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What is an elementary particle?

  1. Jan 11, 2016 #1
    When my eighth grade daughter was studying physics in her class, I watched a movie with her on the Hadron Collider (" Particle Fever "). We discussed atoms, electrons, protons, neutrons; we discussed orbits and statistics, and then down into the standard model and fermions, leptons, yadda yadda yadda.

    Anyway, on her test, she was asked to name "elementary" particles.

    So she wrote something like this: "Most people think electrons, protons and neutrons are elementary, but now we know there are even smaller particles like the Higgs Boson out of which these other particles are made."

    Anyway, the teacher marked her zero. And when she tried to explain, the teacher insisted that my daughter did not understand the word "elementary."

    So I come to you to ask someone to DEFINE the word ELEMENTARY in elementary particle, so that I can figure out where I mis-educated my daughter. Does that word - in the K12 lexicon - have a specific meaning at the atom-level and not below?

    The points on her exam are not an issue -- we let that go. But I need to fix this if I am wrong.
  2. jcsd
  3. Jan 11, 2016 #2
    The electron is an elementary particle. Protons and neutrons have structure, those contain quarks (and gluons).

    The Higgs boson is also an elementary particle.
  4. Jan 11, 2016 #3


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  5. Jan 11, 2016 #4


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    It sounds to me like your daughter knows what the word elementary means, but she doesn't know what elementary particles we know of and how they're all related. Plus she didn't answer the question asked (unless it wasn't just to name some elementary particles).
  6. Jan 11, 2016 #5


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    Elementary particles are particles with no structure. Basically that means that it scatters like a point particle.
  7. Jan 11, 2016 #6


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    What answer to that question is expected in eight grade? Do they learn about quarks, neutrino's, bosons, etc in eight grade?
    If she had named electron, neutron and proton as elementary particles, would that have been an acceptable answer in eight grade?
  8. Jan 12, 2016 #7
    Rigorous definition of elementary particle:
    "An irreducible representation of the Poincaré group in a Hilbert space".
    Probably we have to add: "with a definite mass, charge and spin". But I'm not an expert on this, I ave learned it by heart :smile:

  9. Jan 12, 2016 #8


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    While the Higgs boson is an elementary particle, it is not a part of electrons, protons or neutrons.
    And electrons are elementary*, as mentioned before.

    *at least there are no indications of any substructure, at a precision of Anomalous_magnetic_dipole_moment#Electron [Broken].
    Last edited by a moderator: May 7, 2017
  10. Jan 12, 2016 #9


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    I think it's reasonable to teach eight-graders that protons and neutrons are not elementary particles because they're made up of quarks, which are elementary. "Can't be further decomposed, as far as we know" is a perfectly sensible definition of "elementary" at that level.

    On the other hand, I'd want to hear the teacher's side of the story before I passed judgement on this particular episode. If the student had responded to "name 'elementary' particles" with a list that included "quark" and "electron" but not "neutron" and "proton", then it might be an easier call.
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