Aren't they basically made up of the same stuffs (quarks) in almost the same configuration?
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9$\begingroup$ Short answer: the proton is the lightest baryon. Because of conservation of energy and baryon number, there's nothing it can decay to. $\endgroup$– JavierCommented May 21, 2014 at 1:09
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1$\begingroup$ Related: Why is the (free) neutron lifetime so long? $\endgroup$– John RennieCommented May 21, 2014 at 8:59
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1$\begingroup$ It's not "why," it's "how." First we observe the instability (or, depending on the particle in question, predict it from previous laws and equations derived from other observations) and then we match our models to the observations. $\endgroup$– Carl WitthoftCommented May 21, 2014 at 11:18
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$\begingroup$ Another reason might be the charge conservation and the matter-anti-matter asymmetry (conjecturing here). Neutron do not have to preserve charge thus have less conditions to withhold. There are more things in reality than your philosophy discusses Horatius :) $\endgroup$– Nikos M.Commented May 22, 2014 at 23:31
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1 Answer
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Because quarks come in 3 colors, there is no decay path from a proton to any number of particles whose sum mass is less than that of a proton.
There exist no particles that consist of 2 or 1 quarks (pions consist of a quark and an anti-quark). This leaves only the Δ++ as a reasonable choice, but that works out to be heavier due to the Pauli exclusion principle. It decays into the proton rather than the other way around.
There is hypothetical proton decay in the GUT theories; however this has never been observed. (Something happened a few years ago I can't trace and all the old reports giving proton decay time < bismuth decay time are wrong, and the report of a detector pile big enough to see one proton decay per year is preposterous as 10^8 tons monitored sounds preposterous.)
