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My secondary school physics textbook has mentioned that protons and neutrons are made up of down and up quarks in different amounts. It has also mentioned that other quarks exist. It states that particles from these quarks are unstable. It also lists the charges of these quarks. It doesn't provide much more detail beyond this. My question is this:

Why is (for example) a particle made up of ccs (Wikipedia tells me this is called a Double charmed Omega baryon) any less stable than a proton which has the same charge?

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up vote 10 down vote accepted

Generally speaking, the lowest mass particles are stable against decay because they can't spontaneously decay into more massive particles (energy wouldn't be conserved in the rest frame). So particles not made up of up and down quarks (least massive of the quarks; see figure) will quickly decay into particles made up of only up and down quarks.

Look at those quark masses... the up and down quarks are dwarfed by the others, hence they're more stable

This leaves the neutron (udd) and proton (uud) as the only stable particles of which the neutron is more massive and can hence decay into a proton (by conversion of a more massive down quark to an up quark through the weak interaction) - this is called Beta decay and is an important and well established radioactive process. There is nothing that the proton can spontaneously decay into while conserving energy in the Standard Model.

Beyond the Standard Model, most Grand Unified Theories that try to unify the three non-gravitational forces require proton decay since quarks and leptons can now change into each other. This means that the proton will have a finite (albeit very large) lifetime. Some GUTs, such as SU(5), have already been ruled out because the proton lifetimes they predict are smaller than the observed lifetime.

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What about a combination of 3 up quarks? –  yatima2975 May 24 '11 at 13:33
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Good point. That's a $\Delta^{++}$ baryon and it's more massive than the allowed end state of a proton + pion, which it quickly decays into. The mass of a composite particle is not simply the sum of the mass of its constituents -- most of the mass actually comes from the mass-energy of the strong interaction between the quarks. –  dbrane May 24 '11 at 13:46
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