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If Neutron and Proton have some diff. mass then why?

  1. Aug 21, 2007 #1
    I see that in some books, Proton and Neutron charge differ a bit. Why is it? :confused:

    I thought them to be same. But...

    And I am new to this forum...
    Let's hope we will have a great time together studying science...
  2. jcsd
  3. Aug 21, 2007 #2
    You mean mass, right, not charge?
  4. Aug 21, 2007 #3
    Yes yes!
  5. Aug 21, 2007 #4


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    I'm not quite sure why you've posted this in the tutorials section, but I'm sure it'll be seen and moved soon.

    I'll try and give an answer to your question, although I'm sure the experts will chime in with something more rigourous. Protons and neutrons are both made up of fundamental particles. More specifically, they are both made up of three quarks. These quarks come in different flavours, but protons and neutrons both consist of two different flavours, namely "up" and "down". A proton is made up of two up's and one down quark, whereas a neutron is made up of two down's and one up. It's the slight difference in the masses of these different types of quark that gives rise to the difference in overall mass of the proton and neutron.
    Last edited: Aug 21, 2007
  6. Aug 22, 2007 #5
  7. Aug 29, 2007 #6
    To add a bit to cristo message. If up and down quark had exactly the same mass, protons and neutrons would have the same mass (neglecting other quarks). Then we could introduce a new symmetry with a new quantum number called isospin. In practice, this symmetry is broken.
    You can find a nice article on wikipedia :
  8. Aug 29, 2007 #7


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    Up and down quarks are considered to have the same mass.

    Proton is (uud) while neutron is (udd).

    The neutron is neutral, i.e. has a 0 (or no) charge while the proton has a charge of +1e by convention.

    The masses are slightly different -
  9. Aug 29, 2007 #8

    I do not understand this sentence. Quark masses are very small, much smaller than 1/3 m(neutron) :
    m(up) = [1.5, 4.0] MeV
    m(down) = [3.0, 7.0] MeV
    m(up)/m(down) = 0.3 to 0.6
    according to PDG
  10. Aug 29, 2007 #9
    In fact the proton-neutron mass split is a puzzle. As can be seen in the values given by PDG, the uncertainties in the (current) quark masses themselves are enough so that from everything down to nothing in this split can be explained by the masses of the quark. If you take the extreme, those uncertainties are even compatible with a heavier proton !

    There are many models providing different interpretations for this split. You have noticed that the (current) quark masses do not add up to the nucleon (proton or neutron) masses at all, by far. This (or something slightly more technical) is called the mass-gap problem and is one of the most interesting (important) problems in (pure) mathematics today. From this point of view, it is a little to "easy" to blame the quark masses for the proton-neutron split.

    The charge (electrical and magnetisation) densities in the proton and neutron are rather different, suggesting they do not have quite the exact same structures, even though very similar.

    Our understanding of those problems is poor enough so that, even assuming so called "isospin" symmetry (same neutron and proton structures), we already struggle to make precise sens out of them.

    BTW, talking of "constituent" quark masses as one third (approximately) of the nucleon mass is not only old-fashioned anymore. It is over simplifiying.
  11. Aug 29, 2007 #10
    But if we can reproduce part of the baryon spectrum via lattice qcd, it means we have part of the answer to this puzzle, isn't it ? By the way, this "up" and "down" masses from PDG are coming from lattice results I guess.
    I agree that it is one of the most interesting subject of subatomic physics. Particularly because QCD is non-pertubative and so we have to find alternative mathematical methods.
  12. Aug 29, 2007 #11


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    Thanks for pointing that out. I was oversimplifying the matter.

    Does this help - http://pdg.lbl.gov/2005/reviews/quarks_q000.pdf - on the question of method?
    Last edited: Aug 29, 2007
  13. Aug 29, 2007 #12


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    Hmm the right answer should state mass formulae for the baryons, and this review only quotes (page 6) some for mesons.

    Of course naively if m_u < m_d, then mass(uud)<mass(udd). But given that already in the meson mass formulae there are two terms, for QCD and electromagnetic contributions, one could cast some doubts. Of course historically it happened that the proposed mass formula was enough to fit the SU(3) flavour decuplet with some predictions, but the neutron and the proton are not there; they live in the octet.
    Last edited: Aug 29, 2007
  14. Aug 29, 2007 #13
    There is also the potential mass found in the "quark-gluon sea" found within nucleons such as proton and neutron. It has been known for some time now (since 2000) that strange quark is present within proton sea--that is, the proton is much more interesting that just saying it is made of three quarks (uud)--see this link:

    Here is a link from Jefferson Lab with lots of current research on nucleon internal structure:
  15. Aug 30, 2007 #14


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    Yeah, this is interesting for the question of the absolute value of the nucleon mass, but for the specific cuestion here (calculation of the mass difference between proton and neutron) surely it is not so relevant.
  16. Aug 31, 2007 #15
    I have a question on first topic.

    Clearly there is experimental evidence of strange quark within proton [P]. It is also known from experiment that the neutral Kaon(L) meson has (d-bar, s) quark structure. So, is there any experimental evidence (or current research) looking for neutral Kaon(L) meson within structure of [P]--given that within structure of Kaon(L) meson here we find the (s) strange quark shown to be present experimentally within [P] ? Now, it is also known that Kaon(L) has rest mass of ~ 497.7 MeV/c^2 which goes a long way toward explaining the missing mass of the proton (which is 938.3 MeV/c^2) based only on sum of quarks + gluons. So, are you aware of any research looking for neutron Kaon(L) within quark confinement of the proton ?

    Thanks for any leads on this topic.
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