# What's the difference between fundamental particles and composite particles?

## Main Question or Discussion Point

I'm confused, by composite particle we probably mean when we use something to smash it, something new will come up, right? Then what's so different about fundamental particles? For example if we "smash" a electron with a positron, we also get something new--photon.
I guess I am making some conceptual mistake here, what is it?

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blechman
when we say "composite particle" - we mean that the particle is actually a bound state of "smaller particles". Examples of composite particles are Hydrogen (made of 1 proton and 1 electron), or any other atomic nucleus. Also all the hadrons of particle physics are "composite particles" that are made up of quarks and gluons.

"Fundamental" particles are not composed of smaller things (at least that we know of). The quarks, the photon, the electron are all examples of "fundamental" particles.

Of course, it could be that "fundamental" particles are NOT fundamental, and that we just haven't discovered what they are made of yet. Only more experiments can tell us that.

Drakkith
Staff Emeritus
I'm confused, by composite particle we probably mean when we use something to smash it, something new will come up, right? Then what's so different about fundamental particles? For example if we "smash" a electron with a positron, we also get something new--photon.
I guess I am making some conceptual mistake here, what is it?
The photon is not considered to be "matter" in the normal sense. It has no rest mass and can't occupy a volume of space like protons and electrons and such. Instead, photons are created by excess energy in matter. When you smash two composite particles together, the energy is converted into photons that then transfer that energy elsewhere. The photon is considered a fundamental particle however.

When we smash open atoms, we get other particles that do take up space and have rest mass, the electron, proton, and neutron. When we do deep inelastic scattering, we see that the proton and the neutron have three points of deflection, IE three particles that make them up.

I guess I should rephrase my question as, theoretically or experimentally, what's the criterion of determining whether a particle is fundamental or composite?

bapowell
I guess I should rephrase my question as, theoretically or experimentally, what's the criterion of determining whether a particle is fundamental or composite?
Blechman gave you the distinction between fundamental and composite particles. When you collide fundamental particles, like an electron and a positron, it is still possible to generate other particles, however. For example, LEP, which is an electron-positron collider at CERN, frequently creates massive W and Z bosons in these collisions. Surely the electrons and positrons don't contain these much more massive particles as constituents -- i.e. the collision is not breaking the electrons and positrons up into more elementary particles. Instead, what happens is that the electrons and positrons annihilate upon collision. The energy of this annihilation depends on both the rest energies and kinetic energies of the particles. The energy must be carried by something -- some other particle(s). What happens is that new particles are created as a conduit for this energy -- they are not present inside the colliding particles. In this way, one can get e- + e+ --> photons rather easily. One can also get e- + e+ --> Z, but only if the kinetic energies of the e- and e+ are sufficiently large to create the much more massive Z boson.

arivero
Gold Member
Deep inelastic scattering, surely, is an experimental proof of composite. I can not think of others... colliding and breaking into pieces is not an answer (albeit it was though to be an answer via a different way, "dual models", in the late sixties).

Theoretically, I think that a composite theory needs to argue, besides the components, the force that is joining them. Technically it could mean that the symmetry observed in the particle must be upgraded to a local gauge symmetry, but again I am not sure if this is the only argument.