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About asymptotic freedom

  1. Apr 17, 2009 #1

    I have a simple question related with asymptotic freedom.
    Quarks behave as free particles at small distance (separation between the quarks is small) because strong interaction decrease with decrease in distance or increase in energy. So, one quark feels almost independent of other.
    Now, in DIS where hadron is probed by high energy electron: basically virtual photon interact with quarks inside the hadron. The interaction is electromagnetic. At high energy or short distance the electromagnetic interaction is larger. There is no strong interaction with the projectile. Then, why electron sees the quarks as free? Quark-quark feel them self as free from each other.

    Last edited: Apr 17, 2009
  2. jcsd
  3. Apr 17, 2009 #2


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    The quarks can't separate because the stong force increases as they get further apart. When enough energy is put in to force them apart, new quarks come into being, attached to the original quarks, leaving them still paired.
  4. Apr 18, 2009 #3
    Yes, I know that. But my question is different. Interaction of virtual photon with quarks is electromagnetic, not strong. Then, why the electron in DIS (deep inelastic scattering) sees the quarks as free at high energy?
  5. Apr 18, 2009 #4
    Electromagnetic scattering is well understood, so the effect can be filtered out of the results.

    Imagine that you were in outerspace and 100 yards away was something that looked like a bowling ball, but someone told you it was actually three bowling balls connected together. Then you have an unlimited number of bowling balls to throw at the target, and a lot of patience.

    Fairly quickly you determine that the bowling balls cannot be seperated, the three of them are always connected (in my analogy this corresponds to the property of quark confinement that mathman was mentioning, although in my analogy I am ignoring the possibility of what happens when the flux tubes break).

    Now you want to determine the nature of the bonds that connect the bowling balls. Basically you expect them to be connected by tight springs. Then you do a calculation of the scattering pattern you expect to occur as a function of the spring constant. After doing the experiment it turns out that the spring constant is zero, and so in fact the springs have some slack in them, asymptotic freedom.
  6. Apr 20, 2009 #5
    Imagine two particles bound with a spring. They oscillate around an equilibrium position r_0 but most of the time they spend at extreme (turning) points where their velocity is the smallest. So most of the time they feel themselves interacting rather than free.

    If you push (or pull) one of them very slowly, you will feel the total mass - the system is accelerated as a whole on average.

    If you push one of them suddenly, during delta_t << oscillation period AND the perturbation of the spring propagates with finite velocity from the pushed particle to the second one (delta_t<<r_0/c), then you will "feel" only the pushed particle whatever oscillation phase is.
    So for a very fast projectile, the quarks may look free.

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