extra down quark at the expense of an up quark relative to the
proton, and the greater mass of this down quark gives the total in
forming the neutron a greater mass than the corresponding trio for
a proton; (ii) the electrostatic forces among two ups and a down (as
in a proton) will differ from those between two downs and an up (as
in a neutron). These also contribute to the total energy at the MeV
scale. So the mass difference between a neutron and proton
(experimentally 1.3 MeV) is due to the electrostatic forces between
their constituent quarks and the greater intrinsic mass of a down
quark relative to the up quark.
Up and down are siblings in the quark family. The electron is not
made of quarks, and as far as we know is itself fundamental, like the
quarks. As such it belongs to a different family, known as leptons.
As up and down quarks are paired, with a difference of one unit
between their respective electric charges (in the sense that +2/3 −
(−1/3) = 1), so does the electron have a sibling whose electric charge
differs from the electron’s by one unit. This entity, with no electric
charge, is known as the neutrino.
Neutrinos are produced in radioactive decays of many atomic
nuclei. In these processes they appear along with their sibling, the
electron. For example, so long as it is not trapped in a nucleus, a
neutron turns into a proton by emitting an electron and neutrino in
the process. This is called beta decay, where the instability of the
neutron is due to it having a slightly greater mass than does a
proton. Nature seeks the state of lowest energy, which translates in
this case to the state of lowest mass. It is the small excess mass of a
neutron that makes it (slightly) unstable when left in isolation. If
you had a large sample of neutrons, each of them free of the others,
then after about ten minutes, half will have decayed by beta
radioactivity. If we denote the neutron and proton by the symbols
n, p, and the electron and neutrino by e
−
, ν, then beta decay of the
neutron is summarized by the expression
n → p + e
−
+ ν
40
Particle Physics