Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Experimental Proof that Neutrons are 3 Quarks?

  1. Oct 25, 2009 #1
    I am aware that there is strong evidence for the presence of quarks inside of protons and neutrons through the analysis of deep inelastic scattering experiments.

    The questions I have specifically relate to the data which supports the composition of the neutron versus the proton, how we have determined the difference between the two types of quarks, and that the exact numbers of quarks are three in the case of the proton and the neutron.

    Can someone point me to research which describe the actual experiments which show that:

    a) protons have exactly three quarks
    b) proton quarks are U, U and D
    c) neutrons have exactly three quarks
    d) neutron quarks are U, D and D
    e) the charge of the U quark is 2/3 and D quark is -1/3

    I have been hunting around for months and haven't seen the actual experimental data.
  2. jcsd
  3. Oct 25, 2009 #2
    The orginal work on discovering quarks (aka partons) in protons was done by Friedman Kendall Taylor at SLAC by doing deep inelastic electron scatteriing off of liqiud hydrogen at 20 to 40 GeV in the early 1970's. Worth a Nobel prize. I believe they also scattered electrons off of liquid deuterium.
    Bob S
  4. Oct 25, 2009 #3
  5. Oct 26, 2009 #4
    My read of the experimental data is that we know that:

    a) protons are made up of smaller particles
    b) the data seems to indicate three particles

    but that's about it. The rest seems to come from theory rather than experiment.
  6. Oct 26, 2009 #5
    Sure. If you know any other theory than SU(3) gauge theory predicting that kind of sum rule scaling violations, please let everybody know. It's much, much, much more precise than what you provide in you message, not just "3 particles", SO(3) does not work for instance. Only SU(3) matches all published data.
  7. Oct 26, 2009 #6
    I'm not saying the theory is not good. I'm just trying to differentiate the empirical data from what is deduced.
  8. Oct 26, 2009 #7

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    It might help if you were to ask questions, rather than making brash statements like "The rest seems to come from theory rather than experiment". If you don't know what the data shows (presumably why you asked the question in the first place), you shouldn't then turn around and tell us what the data do not show.

    It would also help if you told us what "the rest" is. The charge assignments? We know from scattering experiments that the proton's constituents have a summed charge squared of 1 and the neutron's have a summed charge squared of 2/3. Given three constituents of two types, that uniquely determines the charges.
  9. Oct 26, 2009 #8
    I don't see how saying that "my read of the data" "seems" to be such and such is a brash statement. I'm certainly not claiming to be an expert here. I'm not claiming that there is no experimental proof of the other points, only that my read of the data provided in the paper cited by humanino and everything I've been able to get my hands on so far is that only point a of my list has experimental direct proof.

    In my question that opened this topic I listed five specific items. The rest would be those parts of the five items that did not pertain to "protons having three parts" namely b, c, d, and e of my original post.

    You state that "We know from scattering experiments that the proton's constituents have a summed charge squared of 1 and the neutron's have a summed charge squared of 2/3." Do you know what specific experiments proved that? I'm also trying to figure out how we could have learned that from scattering experiments.

    I'm well aware that there may be concepts I need to learn before I can understand these interpretations but that's why I'm looking for the experiments themselves so I can learn what methods were used to reach the conclusions that were reached. If I need to learn some new concepts, so be it.
  10. Oct 27, 2009 #9
    You are asking simple questions deserving proper answers, but simplicity here does not entail triviality. Each of those questions requires quite a bit of discussion on its own. There is no 5 line or even 50 line satisfactory answer unfortunately.

    A good and free textbook which would at least partly satisfy your questions can be found here for instance :
    Quark, partons and QCD

    A quick glimpse at the necessary prerequisite to really understand what's going on, and the extent and quality of the data supporting the theory can be found in the DIS scaling plot from the scholaropedia article
    Bjorken scaling
    as well as the two cornerstone related articles
    QCD evolution equations for parton densities
    Shifman-Vainshtein-Zakharov sum rules

    Please note that I am not trying to throw away articles to stop the discussion. I am trying to feed the discussion. I spent 20 minutes trying to find the best suited references I could, and that is less than what it would have taken to attempt to describe here what you can find very well described there. Please skim through. The theory is not trivial, but is mathematically deduced from quite general principle of quantum gauge field theory.
  11. Oct 27, 2009 #10

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    The GLS sum rule gives you (a) and (c). Statements (b) and (d) are statements about what you call something - I could just have easily called them Type-A and Type-B quarks and the physics would be unchanged. Statement (e) stems from (a) and (c) and the charge-squared deeply inelastic scattering measurement I alluded to, or alternatively you can get it from the Gottfried sum rule. In either case, it will take a bit of algebra.
  12. Oct 27, 2009 #11
    Does this discussion mean that there is no scattering data on e-n deep inelastic collisions?
  13. Oct 27, 2009 #12
    Humanino and Vanadium 50,

    Thank you for your answers. This is exactly the kind of information I was looking for.

    I will report back here with further questions and summaries of the information I find that is relevant when I get to the point that I understand the experiments and theory.

    Thanks again.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook