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Lagrangian and fields reprs

  1. Jan 10, 2015 #1

    ChrisVer

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    I was wondering.
    What's the reason for putting objects in low representations in the SM and not higher ones?
    So, why fermions in a doublet of SU(2) and not a multiplet?

    In analogy in SU(5) we put the particles in the 5-plet...
     
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  3. Jan 10, 2015 #2

    Vanadium 50

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    You could do that. But it would predict particles that are unseen. If I put the left-handed electron and neutrino in an SU(2) triplet and not an SU(2) doublet, where is the third particle?
     
    Last edited: Jan 10, 2015
  4. Jan 10, 2015 #3

    ChrisVer

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    Hmmm I was thinking more the representations of the same gauge group.
    For example the SU(2) has the 4-plet or triplet representation
    (
    [itex] 2 \otimes 2 = 3 \oplus 1[/itex]
    [itex] 2 \otimes 3 = 4 \oplus 2[/itex]
    )

    and in a similar way I think you can work up to the 6plet.
    In the 6plet one could put all the leptons.
     
  5. Jan 10, 2015 #4

    Vanadium 50

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    You can't do that and make the quantum numbers come out right. How do the e and mu get the same quantum numbers if they are in different positions in the multiplet?
     
  6. Jan 10, 2015 #5

    ChrisVer

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    I think they would still be color neutral...
    They would still have the same isospins and u(1) charges, and that's enough.
    For example if I had:
    [itex] [6] = \begin{pmatrix} e \\ \nu_e \\ \mu \\ \nu_\mu \\ \tau \\ \nu_\tau \end{pmatrix}[/itex]

    The U(1) transformation matrix would have to be [itex] Y_6 = \begin{pmatrix} Y_{el-flav} & 0 & 0 \\ 0 & Y_{mu-flav} & 0 \\ 0 & 0 & Y_{tau-flav} \end{pmatrix}[/itex]
    (still traceless) with [itex]Y_{i-flav}[/itex] the same 2x2 matrices you have in the SM for the i-th flavor.
    and similarily for the isospin
    [itex]T_6^i = diag ( \tau^i , \tau^i , tau^i ) [/itex]
    The only quantum number which I "feel" this would violate is the lepton number. But the lepton number is an accidental symmetry of SM.

    Maybe I'm terribly wrong with the choices of Y and T matrices?
     
  7. Jan 10, 2015 #6

    Vanadium 50

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    You can't have them in a 6-plet of isospin and have the same isospins.

    Consider angular momentum, also an SU(2). A 6-plet corresponds to J=5/2 which has m = +/- 5/2, +/- 3/2 and +/- 1/2. You can't declare it to have three +/- 1/2 and no +/- 3/2 or +/- 5/2. That's not a J=5/2 state and it's not a 6-plet.
     
  8. Jan 11, 2015 #7

    vanhees71

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    There's no way (yet?) to derive the particle content of the Standard Model theoretically. It's just empirical input to the model. The same holds for the many free parameters (coupling constants/masses).
     
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