Hi
So I'm back again, though I forgot my username and password so I had to make a new account.. that's not the question though. Anyhow! - I started thinking about the nucleus after reading about the island of stability and onwards about the shell model and thinking "what a bunch of bollocks - I could do that better myself! I'm making me my own model of the nucleus"

Problem is, I can't do it better myself (obviously), but it's nice to just keep the brain spinning sometimes. Let me set the stage and tell you what led me to this question:

I was thinking rather naively (like no-one else had thought about it) to simulate the nucleus as a cluster of balls (since I was thinking protons and neutrons are after all rather partically particles), attracting each other through the strong nuclear force and repelling each other through the electric force and just watch how they arranged themselves and at what sort of numbers and combinations of "protons" and "neutrons" the whole thing got unstable. However...

Then I started thinking about what exactly makes a proton and a neutron and how the heck those little guys interact and I suppose this is exactly where I run amock since I know basically nil about it in detail. The quarks in a nucleon pass them little gluons between them as force-carriers. ("rubber-band" force if you will I suppose, not affecting each other a whole lot while near each other but then tuggin the leash if one tries to run away?). This all good and well.. I imagine one could imagine a nucleon almost like a .. vibrating/rotating triangle with a quark in each corner.. I might be stretching it a bit here..

So.. damnit I really wish I had a picture.. ill have to coordinate-systemitise it to you.. U(x,y) marks an up-quark at (x,y) etc.. so:
you have a proton with U(0,0),U(3,2),D(3,-2) and a neutron with D(-1,2),D(-1,-2),U(-4,0). Drawing this on a paper might help visualise it.

How come (or does it??), a new neutron doesnt form spontaneously from D(-1,2),D(-1,-2),U(0,0) ? (also forming a new proton from the rest of the quarkies, if you imagine adding a third dimension to the coordinate-system and allowing the remaining quarks to be closer to each other too) Why doesnt gluons just straight up pass between nucleons like that? (where the hell does them mesons come from and how does the three quarks in the nucleon know that it's those other two that is it's buddies and not that other fine quarks "over there" it should bond with?) - which leads me to my actual question (I think): how come quarks arnt free to roam about the nucleus as long as they stay within, say, a proton diameter of atleast two other suitable quarks? (considering color and all that good stuff) Another way of asking this question is - "does quarks wander between nucleons in a nucleus, forming and re-forming protons and neutrons as they do so"? (I'm thinking "no" because I cant really picture why a nucleus would be more or less stable this way but it would seem as if though it would be just a blob of quark chaos). (which would also jiggle the electric field in the nucleus around and messing with the electrons etc... just a thought.. like some.. intra-van-deer-waals effect)

An additional question I was thinking about is this. can neutrons and protons pass through one another (proton/neutron and neutron/neutron collision) - or more precisely - can they bounce off of one another?! how can a bunch of quarks interact like that and make a bunch of other quarks bounce like so?

I dont doubt for a second that the answer to all these questions lie to find within the QCD.. I dont feel that I have the patience or know-how to learn that in a reasonable time-frame, so I'm trying to condense.. squeeze.. the information out of all you guys.

damn quarks ruined my model of the nucleus We really do have to get rid of all this wave/particle-duality stuff too, it's no fun.

I'll give an explaination of nuclear systems and then if there's anything still you're wondering about it'll be easier to address if it's a matter of not understanding the current model.

Nucleons are made up of quarks, which are held together by gluons. The protons and neutrons are made up of 3 'main' quarks. The up and down quarks in the nucleons have a rest mass of only a few MeV. They make up less than 10% the mass of the nucleons. The rest of made up of binding energy carried by the gluons. With the gluons having so much of the energy it's easy for them to produce more quarks, then then do more interactions with the main quarks. Basically, there's a constantly shifting set of quarks in a proton but at any moment there's the right balance to mean the proton is a colour singlet (the strong force charge is balanced out to zero) and there's 3 primary clusters of electromagnetic charge in the nucleon (ie 3 quark states).

Now gluons also carry colour charge, they are unlike the photon (which doesn't carry electromagnetic charge itself). So gluons self interact but also they aren't allowed to be seen on their own, because colour (ie the strong charge) is 'confined', it always must balance out to zero when it's viewed from afar. So we've got this constantly shifting set of quarks, throwing gluons at one another and the gluons are throwing gluons at gluons which are throwing gluons at.... well you get the idea. But how do protons and neutrons interact if they aren't allowed to throw gluons at one another, because then we'd see gluons on their own?

Well the gluons have so much energy that they can produce a quark-antiquark pair. Such a pair has no colour charge total (they balance out) and so they can travel a bit further than gluons. We call these pairs 'mesons' (compared to baryons, which has 3 quarks which balance out their colour). So a nucleon cannot throw out a gluon but it can throw out a meson. The mesons last just long enough to make it to the next nucleon and those quarks in the meson interact with the gluons in the new nucleon. So nucleons interact by mesons, not by gluons. It's a residual effect.

So when a proton and a proton interact they can do it by passing a meson between them or if they have enough energy their quarks will produce a weak boson (which have 90 times the mass of the proton! It's like a baby giving birth to an elephant!). This then passes to the other proton and interacts with quarks there. The super massive weak boson doesn't last very long so the protons have to get VERY close, which needs all that energy.

QCD explains all of this. Well, it's still a matter of conjecture that QCD explains confinement. It's not a mathematically simple concept. Infact it's one of the Clay Institute millenium prizes. If you can prove what basically boils down to confinement then you would win a million dollars! Calculations within mathematical methods or super computers (several people in my department are UK leaders in this area) give tons of evidence that it's true but showing it's true for lots of cases isn't the same as proving it's true in general.

Does that help? I think it addresses a lot of your misconceptions about nucleus models. What are you unsure about following that?

/edit

Oh and if you don't know how colour works here's an explaination :

EM charge works by + and - some elementary charge. So an electron is - and a positron is +. When an electron emits a photon, which doesn't carry EM charge, it doesn't charge it's EM charge. When it emits a weak boson like a W-, which is charged, it must preserve total EM charge so the electron goes to something EM neutral and the W-, so you get electron -> neutrino + W-.

Quarks and gluons are similar but a bit more complicated. A quark will have EM charge, say +2/3 but also a colour charge. There's 3 charges (called colours) in the strong force, call them a, b and c. a+b+c = neutral and a+(-a) = neutral. So a proton will have 3 quarks with a, b and c colour. A meson will work by having a and -a colours. A gluon carries colour but not EM charge. So when a gluon, say with colour a, emits a gluon it must change 'colour'. So a quark with colour a emits a gluon and changes to colour b. To keep colour total unchanged, the gluon has colour (-+a, so that the quark+gluon has colour b+(-+a = a. It's unchanrged. So suppose you have a meson, where the two quarks have charge c and -c. The first one emits a gluon and changes to b, so the gluon has colour -b+c. This then goes to the other quark and it goes from -c to -c+(-b+c) = -b. So the quarks are now b and -b. Total zero. So quarks are constantly changing colour, provided the total is still zero.

But even worse, the gluons make more quarks. So the first gluon goes from c to b, via a gluon emission with colour -b+c. But on the way, that turns into two gluons, with colour say -b+a and -a+c. Still a total of -b+c, which added to the quarks gives the total of zero. But then those two gluons can turn into more gluons or more quarks.

As you can see, it gets very complicated very quickly. That's what quantum field theory is all about, adding up ALL these quarks and leptons and photons and gluons and weak boson productions, accounting for colour and charge and all the ways they can be produced. It's a staggeringly complicated procedure. Hence why even the simplest computer models need super computers!

Great post - thanks Alpha.
I'll formulate a response, but I also have to get some sleep because I'm leaving for turkey tomorrow and wont be back for three weeks (damnit, I hope they have invented some kind of connection to the series of tubes over there.). So I'll let my dawning confusion simmer for a while

Most of it was known to me and I had a patchy sort of understanding of it all, the small mass-portion of the quarks was quite a surprise though and getting to know about the large mass ratio of the gluons!

Btw, the meson (I suppose there's more than one type too!.. (six?? up/down/strange/charm/bottom/top pairs? or can you have a strange-antibottom meson too?!..making.. uh.. 36.. types of mesons?!)) - anywho - would it be fair to say that they cancel their color out by forming say.. "red" plus "anti-red" making "white" while the baryons have red+green+blue making it "white"?
Also.. does it "matter" whether the meson is red+antired or green+antigreen, or is everything content with it being just plain old "white"?

omg.. does this mean you can have, say a (top , down , charm ) baryon? This is getting out of hand

ah damnit it really is a love-hate thing learning stuff like this. ideas and questions run screaming through my mind, spontaneously forming out of this newfound energy, interacting with each other, shooting off new ideas and questions faster than I can put them in black and white.

(can anti-quarks make baryons? does antiblue + antired + antigreen make white too?!) aargh.. too many permutations!

I have to get some sleep. Thanks for the post, I'll be picking up on this later, for sure.

Have a nice weekend. Cheers!

(btw. you're in the UK, but am I making a correct assumption in that you are not natively from there? Not that it matters I just though I noticed some curious word-sequences. Anyways, again, thanks for the post, it is food for though. g'nite!)

Related interrogation.

All my old arcane texts assert that the strong force is as strong between any nucleon pair (n-p, p-p, n-n).

However, as far as I know, neither a biproton nor a bineutron has been observed, never ever. If such a thing existed, we'd already have seen it, either in a nuclear reactor or in an accelerator. Or at least, we'd have seen its disintegration products.

So could it be that the strong force is important only between a proton and a neutron?

Sorry for interrupting.