must do, it also gave us one more thing to look for, a prediction, there must exists a quite heavy scalar particle -> The Higgs boson... and currently, this is the piece we are looking for.

    In the 80's you might have asked "why is SU(3) force included in the standard model when we have not find experimental proof that it's coupling constant is asymptotic free?". This is just the progress of science, there is always one piece of the puzzle missing ;-)

    If it was possible to include gravity without "pain" in the 70's, we might have called the standard Model the Lagrangian which describes all four known forces, but now one didn't succeed with that, so that's why we call everything which is not EM-weak-strong "beyond" the standard model. This is just one of the anti-logical scientific concepts one has to learn, just as current flows in the opposite direction of the electron flow and that brighter stars are measured with smaller magnitude etc ;-)

    Yes, standard model is a particle physics things, it is a subset (as I wrote earlier) of Quantum Field Theories. The Standard Model is not ad hoc (ad hoc is when you give a suggestion of solution with no further prediction, in fancy words: an non-falsifiable explanation - it can never be proven wrong since it only describes what is known) since it has grown over several decades.... and you should get this Theory vs. Model thinking into your head, this is how physicists think. We make theories, then models, then we do experiments. The Standard Model is just a name, just as "The Hydrogen atom" is....

    ...and of course it is within a limited frame work. The Hydrogen atom is that too, when we go deeper into the experimental structure, we find that we must have relativistic corrections and so on and so forth, hence The Hydrogen atom model within the theory of non relativistic quantum mechanics works really good within a region of energies, then it becomes inaccurate. So is "the hydrogen atom" just an hoc collection of favourite particles and forces in a limited framework and things that don't yet fit are excluded? It's just a name...
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