Well, there are some rules that these particles have to play by. One is
the conservation of electric charge. A down quark has a charge of -1/3
(in units of a proton's charge), and an up quark has a charge of +2/3.
Two up quarks and a down quark give you a total charge of +1 (a proton)
and two down quarks and an up quark give you a total charge of zero (a
In neutron decay, the end products are a proton, an electron, and
an electron antineutrino. To keep the charge adding up the same before
and after the decay, the neutron has to emit something negatively
charged. Seen on the quark level, one of the down quarks emits the
electron and the antineutrino in order to turn into an up quark.
The antineutrino is there to conserve the total number of
electrons in the interaction. It has an "electron-number" of -1 while
the electron has electron-number +1.
The whole interaction is mediated by exchanging a W- boson
(charge: -1). The down quark emits the W- boson and turns into the up
quark, and the W- boson immediately produces the electron and the
antineutrino. W bosons, the carriers of the weak nuclear force, have
masses of 80.5 GeV or so, about 80 times that of a proton. So they
cannot lead honest lives, but must hide under Heisenberg's uncertainty
principle for energy and time (they can only exist for a very short
amount of time, do their interaction, and disappear).
Heavier versions of the down quark also decay in this way, but
they can produce heavier versions of electrons, as well as the
electrons themselves. The heavier down-type quarks are the strange and
bottom quarks, and the heavier cousins of the electron are the muon and
tau. Neutrons can only emit electrons and neutrinos because the amount
of available energy (given by the mass differnece between the proton
and the neutron) is not enough to produce any of the heavier particles.
The W boson was postulated in the 1960's as a way to explain
neuclear beta decay in a way that makes theoretical sense, and it was
discovered at CERN in 1982 in high-energy proton-antiproton collisions.
(republished on 07/20/06)