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Decomposition of SU(3) and particles

  1. Dec 20, 2011 #1
    As we know the algebra of SU(3) consist of two Cartan generators and 6 raising and lowering operators. We define the eigenstates of the Cartan operators as u,d,s, correspoding to the three lightest quarks.
    Now when we study the [itex] 3\otimes 3[/itex] tensor product we can show that the Hilbert space of these states decompose as
    [itex]3\otimes 3 = 8\oplus 1[/itex]
    Which means that we can devide this Hilbert into two invariant subspaces.
    And similar for baryons
    [itex]3\otimes 3\otimes 3=10\oplus 8 \oplus 8\oplus 1[/itex]
    My question is following. We can categorize the mesons in two multiplets of 8 particles and 1 singlet, and the baryons in one multiplet of 10 particles, one of 8 particles and one singlet. So is not obvious to me which is the correspondence between these groups of particles and the above Hilbert spaces. If the decomposition of [itex] 3\otimes 3[/itex] has one 8-plet why we have two mesons diagrams and for baryons why we have only one 8-plet of particles?
  2. jcsd
  3. Dec 20, 2011 #2


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    Look at the [itex]J^{P}[/itex] quantum numbers. Mesons have [itex]0^{-}[/itex] and [itex]1^{-}[/itex] octets; the vector meson octet ([itex]1^{-}[/itex]) carries a single space-time index; transforms as a Lotentz vector hence the name “vector meson”. In terms the [itex]SU_{f}(3)[/itex] quark fields, the vector mesons have the following combinations
    [tex]\{8\}_{\mu}^{i} \sim \bar{q}(x) \lambda^{i}\gamma_{\mu} q(x),[/tex]
    [tex]\{1\}_{\mu} \sim \bar{q}(x)\gamma_{\mu}q(x),[/tex]

    compared with pseudoscalar mesons ([itex]0^{-}[/itex]) singlet and octet;
    [tex]\{1\} \sim \bar{q}(x)q(x) = \bar{u}u + \bar{d}d + \bar{s}s,[/tex]
    [tex]\{8\}^{rs} \sim \bar{q}(x)R_{(a)}( \lambda ) q(x), \ \ r,s = 1,2,3[/tex]
    R_{(a)}(\lambda) = \lambda^{3}, \lambda^{8}, (\lambda^{i} \pm \lambda^{i+1}); i = 1,4,6

    Last edited: Dec 20, 2011
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