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The constituents of a neutron?

  1. Dec 20, 2005 #1
    I'm new to this forum.
    I was hoping someone might be able to help me understand what the constituents of a neutron are.
    Last month I read a book that stated that the neutron was composed of 2 'd quarks' and 1 'u quark'... while the proton is composed of 2 'u quarks' and 1 'd quark'. The 'd quarks' are slightly more massive than the 'u quark'.... giving the neutron the .138 extra mass (compared to the proton).
    The book I'm currently reading states that the neutron is composed of (1proton + 1electron + 1neutrino).... and that this accounts for the larger mass of the neutron compared to the proton.

    Are these 2 descriptions basically saying the same thing, but from different perspectives? Or is one description right while the other is wrong?

    Thanks for any help.
  2. jcsd
  3. Dec 20, 2005 #2

    Meir Achuz

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    The first explanation is essentially correct. The neutron's being heavier is more complicated than just having a heavier d quark. The book you are "currently reading" is completely wrong. Throw it out before reading any more. A neutron does decay into a proton, an electron, and an antineutrino, but IT IS NOT COMPOSED of them. You throw out garbage, but you are not composed of garbage.
  4. Dec 20, 2005 #3
    Thanks alot for the help.

    So the 1st was right: the neutron is composed of 2 'down quarks' and 1 'up quark'.
    But, the 2nd is wrong when it states that the neutron is composed of (1proton + 1 electron + 1 neutrino).... it's just that when the neutron decays, it breaks down to the '1 proton + 1 electron + 1 antineutrino' (was he also wrong about stating neutrino as opposed to an antineutrino?).

    Thanks again.
  5. Dec 21, 2005 #4
    Welcome Jorge !
    The neutron and the proton are very similar. They are dubbed "nucleon" as collective name. They are bound states of quarks : "uud" for the proton and "udd" for the neutron. The "u" quark carries a +2/3 electric charge (in units where electron has -1 charge), and the "d" has -1/3. You can just add up the electric charge, and recover that the proton has +1 charge, whereas the neutron is... neutral :smile:
    So the nucleon is a bound state of quarks. What ties the quarks together is the "strong force". It is indeed very strong : forces of the order of tons at the scale of [tex]10^{-15}[/tex] m... Those kind of forces are usually develloped by trucks, not elementary particles :surprised :biggrin:
    It so happens that the neutron is heavier than the proton. This indeed allows one of the d-quark to "transform" into a u-quark. This transformation is operated thanks to the "weak force" which is responsible for radioactivity. The d-quark itself does not emit a pair eletron-antineutrino in the process of transformation. Rather, it emits a so-called [tex]W^-[/tex] boson. This last guy is very heavy, so it exists only during a very short time, and very rapidely decays into the electron and antineutrino.
    If your book said that "the neutron is composed of" the final particles emited during the process, then it is very wrong indeed (as you saw, there is an additional boson in between anyway...) However, it was right if it emphasised the fact that an antineutrino is emited. This is in order to respect "lepton" number conservation. As opposed to quarks, leptons are the constituents of matter which do not feel the strong force. They only feel the gravity, the eletromagnetic and the weak forces. An electron has "leptonic charge" +1, whereas an antineutrino has leptonic charge -1. Since in the process of neutron decay, there is no lepton in the initial state (which is simply... one neutron sitting here and nothing else !) then the final state must have zero total leptonic charge. (Our final state is composed of 3 particles flying away : the proton, the electron and the antineutrino).
    You can take a look at hyperphysics.
  6. Dec 21, 2005 #5
    Thank you very much for your response!
    It was incredibly useful in helping me better understand the process.
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