I know that a proton is composed of 2 “up” quarks and 1 “down” quark. But just a couple of days ago, I learned that there was another baryon with an exceedingly short lifetime called a “delta-plus resonance”, and that it too is composed of 2 up quarks and 1 down quark. This “resonance” particle also has a spin of 3/2, whereas a proton has a spin of 1/2.
What’s up with that? Does the proton have one of its 3 quarks spinning in the opposite direction of the other two, while the delta+ has all 3 of its quarks spinning in the same direction? Is that the only difference between these 2 hadrons, or is there something more? How many particles are there out there that’re composed of an up-up-down quark combination?
There are many different delta baryons. [DELTA][sup]+[/sup] is made of the same quarks as the proton (uud) and the [DELTA][sup]0[/sup] is made of the same quarks as the neutron (udd). There are other delta baryons as well, all with isospin 3/2. If I’m reading my old particle physics textbook right, the delta baryons exist because the quarks are in some sort of excited states, whereas in the proton and neutron, the quarks are in the lowest energy state. Don’t ask me to explain that, because I don’t really understand.
Having all the quark-isospins pointing in the same direction would, I think, qualify as an “excited state” when compared with having two quarks spinning one way and a third quark spinning the other. But I’m guessing in the dark here. Is there some website I could look at that has a list of all the known baryons and/or mesons, along with their spins, charges, masses, quark compositions, etc.? All I can find when I do a web search on “particle zoo” is lists of the 6 quarks and a few “sample” hadrons like the lambda and omega.
I think, but am not sure, that the difference between the Ns and Deltas (or baryon octet/baryon decuplet particles in general) is that the quarks in the Ns are in the singlet spin state whereas the Deltas are in the triplet. The difference between the N and Delta families with different masses (e.g. Delta(1232), Delta(1600) etc.) is, I believe, due to the orbital angular momentum of the quarks.
Anyway, the definitive and all-encompassing particle physics reference is http://pdg.lbl.gov . Here are summaries for baryons, mesons, etc.
You can also get a free copy of the Particle Physics Booklet which contains all of the above and a lot more here.