Question

Which of the following particle is stable
A. Neutron
B. Proton
C. Pion
D. Muon

Answer 1

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Protons are the most stable. According to some theories, they should decay once in a while, but none has ever been seen to do so. For all practical purposes, if you leave one alone, it will last forever.

Muons are unstable but last much longer than pions.