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The radius of a proton is a more meaningful quantity than the radius of an electron, because as far as we know the electron is pointlike (no internal degrees of freedom) but a quark is a composite particle containing three quarks and so in principle one could assign it a radius just like the hydrogen atom - a composite particle of a proton and an electron can be assigned a radius by studying the electron wavefunction for its ground state.

Then again, the "radius" of a wavefunction, be it the electron orbital in an atom or the wf of a quark in the proton, is only meaningful as an order of magnitude anyway.

We have met the enemy, and he is us — Pogo

by Carrie (migeru at eurotrib dot com) on Fri Feb 22nd, 2008 at 03:22:25 PM EST
[ Parent ]
You are a theory guy, right? ;-)
10^(-18)m is experimentally tested, point is what the theory says.

But more important you have a typo
"but a quark is a composite particle containing three quarks", should be proton is a composite particle...

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers

by Martin (weiser.mensch(at)googlemail.com) on Fri Feb 22nd, 2008 at 04:02:56 PM EST
[ Parent ]
In a previous life I was a theory guy, yes, though I took my graduate Standard Model courses from a pretty good phenomenologist.

I suppose I could find it in the particle data booklet, but can you outline the experiment that tests the electron radius?

We have met the enemy, and he is us — Pogo

by Carrie (migeru at eurotrib dot com) on Fri Feb 22nd, 2008 at 04:51:20 PM EST
[ Parent ]
Well, how do we know that the proton has a fm? When hitting it with a charged particle with a Compton wavelength in the order of 1 fm in a scatter experiment, we start to see a formfactor, e.g. the particle doesn't see the full charge any more.
LEP II had about 100 GeV/c^2 per electron, therefore a Compton wavelength of about 10^(-17)m, and there was not the slightest sign of substructure, we still see absolutely the full structure and one would likely expect already small signs of a formfactor with a somewhat bigger wavelength than the electron size.

I don't know what measurement exactly provides the best measurement, could be a precision measurement of the B sector or whatever, but that's it pretty much.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers

by Martin (weiser.mensch(at)googlemail.com) on Fri Feb 22nd, 2008 at 05:17:09 PM EST
[ Parent ]
There's more than one LEP? that's scary ;-)

Any idiot can face a crisis - it's day to day living that wears you out.
by ceebs (ceebs (at) eurotrib (dot) com) on Fri Feb 22nd, 2008 at 05:32:24 PM EST
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