That's pretty much right, although the counting isn't exactly
consistent, and there are some things we do not yet know for sure.
One is the graviton. We do not yet have a satisfactory quantum
theory of gravitation. If gravitons exist, they should have spin 2.
The other is supersymmetric counterparts. There is no direct
experimental evidence for supersymmetry, although it does make the
theories tidier in many ways. I wouldn't call these Standard Model
particles either. We usually refer to the "MSSM" -- the Minimal
Supersymmetric Standard Model as the supersymmetric extension of the
Standard Model with the minimal number of new particles. It still gets
classified as speculation at this point.
You mention 8 gluons, but these differ only by their color
charges. You could then multiply your quark count by three to account
for the fact that they can have different color charges, too. We often
don't do that when listing particle content. Supersymmetric partners of
quarks get the same color charges and interact with gluons too, so the
same factor of three goes there too. It's important to keep around when
computing production and decay rates.
So that's it for the Standard Model. When doubling this for the
MSSM, you have to be very careful counting states, because the
different spin states of the particles count as different particles! A
left-handed electron has different interactions from a right-handed one
(it interacts with W's while the right-handed one does not). There are
therefore two supersymmetric partners of the electron, with possibly
different masses. These two superpartners each has spin zero, so the
number of states all adds up.
The photon, while it has a spin of 1, has only two spin states
(polarizations) allowd -- most spin-1 objects can be in three states,
+1, 0, and -1, but the photon is massless and is missing the 0-spin
state. The W and Z also have spin one, but they have all three spin
states allowed. The gluon and the graviton are also missing the
In addition, in the MSSM, there are five (!) Higgs bosons, three neutral ones, h, H, and A, and two charged ones, H+ and H-.
Here's the boson counting for superpartners:
Neutral Standard Model bosons (and extra Higgs for the MSSM)
photon (2 spin states)
Higgs (3 particles, each with no spin)
Z(3 spin states)
Their superpartners have the same quantum numbers and can mix.
There are therefore four spin-1/2 "neutralinos" in the MSSM (each has
two states, for a total of eight states, to match the eight total
states above). Each of the four neutralinos can have a different mass.
The gluino (superpartner of the gluon) has spin 1/2, with two
states to match the 2 polarization states of the gluon. These all have
the 8 color states. This one doesn't mix with the others because it has
The gravitino (superpartner of the graviton) has spin 3/2, and so
has four allowed spin states, to match the four spin states of the
graviton. The gravitino doesn't mix either; it has different
interactions from the neutralinos.
The charged bosons are the
W+ (3 spin states)
H+ (1 particle with no spin)
and there are two spin-1/2 "charginos" (mixtures of the
superparters of these four states). The negatively-charged bosons
correspond to negatively-charged charginos.
That just about does it!
For some purposes, what matters is the number of different
adjustable parameters in the model, not the number of different
particles. As you can see the particle count depends a little on
definitions. The general lore is that the current Standard Model has
about 20 different parameters which don't come from some known theory.
An example is the ratio of the mass of an electron to the mass of a
muon. Another is the ratio of the electrical to the gravitational
forces between two electrons./ mike w
(published on 10/22/2007)