The Standard Model

The Standard Model of particle physics is the current theory of the fundamental particles and forces that make up the Universe.

Physicists used to believe that atoms were fundamental particles - that is, they could not be broken down into smaller pieces. However, at the beginning of the 20th century men such as J.J. Thomson, Ernest Rutherford and James Chadwick showed this was not the case. Atoms are in fact made up of electrons, protons and neutrons. These particles were thought to be fundamental at the time. However, we now know protons and neutrons are made up of even smaller particles called quarks. A proton consists of 2 'up' quarks and 1 'down' quark, and a neutron consists of 1 up quark and 2 down quarks. Quarks are currently believed to be fundamental. The electron is also believed to be fundamental. It falls into another category of fundamental particles - the leptons.

While just two types of quark and one type of lepton is enough to account for all everyday matter, there are in fact 6 types of quark and 6 types of lepton, each of which has an associated antiparticle. The 6 quark 'flavours' (as they are called) are grouped into three so-called 'generations':

First generation Second generation Third generation Electric charge
up (u) charm (c) top (t) +2/3e
down (d) strange (s) bottom (b) -1/3e

Lone quarks are never observed. Instead, they always exist in combinations called hadrons. There are two types of hadron - baryons (which are combination of three quarks) and mesons (which are combinations of a quark and an antiquark). The only hadrons which exist in everyday matter are protons and neutrons (which are baryons). All other hadrons are extremely short-lived. To date, hundreds have been discovered. The first hadron (other than the proton or neutron) to be discovered was a type of meson called the pion, which was found by Cecil Powell and others during investigations of cosmic rays in the late 1940s. Most hadrons, however, were discovered in various particle accelerators around the world.

The table below gives a few examples of hadrons:

Quarks Electric charge Name Symbol
uud +e proton p
udd 0 neutron n
or 0 pi zero
+e pi plus
-e pi minus
0 kay zero
0 kay zero bar
+e kay plus
-e kay minus
0 bee zero
0 bee zero bar
0 bee sub ess
0 bee sub ess bar
(Bars over symbols denote antiparticles.)

Leptons, unlike quarks, can exist 'on their own'. There are also three generations of lepton, each consisting of a charged lepton and a neutrino:

First generation Second generation Third generation Electric charge
electron (e-) muon (-) tau (-) -e
electron neutrino () muon neutrino () tau neutrino () 0

The muon and the tau (discovered in 1936 and 1975, respectively) are essentially like massive electrons (the muon is around 200 times more massive, and the tau around 3500 more massive than the electron). Both are unstable and decay to lighter particles, whereas the electron is stable.

These particles interact via four fundamental forces. In increasing order of strength they are the gravitational, weak, electromagnetic and strong forces. The strong force acts between quarks - it holds them together in baryons and mesons, and also holds together atomic nuclei (which would otherwise fly apart due to the electric repulsion of the protons). The electromagnetic force acts between charged particles. It is the force that keeps electrons bound to atoms. The weak force can act between quarks and leptons. It has the important property that it can change quark flavour. As a result, it is the force responsible for (among other things) -decay in atoms, in which a neutron is converted to a proton with the emission of an electron and an antineutrino. Essentially, the weak force converts one of the down quarks in the neutron to an up quark.

Physicists are currently working on theories that unify all these forces, but that is another story. Now we have this brief outline of the Standard Model, we are ready to start looking at parity, CP violation, and ultimately the BaBar experiment itself.


Last modified Tue 11 April 2000 . View page history
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