The Standard Model relies on several assumptions, among which the existence of a certain fields, whose quanta are point-like particles, divided first in two categories : matter and force particles.
Matter particles
Those fundamental particles are fermionic, which implies one can fill a box with them until none can be added (as long as the box is strong enough to resist electric repulsion for instance). Fermi-Dirac statistics is a deep phenomenon, linked to the intrinsic angular momentum called spin : fundamental fermions are spin 1/2 (spinor) particles, and it implies that
after a rotation of 2\pi, their wavefunction changes sign !. This is not a real problem, since only the (hermitean) square of the wavefunction is observable, that is the probability density. Notice also that after 4\pi the double sign reverses cancel, and there are deep reasons why 4\pi rotations are
always equivalent to no rotation. Let us first stare at a list of them :
First family
Second family
Third family
electron e^-
muon \mu^-
tau \tau^-
electronic neutrino \nu_{e}
muonic neutrino \nu_{\mu}
tauonic neutrino \nu_{\tau}
up quark u
charm quark c
top quark t
down quark d
strange quark s
bottom (beauty) quark b
Botanistic classification : 1 and 2 are leptons, while 3 and 4 are hadrons. Leptons do not feel the strong force (on which more latter). Etymology : "hadron" comes from a greek word sounding like "hadros" and meaning "strong, robust, bulky, thick, or stout", and "lepton" from "leptos" : "fine, thin, slender, small". To this list you could add anti-particles, or consider they are ordinary particles traveling backwards in time. Anti-particles have opposite charges except mass and spin, are denoted by an upper bar on the particle symbol, and sometimes have a special name, such as the positron \bar{e}^+. Each of those might have a supersymetric bosonic partner (supersymetry relates bosons and fermions) but this is not part of the standard model. Although supersymetry has shown relevance in nuclei, it has not been observed for fundamental particles. Some basic facts :
- Carry minus one unit of electrical charge : -e.
- Zero electric charge, and at most a very small mass (zero in the standard model)
- +2/3 e, and color charge (red, blue or green) which is "hidden" : free particles are "white", or more accurately "invariant under color rotations".
- -2/3 e. Also colored. It is difficult to define mass for 3 and 4 guys because they are never free, especially difficult for the lightest, first family members.
The quarks are "contained in a white bag", either they pair in mesons (quark/antiquark bound state) or in baryons (three quarks bound states). Recent proposal of "pentaquarks" seem nowadays unlikely : only intermediate energy have published low statistics evidences, high energy experiments lead to negative results. Confirmation or rejection of this hypothesis will be available soon, when the analyzing process of data from dedicated experiments will be over.
Force particles
There are 12 bosonic fundamental particles with spin 1 (vector) in the standard model, plus one additional not yet confirmed and spin zero (scalar) particle. They are beautifully unified by a so-called gauge model, formulated entirely in terms of
symmetries. One would contemplate a U(1)\otimes SU(2)\otimes SU(3) gauge group. Let us emphasized here that gauge symmetry is not a real symmetry, but rather a
very elegant mean to deal with constrained systems
- The photon is the quantum of light, it is usually denoted \gamma and carries the electromagnetic interaction. It is massless, and this fact is related to the U(1) Quantum ElectroDynamic (QED) part of the standard gauge group which is not broken. Broken symmetries occur when the laws have a certain symmetry, but the ground state (vacuum) does not. So all the tower of states constructed form the vacuum do not exhibit this symmetry.
- The 3 massive vector bosons W^+, W^- and Z^0 are responsible for the weak interaction. This is for instance what causes \beta decay of the proton into a neutron, at the quark level : d\rightarrow u + e^- + \bar{\nu}_e. There are also so-called "neutral currents" with the Z^0 able to go from one family to another. The gauge part is SU(2), but this symmetry is spontaneously broken, giving mass to the vector bosons by the so-called Higgs mechanism. The weak interaction is very peculiar, in that is does not respect some basic symmetries of Nature the other interactions seem to : especially P symmetry, the parity which consists in "taking the mirror image", is maximally broken : the \nu (if massless) is always left handed, while the \bar{\nu} (if massless) is always right handed. So physicists first hoped CP would not be broken, with addition of the exchange particle-antiparticle exchange C. It appears not to be the case, but then it is only slightly broken, such as in the decay modes of the some mesons. Today, we strongly believe that by adding time reversal, the all-important CPT symmetry is the fundamental for the theory. The spin-statistics theorem relies on this symmetry.
- The 8 massless gluons, with gauge group the color SU(3). Gluons are also peculiar in that themselves carry color charge : a red gluon turn blue by emitting a red/antiblue gluon absorbed by a blue quark turning red for instance. This fact makes gluons interact with each other. There are three gluons and four gluons vertices in the standard model. The non-linearity makes the equation hopelessly unsolvable until now. Also, this is likely the cause of confinement, the property that color is always hidden in color-invariant bound states. Another striking feature is asymptotic freedom, the fact that at very low distances, or alternatively at high energy, quarks look like free particles.
The Glashow-Weinberg-Salam model has already unified the first two in the so-called
electroweak unification U(1)\otimes SU(2)=U(2). This leads us to the Higgs mechanism : for each broken symmetry, a massless Goldstone boson appears. The Higgs mechanism permits this degree of freedom to be "eaten" by the massless gauge boson thus acquiring mass. We can even refine the model to make all fundamental particles massless acquiring mass by interaction with the Higgs scalar boson. This particle should soon be discovered at the LHC facility (Geneva)
Additional informations :
A good source of general information is
wikipedia
Another specifically on physics, somewhat at lower level but good illustrations is
hyperphysics
The http://particleadventure.org/particleadventure/ is the source of the link provided by Marlon, and this link leads to the gateway to it, a very friendly introduction.
The http://public.web.cern.ch/Public/Content/Chapters/AboutCERN/WhyStudyPrtcles/WhyStudyPrtcles-en.html is the main European research center.
Of course, the
Particle Data Group has a reference site with all particles informations, and more, quite up-to-date and reliable,
The less technical particle adventure is also from the PDG.
http://laser.physics.sunysb.edu/~wise/wise187/janfeb2001/weblinks/physics_words.html
