Neutron= proton + electron?

  1. Chi Meson

    Chi Meson 1,772
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    Wow. Clean slate! I like the format! And I've got a question.

    In elementary physics, it is said that the neutron can "sorta" be thought of as a proton plus an electron together. The mass of the neutron is slightly higher than a proton, by approximately the mass of an electron; in beta decay, a neutron decays into a proton, electron and neutrino; the neutron is neutral, since the two charges cancel.

    My question is: I know it is incorrect to say that a neutron is a "Proton plus Electron and Neutrino," but why is it wrong to say this? I'm familiar with the HUP explanation that since the mass of an electron is so small, then it's position is too vague to be contained in the tiny neucleus, but is there some other reason?
     
  2. jcsd
  3. For a start, because neutron decay produces a proton, and electron and an electron ANTI-neutrino?
     
  4. mathman

    mathman 6,621
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    For starters: A proton consists of 2 up quarks, 1 down quark, and gluons holding it together. A neutron consists of 1 up quark, 2 down quarks, and gluons.

    The decay process in simple terms occurs because a down quark changes into an up quark and emits an electron and an anti-neutrino. The important thing to note is that the electron and anti-neutrino weren't there before. They were created at the time this (weak force) reaction took place.
     
  5. chroot

    chroot 10,426
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    It it correct to say:

    Proposition 1: The neutron is composed of two up and one down quark.

    but NOT correct to say:

    Proposition 2: The neutron is composed of a proton, electron, and anti-electron-neutrino.

    Why is this incorrect? It's simple, really. Let's say you put the neutrons in a particle accelerator and bounce electrons off of them. You will see scattering events. If Proposition 1 is correct, you will see by analyzing the scattering events that there are three different scattering centers inside the neutron -- each is one of the quarks. If Proposition 2 is correct, you would instead see only two scatterting centers, one of which is much, much "stronger" than the other, corresponding to the internal electron and proton. You would probably not get enough scattering off of the neutrino to be measurable.

    Experiment confirms Proposition 1, and disproves Proposition 2. The neutron is really composed of three quarks.

    The electron and anti-electron-neutrino appear when one of the down quarks decays via the weak force into an up quark, and a W- boson. The W- boson then decays into an electron and an anti-electron neutrino, conserving charge and lepton number.

    - Warren
     
  6. Chi Meson

    Chi Meson 1,772
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    yes, okay, for someone as picky as me, I should have caught that.

    Thanks to Chroot & Mathman. I was searching my old texbooks, but I could not find a direct answer. Now, a follow up question:

    If I were explaining this to advanced high school students, would it be a bad analogy if I were to say that this explanation is similar to: "It is incorrect to say that the uranium atom is composed of a barium plus a krypton atom and a few neutrons" ?

    In both situations (beta decay and uranium fission) there is a conservation of a set of quantum numbers, and in both situations there are smaller components that are "shuffled" to create significantly different larger particles. Should I just avoid this analogy?
     
    Last edited: Sep 18, 2003
  7. selfAdjoint

    selfAdjoint 8,147
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    Well about "It is incorrect to say that the uranium atom is composed of a barium plus a krypton atom and a few neutrons" it is! Because you could think of many other ways to compose it = uranium is made up of 92 Hydrogen atoms, plus some neutrons. If you mean that Barium and Krypton were found in the first (recognized) fission experiments, be aware that there are many ways fission can happen.
     
  8. Chi Meson

    Chi Meson 1,772
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    I was about to elaborate further, but I'm seeing now that this analogy is gettingout of hand. I'm leaning to the explanation that the composition of fundamental particles as an arrangement of quarks is NOT analogous to the compsition of nuclei as an arrangement of protons and neutrons.

    Would you agree?
     
  9. chroot

    chroot 10,426
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    In atomic fission, no particles are changing identities. There is no decay or transmutation. A big nucleus splits up into some smaller ones. While I don't particularly care for the explanation that uranium = barium + krypton + neutrons, it's really no more incorrect than a dozen eggs = four eggs + eight eggs.

    However, it is entirely incorrect to talk about a neutron = proton + electron + anti-electron-neutrino, since these particles only appear after a down quark decays.

    - Warren
     
  10. what is the charge of an anti-electron-neutrino,anti-muon-neutrino,and the anti-tau-neutrino?
     
  11. chroot

    chroot 10,426
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    The neutrinos and and their antiparticles all have no electrical charge.

    - Warren
     
  12. Chi Meson

    Chi Meson 1,772
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    Thank you chroot. That clears it up.
     
  13. what's the difference between the neutrinos and their anti particles.
     
  14. chroot

    chroot 10,426
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    Their interaction with the weak force.

    - Warren
     
  15. Okay,thank you very much.
     
  16. jcsd

    jcsd 2,226
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    That's if neutrinos aren't their own antiparticles which is still a matter of conteion among particle physicists.
     
  17. hey mathman, you said that the elctron and the electron-antineutrino were created when the weak force decay took place. How can matter be actually created? Mustn't there be some sort of energy being converted in the particles?
     
  18. chroot

    chroot 10,426
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    The neutron's rest mass is equal to the sum of the kinetic energy and rest masses of the decay products.

    - Warren
     
  19. no I mean where does the electron come from?

    can't have been released from the neutron because the neutron is composed of 2 down quarks and 1 up quark. therefore it is not from the neutron.
     
  20. Nereid

    Nereid 4,014
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    The electron is a lepton, and the anti-neutrino is an anti-lepton, so lepton number is conserved. Baryon number is also conserved. Energy is conserved. Since no conservation laws are broken, the neutron must decay (ergodic principle; what is not forbidden is compulsory).

    A down quark is transformed into an up quark, via the weak interaction (force carrier is the W- particle).

    So, yes, it did 'come from' the neutron; but it didn't exist until the decay happened!
     
  21. chroot

    chroot 10,426
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    Did you not read this thread? The beta decay is a result of a down quark decaying into an up quark.

    - Warren
     
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