The strong force acts between quarks, and is mediated by gluons (collectively, "glue"). Neutrons are composed of quarks (two down and an up). But so are protons. In fact, as far as the strong nuclear force is concerned there's very little difference between a neutron and a proton (this similarity is called isospin symmetry).

The interesting thing here is the implication that the the two negatively charged particles (the down quarks) are in orbitals above and below the up (positively-charged) quark. Maybe. I haven't read the paper.

You beat me too it. :) Quarks are -1/3 charge particles:down, strange and bottom and +2/3 charge particles:up, charm and top. The neutron consists of ddu = 2x(-1/3) + (+2/3) = a net charge of zero. As you rightly said the gluon, a gauge boson is responsible for the strong force. The deeper one looks the more colourful this theoretical world becomes.

It is a fascinating subject even though the understanding of the maths behind QCD field equations is way beyond the capabilities of my poor skull :)

From Adam: "Neutrons are composed of quarks (two down and an up). But so are protons." I assume by you mean that protons are composed of quarks like neurons, not that they are composed of two down and an up quark like neutrons.

My grasp of quarks is tenuous at best, so I hope that is correct.

But at least it goes some way to explaining why - in the classic 'for idiots' model of the atom - neutrons and protons huddle up together.

The only other explanation I could find was that they were scared of the surrounding electrons.

Protons consist of two up and one down quark:uud 2x(+2/3)+(-1/3) which gives a net charge of +1. The exchange of gluons between these quarks is what is responsible for the strong force. The strong force acts over very small distances around the diameter of an atomic nucleus. Contrary to common sense the strong force increases with distance.

No quark or gluon has ever been detected by itself. Although current theory suggests that gluons can exist without quarks in what is know as a glueball, although this has yet to be observed. Recently a quark-gluon plasma was observed at the RHIC(relativistic heavy ion collider) at Brookhaven National Labs.

The electromagnetic force (i.e. 'charge') is independant of the strong force, at less than GUT energies. So why even bring the strong force into it?

What I really wanted to know was whether this conflicts with the Standard Model.

At nuclear energies it is pions which typically transmit the strong force, not neutrons. As previous posters have also pointed out at the fundamental scale it is really the gluons which are responsible.

The charge distribution within a neutron is interesting partly because it is expected to be almost perfectly spherically symmetrical (though not quite perfectly due to the difference between matter and anti-matter, called CP violation). Evidence of a larger than expected non-spherical distribution would be indication of new physics.

"I like to say 'Quark'! Quark Quark Quark."

Lust for gold? Power? Or are they just born with a heart full of neutrality?

I thought that was the midichlorians. I don't ever recall Anakin's neutron count being taken...

I think I'm going to give up scientific rationality and go for creationism or hobbits and elves - at least I can stop thinking that I am missing something important.

Let me rephrase that - ...at least I can stop thinking.

Think about it this way: When previous model of neutron was released in '47, there was bound to be a some boffin going: "This is important, as we didn't know about this fact of nature before, now we know!".

Pack of popcorn and a soda and I am ready to be entertained by people using word 'Quark' in conversation not related with food.

Surely the total charge over the surface of a neutron - if it has such a thing - has to be neutral. Or am I missing something?