The material objects which surround us during our everyday life are made
of protons, neutrons, and electrons. 5 Protons and neutrons are both examples
of baryons, where a baryon is defined as a particle made of three quarks. A
proton (p) contains two “up” quarks, each with an electrical charge of +2/3,
and a “down” quark, with charge − 1/3. A neutron (n) contains one “up”
quark and two “down” quarks. Thus a proton has a net positive charge
of +1, while a neutron is electrically neutral. Protons and neutrons also
differ in their mass – or equivalently, in their rest energies. The proton mass
is m p c 2 = 938.3 MeV, while the neutron mass is m n c 2 = 939.6 MeV, about
0.1% greater. Free neutrons are unstable, decaying into protons with a decay
time of τ n = 940 s, about a quarter of an hour. By contrast, experiments
have put a lower limit on the decay time of the proton which is very much
greater than the Hubble time. Neutrons can be preserved against decay by
binding them into an atomic nucleus with one or more protons.