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Particle confusion

  1. Jan 6, 2009 #1
    So I have just started learning a bit about particle physics and I am confused about some small details...

    As I understood it fermions and bosons are two different types of particles that have different behaviours. They are not related other than the fact that they are both particles.

    But now I have heard that there is a boson called a pion which holds together neutrons and protons in the nucleus. The pion is made up of an antiquark and a quark but quarks and anti quarks are fermions... right?

    Are all bosons made of quarks (fermions) and if so how can bosons have properties that fermions can not have like existing in the same space which the Pauli exclusion principal states that fermions (quarks) can not do?

    Another thing that is confusing...

    I found out now that a boson is a meson which I know is a type of hadron but I thought hadrons are groups of particles bound by the strong nuclear force and particles that are bound by forces are matter particles (fermions) so how can a boson be a hadron?

    Ok, so maybe I am very confused and I know that this could be kinda difficult to explain over the net but I would appreciate it a lot if someone could try.

  2. jcsd
  3. Jan 6, 2009 #2


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    Gauge bosons carry forces between other particles, photons carry the electromagnetic force, W+Z the weak nuclear and gluons the strong nuclear.
    Composite bosons are different, they are pairs of quarks and so behave more like 'real particles'

    I always found this confusing (I'm not a particle physicist). I don't know why they are put together, wether it's some deeper part of the standard model or just historical accident.
  4. Jan 6, 2009 #3


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    Bosons are particles that have integral spin quantum numbers (0, 1, 2, ...) and do not obey the Pauli exclusion principle.

    Fermions are particles that have half-integral spin quantum numbers (1/2, 3/2, ...) and obey the Pauli exclusion principle.

    This classification is different from hadrons versus leptons, or fundamental versus composite, etc.
  5. Jan 6, 2009 #4


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    jtbells answer is the most general one and was proven by Pauli in the "Spin-statistics theorem"
  6. Jan 6, 2009 #5
    The first part of the explanation was spot on, thanks but...

    Can you explain what you mean by this...

    Which classification are you refering to? And fundamental and composite, haven't heard of those.... are they diffrent types of bosons?
  7. Jan 6, 2009 #6
    And one more thing...

    If this is the 'standard definition'...

    Does this mean that not all fermions are 'matter particles' and not all bosons are 'force particles' and that bosons can make up matter and fermions can make up forces...?
  8. Jan 6, 2009 #7


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    Forget about mgb_phys answer for a moment, it is not the most general one and can therefore be confusing to you right now.

    Jtbell meant that the classification he gave is the most general one.

    But infact, the most general one is that fermions obey Fermi-Dirac statistics, i.e the Pauli principle and that bosons obey Bose-Einstein statistics, i.e the no not obey Pauli principle. Then it was shown by Pauli that this is connected with the intrinsic QM-spin of the particles. Fermions have half integer spin, bosons have integer spin.

    Now let's do some application of this.

    Fundamental particles (no internal structure) like leptons (electrons, muons, tau, neutrinos) are fermions. Fundamental particles like the photon, W and Z are bosons.

    {The forces between the leptons are described in quantum field theory by exchange of VIRTUAL photons, Z and W - hence called gauge BOSONS, but these particles are virtual, they don't obey E^2 = P^2 + M^2}

    Composite particles (have intenral structure of one looks really close) like protons, neutrons are fermions. Pions are bosons. He-3 nuclei is fermion, He-4 nuclei is boson.
  9. Jan 6, 2009 #8


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    By "this classification" I mean bosons versus fermions.
  10. Jan 7, 2009 #9
    Maybe the simple answers are nicer.

    There are a lot of elementary particles. In the standard model the most basic particles that are not made of anything else are of three kinds:

    1)leptons (the electron and its brothers and neutrinos)
    2)quarks (which we have never really seen)
    3)gauge bosons (thingies which fly around and transport the forces)

    Quarks are never seen alone, but the hadrons proton, neutron and their relatives behave as if they are made of quarks.

    Most of these things have spins. If you throw them together they form a new particle with a total spin. If the spin is half integer the particle behaves as a fermion (statistical effect), otherwise as a boson.
  11. Jan 7, 2009 #10
    Thanks for the help, after your comments and some quality time on Wikipedia I think I got everything worked out now....

    I think that it made it difficult for me that, when I first heard about fermions and bosons they were presented as 'matter particles' and 'force particles' and I was focusing too much on that.

    What helped was to understand that mesons, despite being made up of fermions (a quark and a anti-quark) that they are bosons because they have a integer spin value.

    Thanks for the help!!
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