Our current understanding of elementary particles and most recent experimental results are well described by the Standard Model (SM) of particle physics. The SM is a model of elementary particles (fields) and interactions (forces) that explains their behavior in terms of symmetries and the destruction of symmetries. It explains the observed particles in terms of the fundamental particles (fields) and describes these particles and the interactions between them using force carriers, gauge bosons (Table 2).

According to the SM, matter is composed of a dozen fermions (6 quarks and 6 leptons) along with their antiparticles and the gauge particles (5 vector bosons) which mediate their interactions,

Fermions (spin 1/2 particles), Table 1 6 Quarks: three doublets of quarks, (u,d), (s,c), (t,b).                       6 Leptons: three doublets of leptons, (e, νe), (μ, νμ),(τ, ντ). Bosons (spin 1 particles), table 2     Five force-carrying gauge bosons: γ, W+, W−, Z0, and g (gluon).

these types of matter, leptons and the vector bosons, are considered structurless and treated as interacting fields appearing in lagrangians which describe the dynamics of their interactions. The above particles (fields) and their quantum numbers are shown in Tables (1 and 2). The known particles are excitations of these fields. In particular, hadrons (baryons and mesons), the strongly interacting particles, have a finite size (of the order of 1 fm) and can not be considered elementary. There are two types of hadrons,

• Mesons have integer spins (S = 0, 1, ...) and they are bound states of a quark and an anti-quark, e.g., the pion triplet:  
  • π⁺ |ud> , π⁻ |du> , and the neutral one π⁰ is the combination 1/√2 (|u u + d d >) . They are pseudoscalar mesons which are ground states (|L| = = 0) with the quark and anti-quark spins antialigned resulting in total angular momentum |J| = 0, see Fig. (2).
  • There are other states for the above| L| = 0 states, but with the quark and anti-quark spins aligned resulting in |J| = 1, and these are called vector mesons, e.g., the rho meson triplet, ρ⁺ |ud> , ρ⁻ |du> , and the neutral one ρ⁰ is the combination 1/√2 (|u u + d d >)
• Baryons have half-integer spins (S = 1/2, 2/3, ...). A baryon is a state of three quarks |qqq>, e.g. the proton |uud> and the neutron |udd>, and their anti-particles, p |u u d> and n |d d u> .
  
  So every baryon is composed of three quarks and every meson is made up of a quark and an anti-quark. However, according to Pauli Exclusion Principle (two identical particles can NOT occupy the same state) one particle can't contain two identical quarks (e.g. the proton contains two up quarks!). Because of this it was proposed that quarks should have a new property, a new quantum number, called color (also called a color charge): red, blue, green, and the corresponding anti-colors (for anti-quarks). Important to know that color here is not the common visual one. Therefore, the three quarks in a baryon should be of different colors and a meson must contain a colored quark and the corresponding anti-colored one (making hadrons colorless, i.e. have neutral color charge, exactly as atoms have neutral electric charge). Colored quarks interact through exchanging gluons (the strong force carriers) and and unlike the photon (the carrier of the EM force) there are eight kinds of gluons. This number of gluons is determined from the color-anticolor combinations, and one would expect nine gluons (3 colors × 3 anti-colors). However, real gluons are orthogonal linear combinations of the above 9 states and the following combination of states
  
  1/√3 (RR + GG + BB)
  is colorless and does not contribute to the strong interaction and thus we are left with only 8 color combinations (i.e. 8 gluons). The gluon contains an octet of fields, belongs to the adjoint representation (8),

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              http://hyperphysics.phy-astr.gsu.edu/hbase/particles/quark.html

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Table 1: Fundamental fermions: both Quarks and Leptons have six flavors.

Quarks:      Flavor	Charge
u    c    t       d    s    b	2/3 − 1/3
Leptons:
e     μ     τ        νe    νμ    ντ	− 1 0

Basic Forces: EM force, Weak force, Strong force; Gravity.
According to the SM, there are four basic forces among the above particles: the Strong force , which affects only the hadrons, and the electromagnetic and weak forces as well as Gravity. In field theory, each force is governed by exchanging field particles (quanta) which are themselves elementary particles of integer spin (bosons). Apart from gravitation, which is too weak to effect their interaction, the other three are all gauge interactions. They are all mediated via spin 1 gauge bosons, whose interactions are completely specified by the corresponding gauge group.

Table 2: Fundamental interactions and their carriers (Gauge Bosons; S = 1).

Force	Strong	EM             Weak
Carrier	gluon	photon        W−  W+  Z0
Mass (GeV)	0	0           80.4  80.4   91.2
Elec. charge	0	0            −1   +1    0
Range (m)	10−15	∞              10−17
Strenght	αs≈1	α≈10−3          αw≈10−6
Gauge group	SU(3)	U(1)           SU(2)

The photon and gluon are massless and therefore stable. That is, they don't decay into other particles. The W⁺, W⁻, and the Z are massive and unstable; they decay after a very short time into lighter particles.

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