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A Are valence quarks real physical entities?

  1. Sep 30, 2018 #1
    Im carefully studying Deep Inelastic Scattering and I have the doubt if valence quarks are real physical entities or if they are just a way we can think hadrons are built. I understand that inside hadron we have the Parton Distribution Functions that say that there are an infinite number of quarks, antiquarks and gluons. In many lectures (https://gsalam.web.cern.ch/gsalam/repository/talks/2009-Bautzen-lecture2.pdf, slide 27) I can see that valence quarks are defined as the integral of the quark minus antiquark PDF distributions. This makes me wonder if the valence quarks are just a way to imagine the composition of hadrons, in the same way the Bohr model hepls to visualize the atom. Thanks for your help!
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  3. Sep 30, 2018 #2


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    If you want an answer to this you will first have to define what you mean by something being a "real physical entity".

    This is not really what the pdfs say. They tell you the probabilities to encounter a quark with a particular momentum. A hadron does not have a fixed number of particles inside it - it is not an eigenstate of an operator that could be dubbed a "total particle number" operator.

    This encodes the information about the "quark - antiquark content". While a hadron is not an eigenstate of a "total particle number" operator - it is an eigenstate of an quark - antiquark operator.
  4. Sep 30, 2018 #3
    Thanks for your reply Orodruin. What do I mean by physical real quantity? If I understand correctly, deep inelastic scattering experiments tell us protons are made of charged particles...but Im not sure DIS can tell protons are made of only uud quarks. Am I right?
  5. Oct 2, 2018 #4
    What DIS reveals is that, as you increase the energy of the probe particle, it starts to scatter off lighter and lighter constituents of the proton. At higher energies it starts to scatter off virtual quarks and anti-quarks that are fleetingly created from the gluons that mediate the strong force - and sometimes these virtual, "sea" quartks can have other flavours such as strange.
  6. Oct 6, 2018 #5


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    Strictly speaking there are no Quark states in the QCD hilbert space and the QCD lagrangian could be written using only hadron fields.

    The quark fields are used because they provide local fields that carry the fundamental rep of the colour group. Thus making the group algebra simpler and giving the integrals a more tractable form, but at the cost of unphysical states.
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