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Thread: freeproton decay

  1. #1 freeproton decay 
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    I'm trying to understand the weak force, is it somewhat obscure [or it's just me]?

    can someone explain what happens in a free proton?
    What starts the decay ? the weak force between the quarks or what?


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    Quote Originally Posted by monalisa View Post
    I'm trying to understand the weak force, is it somewhat obscure [or it's just me]?
    It is not just you:Proton decay - Wikipedia, the free encyclopedia

    Granted, the wikipedia pages are helpful and they maintain accuracy in reporting pretty well- But it is not an education.

    No one on the forum, even those that are very knowledgeable on the topic, can really teach it to you simply. The best way to learn, I'm sorry to say, is to attend a University.
    I am not saying a person cannot be self taught but these topics really do seem to require guidance in what to learn and what order to learn it in.
    This really is not stuff you read up about and understand it in a day.
    Maybe if you clarify what your goal is and where you're current knowledge is, members can point you in a better direction in your learning.


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    Thanks, I'll sum up what is obscure:

    1) We have a neutron, made up by three quarks, and the weak force keeps them together. Everywhere I read it is not like a classical force, but I couldn't find a formula, a force-law like we have with gravitation and Coulomb, I learned only that it works in a rance of m^-18.
    Even if it is not like a classical force, it keeps the quarks together, no matter of thei charge (flavour), how does that happen?

    2) then, I read, the same weak force causes a down quark to turn into an up quark, how does that work?
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  6. #5  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by monalisa View Post
    can someone explain what happens in a free proton?
    What starts the decay ? the weak force between the quarks or what?
    Protons do not decay - according to the standard model and current evidence, anyway.

    Quote Originally Posted by monalisa View Post
    1) We have a neutron, made up by three quarks, and the weak force keeps them together.
    That is the strong force, isn't it?
    Strong interaction - Wikipedia, the free encyclopedia

    Everywhere I read it is not like a classical force, but I couldn't find a formula, a force-law like we have with gravitation and Coulomb
    I don't think there is any simple formula for the force like there is for gravity or electric charge. It may be better to think of it as an interaction between particles of different types.

    2) then, I read, the same weak force causes a down quark to turn into an up quark, how does that work?
    Flavour changing processes

    I don't think there is any easy way to understand this stuff without at least a degree-level study of physics and probably some postgraduate level education as well.
    ei incumbit probatio qui dicit, non qui negat
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    The strong force keep nucleons together, weak force keeps quarks together.
    Maybe a degree is needed to understand the math or other, but a physics principle can always be explained and so can a process.
    If the formula is complicated we can skip it.

    It was a typo , I meant "free neutron" can you explain how the weak force produces beta/freeneutron decay?
    It seems to me that the process is complex and is rather impossible to ascribe it to a single phenomenon (force)
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    Quote Originally Posted by monalisa View Post
    The strong force keep nucleons together, weak force keeps quarks together.
    Quote Originally Posted by Wikipedia
    On the smaller scale (less than about 0.8 fm, the radius of a nucleon), it is the force (carried by gluons) that holds quarks together to form protons, neutrons, and other hadron particles.
    Strong interaction - Wikipedia, the free encyclopedia

    can you explain how the weak force produces beta/freeneutron decay?
    No. Except it is due to quark flavour changing (see above). That is as far as my understanding goes.
    ei incumbit probatio qui dicit, non qui negat
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    Quote Originally Posted by monalisa View Post
    The strong force keep nucleons together, weak force keeps quarks together.
    Maybe a degree is needed to understand the math or other, but a physics principle can always be explained and so can a process.
    If the formula is complicated we can skip it.

    It was a typo , I meant "free neutron" can you explain how the weak force produces beta/freeneutron decay?
    It seems to me that the process is complex and is rather impossible to ascribe it to a single phenomenon (force)
    No. Strong force keeps quarks together. So caled residual strong force keeps nucleons together (pion field instead of gluons). What weak force does is it changes the flavour of one quark in neutron therefore fliping its isospin and changing into proton. The W boson then decays into electron and antineutrino or positron and neutrino. Read my post in your other thread it will help.
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    Thanks for your sincerity, I'll tell what baffles me, probably in some way you can help:

    - WF changes a down quark to an up making it a proton (in a mysterious way)
    - then WF changes some mass into a W boson
    - the WF changes the boson into an electron - antineutrino

    rather omnipotent , would you agree?

    Besides this
    - the electron is created after the neutron has been turned into a proton so, as soon as it is born it feels a huge attractive force (r = mx10^-17-18)
    - EMR , energy of the boson gives place to pair formation of a charged vs. uncharged particle one with a mass billions of billions smaller than the other.
    - KE of the electrons vary a lot , from slow to near C, how is that possible neutrons are different? How can a slow electron escape the huge electrostatic attraction of the newborn proton?
    Last edited by monalisa; September 26th, 2013 at 08:07 AM.
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    Quote Originally Posted by monalisa View Post
    WF changes a down quark to an up making it a proton (in a mysterious way)
    Yes, this is called flavour changing, and there is nothing mysterious about it.

    then WF changes some mass into a W boson
    No it doesn't. W-Bosons are the gauge bosons for the weak interaction ( along with the neutral Z-Boson ), and they have mass since they interact with the Higgs field.

    the WF changes the boson into an electron - antineutrino
    No, what happens is that a down quark changes into an up quark by emitting a virtual W(-) boson, which, due its large mass, then decays into an electron+electronantineutrino. Again, there is nothing mysterious or omnipotent about this.

    the electron is created after the neutron has been turned into a proton so, as soon as it is born it feel a huge attractive force (r = mx10^-17-18)
    So what ?

    EMR , energy of the boson gives place to pair formation of a charged vs. uncharged particle one with a mass billions of billions smaller than the other.
    Charge is conserved here, because the original W(-) boson carried one unit of negative charge. Total energy-momentum is also conserved in this process. Where is the issue, exactly ?

    KE of the electrons vary a lot , from slow to near C, how is that possible neutrons are different?
    What do you mean by this ?

    How can a slow electron escape the huge electrostatic attraction of the newborn proton?
    Why do you think it is slow ?
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    Quote Originally Posted by monalisa View Post
    Thanks for your sincerity, I'll tell what baffles me, probably in some way you can help:

    - WF changes a down quark to an up making it a proton (in a mysterious way)
    - then WF changes some mass into a W boson
    - the WF changes the boson into an electron - antineutrino

    rather omnipotent , would you agree?

    Besides this
    - the electron is created after the neutron has been turned into a proton so, as soon as it is born it feel a huge attractive force (r = mx10^-17-18)
    - EMR , energy of the boson gives place to pair formation of a charged vs. uncharged particle one with a mass billions of billions smaller than the other.
    - KE of the electrons vary a lot , from slow to near C, how is that possible neutrons are different? How can a slow electron escape the huge electrostatic attraction of the newborn proton?
    Of course it must seems mysterious to you but there isn`t a way to describe it nonmysteriously without some knowledge of quantum field theory. It`s like describing Mona Lisa to people blind from birth. There are strong arguments that weak force must exist from requiered symmetries of lagrangian. Lets say we have leptons and quarks. Now we require some symmetries (gauge symmetries) on their lagrangians that will inevitably result in charged and noncharged vector fields (without mass but thats for another discussion) that couple to lepton and quark fields. We take all this and name it electroweak interaction. These mysterious interactions that change flavours etc. can be directly read from interaction terms that have to look exactly like they do to preserve gauge symmetries! Thats the underlying reason for weak and electromagnetic interaction.

    Quote Originally Posted by monalisa View Post
    - the electron is created after the neutron has been turned into a proton so, as soon as it is born it feel a huge attractive force
    Well yeah but in atom you don`t have any avaiable bound state free so electron cannot stay close to nucleus but if beta decay happens in some strongly ionized atom created electron may actualy occupy some bound state of the same atom.
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    Quote Originally Posted by Markus Hanke View Post
    No, what happens is that a down quark changes into an up quark by emitting a virtual W(-) boson, which, due its large mass, then decays into an electron+electronantineutrino.
    how can a neutron emit a particle whose mass is 100 times larger than itself?
    What do you mean by this ?
    I would expect that all neutrons decay in the same way, i.e. that emitted electrons have same speed...I read that this does not happen, how come?
    so what?
    ... also because the have to escape same attractive force from the newborn proton. What is the strength of the electrostatic force at the distance of mx10^-17/18, what KE do you require to escape such a field?
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    Quote Originally Posted by monalisa View Post
    how can a neutron emit a particle whose mass is 100 times larger than itself?
    This may seem like a contradiction in terms of classical mechanics, but is perfectly permissible in the context of quantum field theory. Remember that the W-boson is a virtual particle, and decays very quickly due to its large mass.

    I would expect that all neutrons decay in the same way, i.e. that emitted electrons have same speed...I read that this does not happen, how come?
    Can you provide the source where you read that, so we have some context ?

    ... also because the have to escape same attractive force from the newborn proton.
    This was answered by Gere already - it is emitted because there isn't any quantum mechanical state the electron can occupy near the nucleus. Again, you cannot think in terms of classical mechanics here- these process are deeply rooted in the quantum world, so you have to employ its rules.
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    Quote Originally Posted by monalisa View Post
    Quote Originally Posted by Markus Hanke View Post
    No, what happens is that a down quark changes into an up quark by emitting a virtual W(-) boson, which, due its large mass, then decays into an electron+electronantineutrino.
    how can a neutron emit a particle whose mass is 100 times larger than itself?
    In this case the W boson is usualy virtual therefore it does not have to satisfy equations of motion. We say it is off mass shell. The idea of exchanging particles is just convenient for mathematical reasons resulting from perturbative expansion of S matrix. In reality it is just W field mediating the interaction. For example two electrons repulse each other by exchanging photons. But there isn`t any light between them, photons in this case are virtual. Also you have to distinguish between rest mass and mass. Whole mass (energy) of the particle is given not only by rest mass but also by its kinetic energy. The only thing that has to beconserved is basicaly relativistic energy given in natural units by



    where M is rest mass in eVs, P is momentum in eVs and E is energy in eVs.

    Quote Originally Posted by monalisa View Post
    What do you mean by this ?
    I would expect that all neutrons decay in the same way, i.e. that emitted electrons have same speed...I read that this does not happen, how come?
    This is because you have neutrino there therefore additional degree of freedom. Conservation of energy and momentum then does not give you almost any restriction on speed of electron. This is actually how Pauli postulated neutrino, he knew there has to be another particle for conservation laws to hold localy.

    Quote Originally Posted by monalisa View Post
    so what?
    ... also because the have to escape same attractive force from the newborn proton. What is the strength of the electrostatic force at the distance of mx10^-17/18, what KE do you require to escape such a field?
    This interaction is described by Fermi function. Also thanks to uncertaintity the distance isn`t so small. Since you have pretty well defined momentum from conservation laws the probability distribution in coordinate space will be large, larger than say size of proton. In QFT it will almost always be described by plane wave nevertheless. You cannot really assume particles are little balls at such scales.
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    Quote Originally Posted by Markus Hanke View Post
    . Remember that the W-boson is a virtual particle, and decays very quickly due to its large mass.
    .
    Is there any evidence of the existence of W-bosons and quarks, or is it just a speculation?
    I suppose a free quark is not observable, but a supermassive W-boson should be traceable.

    What happens to the emitted electron?, is it re-captured to form a H-atom?
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    Quote Originally Posted by monalisa View Post
    Is there any evidence of the existence of W-bosons and quarks, or is it just a speculation?
    Experimental Observation of the Intermediate Vector Bosons W+, W- and Z0

    Quarks are observed, indirectly, from examining the behaviour of hadrons.
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    A W boson has mass 8000 MeV,
    an electron* neutrino 0.5 MeV what happens to the remaining mass/energy?
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    That goes to kinetic energies.
    Emitted electron is free to do whatever it wants.
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    Quote Originally Posted by Gere View Post
    That goes to kinetic energies.
    Emitted electron is free to do whatever it wants.
    If I am not wrong at the distance of mx10^-15 it takes billions of MeV to escape the electrostatic field.
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