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Nucleon and Delta baryons (resonances)

  1. Feb 27, 2007 #1

    CarlB

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    Okay, I decided to start trying to understand the nulceon and delta resonances. Does anyone have suggestions for a good explanation of how these work? How many resonances are there? How are they distinguished? Etc.

    Here's the PDG information:

    http://pdg.lbl.gov/2006/listings/bxxxcomb.html

    I'll add more information as I dredge it up from the web. What I'm really looking for is suggestions on a good reference book.

    Okay, here's a good place to get things:
    http://www.jlab.org/div_dept/physics_division/events/campaign_blocks_1B.html
    In particular, the 6MB 154 page document:
    http://www.jlab.org/div_dept/physics_division/talks/Background/Hall_B/Burkert_2004_StAndrews.pdf
     
    Last edited: Feb 28, 2007
  2. jcsd
  3. Feb 28, 2007 #2
    That last document you linked to is nice. The nucleon resonances are understood as (someone correct me if I am wrong) spin and/or isospin excitations of the nucleon. The earliest understandings of the Delta were that it could be seen as a composite state of the nucleon and a pion.
     
  4. Mar 16, 2007 #3
    This is a good start you guys have here... good selection of articles too.

    The Delta baryons are a spin excitation of the nucleon doublet. Since the nucleon is the non-strange isospin-1/2 group in the JP = 1/2+ octet (ground state), the Delta resonances are the spin-excited equivalent in the JP = 3/2- decuplet. While the nucleon will have two quark spins aligned and one opposite, and hence only two possible flavor states (uud, udd) corresponding to isospin-1/2, the Delta baryons will have all three quark spins aligned. This allows for the isospin to extend to 3/2 for the Delta's, so there are four possible flavor states (uuu, uud, udd, ddd). The bag model gives a good estimate for the masses of the Delta's in relation to the nucleons, and so does the basic vector-gluon exchange model, both with simple expressions.

    The Delta baryon has an interesting historical note attached to it. The Delta++ and Delta- were the first baryons to grab attention when theorists were first considering the problem of partons. You see, the problem is that the Pauli Exclusion Principle does not allow for identical Fermionic particles to fall into the same energy state. As quarks are fermions, and they are grouped in three's to form baryons, then there should be no more than two identical in any grouping of three. But the Delta++ has three identical up quarks in it! This meant that, in order to preserve the Pauli Principle in the Delta++ baryon, there needed to be another quantum number or property that distinguished each of the identical quarks from one another. This became the new SU(3) gauge group of "color", which went along with the non-Abelian "colored" vector gluons.

    Since the advent of the non-Abelian gauge group of QCD, the masses of the baryons and mesons alike have been much better understood in such frameworks as the MIT bag model and others. The Delta baryons allowed physicists to discover this important element of nature.
     
  5. Mar 17, 2007 #4

    CarlB

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    Try this link for a good article, putting together the data more conveniently than the PDG, though it might be a few years out of date:

    Baryon resonances and strong QCD
    E. Klempt
    http://www.arxiv.org/abs/nucl-ex/0203002

    Carl
     
    Last edited: Mar 17, 2007
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