Page 312 - Radiochemistry and nuclear chemistry

Cosmic Radiation and Elementary Particles 295

The spin of a charged particle leads to the formation of a magnetic moment directed along
the axis of rotation. It was discovered in the late 1930s that the magnetic moment of the
proton spin (Mp = 1.41 x 10-26 j T- 1) is about 1/700 of the electron spin (M e =
9.27 x 10 -24 JT-1), although theory predicts a ratio of 1/1836 (= MplMe; see w Also,
the neutron has a negative magnetic moment (M n = -0.97x 10 -26 J T-l). The only
explanation scientists could offer for this deviation was that the proton is not an evenly
charged rotating sphere, but contains some "internal electrical currents', and also the
neutron must contain some internal charges, which balance each other to appear uncharged.
Thus, it was doubtful that protons and neutrons were truly elementary.
Around 1960 Hofstadter and co-workers at the large Stanford Linear Accelerator Center
(SLAC, Ch. 13) proved that both the proton and the neutron have an uneven internal
nuclear charge density. This came from studies of the scattering of high energy electrons
(-~ 1 GeV) against protons and neutrons. It was suggested by Gell-Mann that this could
mean that the proton and neutron were composed of smaller particles with fractional charge
and mass which he called quarks. The intense search for such particles (leading to the
discovery of many new "elementary" particles) culminated in the late 1970s in experiments
in which still higher electron energies (4 - 21 GeV) were used and the energy and scattering
angle of the electrons measured. These revealed that the nucleons had a hard internal
scattering center with charges 1/3 that of the electron and masses 1/3 that of the nucleon.
These particles, quarks, are held together by gluons, which are carriers of the nuclear
force.
These results have led to the Standard Model of the building blocks of matter. According
to this model all matter on Earth - and likely in the Universe - (and including our own
bodies) consists of > 99% of quarks with associated gluons. The rest is electrons.
Elementary particles come in only two kinds: quarks and leptons. There are only six
quarks and six leptons, see Table 10.2. The leptons are the electron, e, the muon, #, and
the tauon (tau particle), 7", and their respective neutrinos. The quarks and leptons are
grouped together in three families (or generations) of two quarks and two leptons each.
This makes 12 elementary building blocks, or 24 if one counts their anti particles; Table
10.2 only refers to our matter (i.e. koino matter). The leptons and quarks all have different
properties and names, sometimes also referred to as colors. The physical theory relating
these particles to each other is therefore named Quantum Chromo Dynamics (QCD).
All matter in nature belongs to the first family, which consist of two leptons, the electron
and electron-neutrino, and the up-quark and the down-quark. The proton is made up of 2
up- and 1 down-quark, giving it a charge of + 1 and mass 1, while the neutron is made up
of 1 up- and 2 down-quarks giving it a charge of 0 and mass of 1"
n = u+2/3 + d-1/3 + d-1/3

p=u +2/3 + U +2/3 + d- 1/3