Understanding Massless Quarks in QCD and the Nambu-Jona-Lasinio Model

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    Massless Quarks
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Discussion Overview

The discussion revolves around the concept of massless quarks in Quantum Chromodynamics (QCD) and their treatment in the Nambu-Jona-Lasinio model. Participants explore the implications of massless quarks in different contexts, such as constituent versus current quarks, and the relevance of these distinctions in particle physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether "massless quarks" refers to a scenario where the energy-momentum relation simplifies to E ∼ p due to a negligible mass term.
  • Another participant clarifies that massless particles have momentum greater than zero and a mass of zero, leading to E = p when the speed of light is set to one.
  • It is noted that quarks are not massless in the strict m=0 sense, indicating a distinction in how mass is perceived in different contexts.
  • A participant explains that constituent quarks, which form baryons, have an effective mass around 300 MeV, while current quarks have masses of 5 to 10 MeV, suggesting that neglecting this mass can be a reasonable approximation in certain scenarios.
  • The concept of massless current quarks gaining constituent quark masses through spontaneous breaking of chiral symmetry is introduced, referencing the Nambu-Jona-Lasinio model.
  • One participant expresses frustration with the complexity of model names, specifically the Nambu-Jona-Lasinio model, highlighting a humorous take on the difficulty of pronunciation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the interpretation of massless quarks, with differing views on the implications of mass in various contexts and the relevance of the Nambu-Jona-Lasinio model.

Contextual Notes

The discussion touches on the distinction between constituent and current quarks, the implications of chiral symmetry breaking, and the varying mass scales involved, but does not resolve the complexities or assumptions underlying these concepts.

StatMechGuy
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I've been doing some reading on QCD, and I keep running into the notion of "massless quarks". Are they massless in the sense that if you look at
<br /> E = \pm \sqrt{p^2 + m^2}<br />
the p^2 term dominates and E \sim p or is this something completely different?
 
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Massless particles have p > 0 but m = 0, so E = p (assuming you have set c = 1).
 
Yes, but quarks are decidedly not massless in the m=0 sense.
 
If you are talking about constitutent quarks, where three of them make up a baryon in the non-relativistic potential models, this is true. Then, mass is typically around 300 MeV.

However, if you are talking about current quarks - which are the relevant ones if you discuss partonic scattering, for example, the masses are 5 to 10 MeV for up and down quarks, and then, neglecting this mass may be quite a good approximation.

The idea is, for example, that you start with masseless current quarks which gain contituent quark masses by the spontaneous braeking of chiral symmetry, as, for example, in the Namb-Jona-Lasinio model.
 
milou said:
as, for example, in the Namb-Jona-Lasinio model.
My god... what happened to simple names for models these days? I can't even pronounce that model.
 
kcrick said:
milou said:
as, for example, in the Namb-Jona-Lasinio model.
My god... what happened to simple names for models these days? I can't even pronounce that model.

That should be http://en.wikipedia.org/wiki/Nambu-Jona-Lasinio_model" . "Namm-boo Joe-nah La-sin-eey-oh". How do you pronounce kcrick? :)
 
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