Relation between momentum and mass of quarks

  1. We say on-shell and off-shell mass of quarks. 1) What is the difference on-shell and off-shell mass of quarks. 2) At lab. center of mass frame for lepton particles p2= -m2. Can we apply this equation for quarks.
    Thank you!
  2. jcsd
  3. tiny-tim

    tiny-tim 26,016
    Science Advisor
    Homework Helper

    Hi Hluf! :smile:
    a quark always has on-shell mass (usually just called "mass")

    a quark never has off-shell mass

    off-shell mass is a mathematical trick which helps in the calculations for Feynman diagrams
    (p is the four-momentum)

    yes this applies to everything (and in any frame): leptons hadrons and photons

    you can regard it as the definition of m (the mass) :wink:
  4. Bill_K

    Bill_K 4,157
    Science Advisor

    If it works once, it's a trick. If it works twice, it's a method!
    1 person likes this.
  5. tiny-tim

    tiny-tim 26,016
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    But if you don't know how it works, it's magic! o:)
  6. Meir Achuz

    Meir Achuz 2,076
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    Gold Member

    If you don't know how it works, go to school to learn how.
    Quarks can appear as intermediate particles in Feynman diagrams, where they are
  7. mfb

    Staff: Mentor

    Well, there is no center of mass frame for (single) photons.
  8. Some people would argue the complete opposite. The longer time a particle lives for, the more probably that its mass would be measured to be almost exactly the on-shell mass, but really the whole universe is quantum mechanical and every process a "little bit" virtual, so everything is always off-shell, the exact on-shell mass occupying measure zero of the probability distribution :p.
  9. Vanadium 50

    Vanadium 50 18,498
    Staff Emeritus
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    Yes, some people will make this argument. Some people find quibbling entertaining. It's not a very good argument for promoting understanding of QFT to people new to it, though.

    There really is a difference between real and virtual, and to quote Potter Stewart, "I know it when I see it"
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