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Why masons can be massless but their composite quark and antiquark both massive?

  1. Apr 28, 2009 #1
    How masons can be massless while their composites quark (q) and antiquark (\bar{q}) are both massive?

    Is there any clear physical senario to understand this?
     
  2. jcsd
  3. Apr 28, 2009 #2

    sylas

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    Do you mean "mesons"? They are made up of a quark and an antiquark, but they are not massless.
     
  4. Apr 28, 2009 #3
    Sylas is right, mesons actually do have mass. If I remember correctly, the mass is positive, the value being somewhere in-between that of a proton and electron.
     
  5. Apr 29, 2009 #4

    tom.stoer

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    This question is tricky.

    If we had a world with exactly massles quarks, then we would have some exactly massless mesons, too. The meson mass zero comes from the fact that these mesons are the so-called Goldstone-bosons of the chiral symmtrey. [the Goldstone theorem says that spontaneous breaking of a continuous symmetry generates massles scalar particles; in case of QCD with massles quarks u,d you would have three massles mesons, namely the familiar pions; in case of QCD with massles quarks u,d,s you would have eight massles mesons, three of them are the familiar pions; ...]

    Attention: other particles like vector mesons, nucleon and other stay massive even with massles quarks! Their masses are generated dynamically due to the QCD interaction.

    If you now turn on some quark masses to become non-zero, the corresponding Goldstone bososn acquire a small non-zero mass.

    Do you know what chiral symmetry of QCD is?
    Do you understand the Goldstone theorem?
    Do you know what spontanous symmetry breaking means?
     
  6. Apr 29, 2009 #5

    Vanadium 50

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    Tom, while what you wrote is correct, it's probably not helpful to be discussing the Nambu-Goldstone theorem to someone who doesn't know that mesons all have mass, or even how to spell mesons. It's true...but probably not the best place to start.
     
  7. Apr 29, 2009 #6

    tom.stoer

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    Maybe you are right - but that's why I am asking questions as well. If Osiris comes back with more basic questions, we can continue, so let's wait and see ...
     
  8. Apr 29, 2009 #7
    Vanadium_50 is right. I can add only that apart from knowing 2+2=4, one must know that 2x2=3+1 here.

    Bob.
     
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