Why is neutron slightly heavier than the proton?












2














With latest knowledge of QCD, is there any explanation for why the neutron is slightly heavier than the proton? Can it be boiled down to a simple formula?






mass standard-model neutrons protons isospin-symmetry







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  • Are you asking for a complete and proven rigorous explanation? Or will "the different valence quark combinations interact with the QCD vacuum in different ways, which produces states with different energies, as has been demonstrated using lattice QCD" be sufficient?
    – probably_someone
    3 hours ago












  • "Different valence quark makeups" - proton is uud, neutron is udd. "Interact with the QCD vacuum in different ways" - since the interactions are non-perturbative at these energies, there's not a whole lot more I can say here.
    – probably_someone
    3 hours ago






  • 2




    Frankly, if you aren't up to speed on the differences between the particles in the constituent quark model, there is no real meaning to asking about the "latest knowledge of QCD": the answer will rely on things you don't (yet) understand.
    – dmckee♦
    3 hours ago










  • Well everyone knows a proton is uud and neutron is ddu. But the mass differences in quarks doesn't explain the N/P mass difference. I mean if the only way to calculate the difference is computer simulations then that means humans don't really know why. Or at least it is beyond human understanding.
    – zooby
    3 hours ago








  • 1




    If protons were heavier than neutrons then protons would decay. The universe would be very different if hydrogen were unstable. I guess the only kind of stars would be neutron stars, and complex atoms wouldn't be produced.
    – PM 2Ring
    2 hours ago
















2














With latest knowledge of QCD, is there any explanation for why the neutron is slightly heavier than the proton? Can it be boiled down to a simple formula?






mass standard-model neutrons protons isospin-symmetry







share|cite|improve this question
























  • Are you asking for a complete and proven rigorous explanation? Or will "the different valence quark combinations interact with the QCD vacuum in different ways, which produces states with different energies, as has been demonstrated using lattice QCD" be sufficient?
    – probably_someone
    3 hours ago












  • "Different valence quark makeups" - proton is uud, neutron is udd. "Interact with the QCD vacuum in different ways" - since the interactions are non-perturbative at these energies, there's not a whole lot more I can say here.
    – probably_someone
    3 hours ago






  • 2




    Frankly, if you aren't up to speed on the differences between the particles in the constituent quark model, there is no real meaning to asking about the "latest knowledge of QCD": the answer will rely on things you don't (yet) understand.
    – dmckee♦
    3 hours ago










  • Well everyone knows a proton is uud and neutron is ddu. But the mass differences in quarks doesn't explain the N/P mass difference. I mean if the only way to calculate the difference is computer simulations then that means humans don't really know why. Or at least it is beyond human understanding.
    – zooby
    3 hours ago








  • 1




    If protons were heavier than neutrons then protons would decay. The universe would be very different if hydrogen were unstable. I guess the only kind of stars would be neutron stars, and complex atoms wouldn't be produced.
    – PM 2Ring
    2 hours ago














2












2








2







With latest knowledge of QCD, is there any explanation for why the neutron is slightly heavier than the proton? Can it be boiled down to a simple formula?






mass standard-model neutrons protons isospin-symmetry







share|cite|improve this question















With latest knowledge of QCD, is there any explanation for why the neutron is slightly heavier than the proton? Can it be boiled down to a simple formula?






mass standard-model neutrons protons isospin-symmetry




mass standard-model neutrons protons isospin-symmetry






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited 1 hour ago









Qmechanic♦

101k121831144




101k121831144










asked 3 hours ago









zooby

1,233514




1,233514












  • Are you asking for a complete and proven rigorous explanation? Or will "the different valence quark combinations interact with the QCD vacuum in different ways, which produces states with different energies, as has been demonstrated using lattice QCD" be sufficient?
    – probably_someone
    3 hours ago












  • "Different valence quark makeups" - proton is uud, neutron is udd. "Interact with the QCD vacuum in different ways" - since the interactions are non-perturbative at these energies, there's not a whole lot more I can say here.
    – probably_someone
    3 hours ago






  • 2




    Frankly, if you aren't up to speed on the differences between the particles in the constituent quark model, there is no real meaning to asking about the "latest knowledge of QCD": the answer will rely on things you don't (yet) understand.
    – dmckee♦
    3 hours ago










  • Well everyone knows a proton is uud and neutron is ddu. But the mass differences in quarks doesn't explain the N/P mass difference. I mean if the only way to calculate the difference is computer simulations then that means humans don't really know why. Or at least it is beyond human understanding.
    – zooby
    3 hours ago








  • 1




    If protons were heavier than neutrons then protons would decay. The universe would be very different if hydrogen were unstable. I guess the only kind of stars would be neutron stars, and complex atoms wouldn't be produced.
    – PM 2Ring
    2 hours ago


















  • Are you asking for a complete and proven rigorous explanation? Or will "the different valence quark combinations interact with the QCD vacuum in different ways, which produces states with different energies, as has been demonstrated using lattice QCD" be sufficient?
    – probably_someone
    3 hours ago












  • "Different valence quark makeups" - proton is uud, neutron is udd. "Interact with the QCD vacuum in different ways" - since the interactions are non-perturbative at these energies, there's not a whole lot more I can say here.
    – probably_someone
    3 hours ago






  • 2




    Frankly, if you aren't up to speed on the differences between the particles in the constituent quark model, there is no real meaning to asking about the "latest knowledge of QCD": the answer will rely on things you don't (yet) understand.
    – dmckee♦
    3 hours ago










  • Well everyone knows a proton is uud and neutron is ddu. But the mass differences in quarks doesn't explain the N/P mass difference. I mean if the only way to calculate the difference is computer simulations then that means humans don't really know why. Or at least it is beyond human understanding.
    – zooby
    3 hours ago








  • 1




    If protons were heavier than neutrons then protons would decay. The universe would be very different if hydrogen were unstable. I guess the only kind of stars would be neutron stars, and complex atoms wouldn't be produced.
    – PM 2Ring
    2 hours ago
















Are you asking for a complete and proven rigorous explanation? Or will "the different valence quark combinations interact with the QCD vacuum in different ways, which produces states with different energies, as has been demonstrated using lattice QCD" be sufficient?
– probably_someone
3 hours ago






Are you asking for a complete and proven rigorous explanation? Or will "the different valence quark combinations interact with the QCD vacuum in different ways, which produces states with different energies, as has been demonstrated using lattice QCD" be sufficient?
– probably_someone
3 hours ago














"Different valence quark makeups" - proton is uud, neutron is udd. "Interact with the QCD vacuum in different ways" - since the interactions are non-perturbative at these energies, there's not a whole lot more I can say here.
– probably_someone
3 hours ago




"Different valence quark makeups" - proton is uud, neutron is udd. "Interact with the QCD vacuum in different ways" - since the interactions are non-perturbative at these energies, there's not a whole lot more I can say here.
– probably_someone
3 hours ago




2




2




Frankly, if you aren't up to speed on the differences between the particles in the constituent quark model, there is no real meaning to asking about the "latest knowledge of QCD": the answer will rely on things you don't (yet) understand.
– dmckee♦
3 hours ago




Frankly, if you aren't up to speed on the differences between the particles in the constituent quark model, there is no real meaning to asking about the "latest knowledge of QCD": the answer will rely on things you don't (yet) understand.
– dmckee♦
3 hours ago












Well everyone knows a proton is uud and neutron is ddu. But the mass differences in quarks doesn't explain the N/P mass difference. I mean if the only way to calculate the difference is computer simulations then that means humans don't really know why. Or at least it is beyond human understanding.
– zooby
3 hours ago






Well everyone knows a proton is uud and neutron is ddu. But the mass differences in quarks doesn't explain the N/P mass difference. I mean if the only way to calculate the difference is computer simulations then that means humans don't really know why. Or at least it is beyond human understanding.
– zooby
3 hours ago






1




1




If protons were heavier than neutrons then protons would decay. The universe would be very different if hydrogen were unstable. I guess the only kind of stars would be neutron stars, and complex atoms wouldn't be produced.
– PM 2Ring
2 hours ago




If protons were heavier than neutrons then protons would decay. The universe would be very different if hydrogen were unstable. I guess the only kind of stars would be neutron stars, and complex atoms wouldn't be produced.
– PM 2Ring
2 hours ago










2 Answers
2






active

oldest

votes


















6














There is an easy partial answer to the question. The proton is (uud) and the neutron is (udd). The up quark is 2.2MeV and the down quark is 4.7MeV. So there is a 2.5MeV mass increase with the neutron. The proton is 938.272MeV and the neutron is 939.565MeV which is then 1.293MeV heavier. That is odd! The neutron has a smaller mass difference than just based on quark masses.



The complete answer is a massively difficult problem. The QCD gauge bosons or gluons are self-trapped so that while they have no mass their self-interaction confines then and their energy in a form of mass. In fact this is majority of mass in hadrons and baryons. This forms the basis of the mass-gap problem. Lattice gauge theory on computers has shed some light on this and predicted the mass of baryons pretty well. An exact math-physics answer is waiting in the wings, and ClayMath as a million dollar prize for an answer. With the proton there is more of a mass-gap due to interactions with gluons. Also the proton being charged may have some renormalized mass correction just from QED that off sets the increase in mass just from quarks.






share|cite|improve this answer





















  • I've just thought if all quarks had the same average momentum, then they're energy differences would be smaller than their rest energy differences. I wonder if that has something to do with it.
    – zooby
    48 mins ago



















0














For years the expected explanation has been, that a strong-force contribution to the mass difference, that originates in the mass difference between down and up quarks, outweighs the electromagnetic contribution to the mass difference.



There were two theory papers on this theme, earlier this year 1 2. The mechanism I get from skimming them, is that the eta meson mixes with the neutral pion (see first paper, page 7), and then the pion interacts with the omega meson (see second paper, end of introduction). That is, I think these are the specific interactions which produce the strong-force contribution to the nucleon mass difference.






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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    6














    There is an easy partial answer to the question. The proton is (uud) and the neutron is (udd). The up quark is 2.2MeV and the down quark is 4.7MeV. So there is a 2.5MeV mass increase with the neutron. The proton is 938.272MeV and the neutron is 939.565MeV which is then 1.293MeV heavier. That is odd! The neutron has a smaller mass difference than just based on quark masses.



    The complete answer is a massively difficult problem. The QCD gauge bosons or gluons are self-trapped so that while they have no mass their self-interaction confines then and their energy in a form of mass. In fact this is majority of mass in hadrons and baryons. This forms the basis of the mass-gap problem. Lattice gauge theory on computers has shed some light on this and predicted the mass of baryons pretty well. An exact math-physics answer is waiting in the wings, and ClayMath as a million dollar prize for an answer. With the proton there is more of a mass-gap due to interactions with gluons. Also the proton being charged may have some renormalized mass correction just from QED that off sets the increase in mass just from quarks.






    share|cite|improve this answer





















    • I've just thought if all quarks had the same average momentum, then they're energy differences would be smaller than their rest energy differences. I wonder if that has something to do with it.
      – zooby
      48 mins ago
















    6














    There is an easy partial answer to the question. The proton is (uud) and the neutron is (udd). The up quark is 2.2MeV and the down quark is 4.7MeV. So there is a 2.5MeV mass increase with the neutron. The proton is 938.272MeV and the neutron is 939.565MeV which is then 1.293MeV heavier. That is odd! The neutron has a smaller mass difference than just based on quark masses.



    The complete answer is a massively difficult problem. The QCD gauge bosons or gluons are self-trapped so that while they have no mass their self-interaction confines then and their energy in a form of mass. In fact this is majority of mass in hadrons and baryons. This forms the basis of the mass-gap problem. Lattice gauge theory on computers has shed some light on this and predicted the mass of baryons pretty well. An exact math-physics answer is waiting in the wings, and ClayMath as a million dollar prize for an answer. With the proton there is more of a mass-gap due to interactions with gluons. Also the proton being charged may have some renormalized mass correction just from QED that off sets the increase in mass just from quarks.






    share|cite|improve this answer





















    • I've just thought if all quarks had the same average momentum, then they're energy differences would be smaller than their rest energy differences. I wonder if that has something to do with it.
      – zooby
      48 mins ago














    6












    6








    6






    There is an easy partial answer to the question. The proton is (uud) and the neutron is (udd). The up quark is 2.2MeV and the down quark is 4.7MeV. So there is a 2.5MeV mass increase with the neutron. The proton is 938.272MeV and the neutron is 939.565MeV which is then 1.293MeV heavier. That is odd! The neutron has a smaller mass difference than just based on quark masses.



    The complete answer is a massively difficult problem. The QCD gauge bosons or gluons are self-trapped so that while they have no mass their self-interaction confines then and their energy in a form of mass. In fact this is majority of mass in hadrons and baryons. This forms the basis of the mass-gap problem. Lattice gauge theory on computers has shed some light on this and predicted the mass of baryons pretty well. An exact math-physics answer is waiting in the wings, and ClayMath as a million dollar prize for an answer. With the proton there is more of a mass-gap due to interactions with gluons. Also the proton being charged may have some renormalized mass correction just from QED that off sets the increase in mass just from quarks.






    share|cite|improve this answer












    There is an easy partial answer to the question. The proton is (uud) and the neutron is (udd). The up quark is 2.2MeV and the down quark is 4.7MeV. So there is a 2.5MeV mass increase with the neutron. The proton is 938.272MeV and the neutron is 939.565MeV which is then 1.293MeV heavier. That is odd! The neutron has a smaller mass difference than just based on quark masses.



    The complete answer is a massively difficult problem. The QCD gauge bosons or gluons are self-trapped so that while they have no mass their self-interaction confines then and their energy in a form of mass. In fact this is majority of mass in hadrons and baryons. This forms the basis of the mass-gap problem. Lattice gauge theory on computers has shed some light on this and predicted the mass of baryons pretty well. An exact math-physics answer is waiting in the wings, and ClayMath as a million dollar prize for an answer. With the proton there is more of a mass-gap due to interactions with gluons. Also the proton being charged may have some renormalized mass correction just from QED that off sets the increase in mass just from quarks.







    share|cite|improve this answer












    share|cite|improve this answer



    share|cite|improve this answer










    answered 2 hours ago









    Lawrence B. Crowell

    10.9k11024




    10.9k11024












    • I've just thought if all quarks had the same average momentum, then they're energy differences would be smaller than their rest energy differences. I wonder if that has something to do with it.
      – zooby
      48 mins ago


















    • I've just thought if all quarks had the same average momentum, then they're energy differences would be smaller than their rest energy differences. I wonder if that has something to do with it.
      – zooby
      48 mins ago
















    I've just thought if all quarks had the same average momentum, then they're energy differences would be smaller than their rest energy differences. I wonder if that has something to do with it.
    – zooby
    48 mins ago




    I've just thought if all quarks had the same average momentum, then they're energy differences would be smaller than their rest energy differences. I wonder if that has something to do with it.
    – zooby
    48 mins ago











    0














    For years the expected explanation has been, that a strong-force contribution to the mass difference, that originates in the mass difference between down and up quarks, outweighs the electromagnetic contribution to the mass difference.



    There were two theory papers on this theme, earlier this year 1 2. The mechanism I get from skimming them, is that the eta meson mixes with the neutral pion (see first paper, page 7), and then the pion interacts with the omega meson (see second paper, end of introduction). That is, I think these are the specific interactions which produce the strong-force contribution to the nucleon mass difference.






    share|cite|improve this answer


























      0














      For years the expected explanation has been, that a strong-force contribution to the mass difference, that originates in the mass difference between down and up quarks, outweighs the electromagnetic contribution to the mass difference.



      There were two theory papers on this theme, earlier this year 1 2. The mechanism I get from skimming them, is that the eta meson mixes with the neutral pion (see first paper, page 7), and then the pion interacts with the omega meson (see second paper, end of introduction). That is, I think these are the specific interactions which produce the strong-force contribution to the nucleon mass difference.






      share|cite|improve this answer
























        0












        0








        0






        For years the expected explanation has been, that a strong-force contribution to the mass difference, that originates in the mass difference between down and up quarks, outweighs the electromagnetic contribution to the mass difference.



        There were two theory papers on this theme, earlier this year 1 2. The mechanism I get from skimming them, is that the eta meson mixes with the neutral pion (see first paper, page 7), and then the pion interacts with the omega meson (see second paper, end of introduction). That is, I think these are the specific interactions which produce the strong-force contribution to the nucleon mass difference.






        share|cite|improve this answer












        For years the expected explanation has been, that a strong-force contribution to the mass difference, that originates in the mass difference between down and up quarks, outweighs the electromagnetic contribution to the mass difference.



        There were two theory papers on this theme, earlier this year 1 2. The mechanism I get from skimming them, is that the eta meson mixes with the neutral pion (see first paper, page 7), and then the pion interacts with the omega meson (see second paper, end of introduction). That is, I think these are the specific interactions which produce the strong-force contribution to the nucleon mass difference.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered 54 mins ago









        Mitchell Porter

        7,38111243




        7,38111243






























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