5.3. WEAKTON MODEL OF ELEMENTARY PARTICLES 287
In 1974, Ting and Richter discovered independentlyJ/ψparticle, which implies the ex-
istence of a new quark, named as thecquark. Thus, the quark family then was extended to
four members:
u,d,s,c.
As fundamental particles ofSU( 4 ), the irreducible representations (5.2.61), written as
(5.2.72)
4 ⊗ 4 = 15 ⊕ 1 ,
4 ⊗ 4 ⊗ 4 = 20 ⊕ 20 ⊕ 20 ⊕ 4
suggest that the meson groups with 8 particles should be extended to 15 particles, and the
baryons be extended to 20 particles in a group. The extended group of mesons with 15
particles has been verified, which is given by
π±,π^0 ,K±,K^0 ,K
0
,η,η′,D 0 ,D 0 ,D±,D±s.However, the 20 baryons corresponding to the irreducible representation (5.2.72) have not
been discovered now.
Up to now, the quark family has six members:
u,d,s,c,b,t.Their irreducible representation is given by
(5.2.73)^6 ⊗^6 =^35 ⊕^1 ,
6 ⊗ 6 ⊗ 6 = 70 ⊕ 70 ⊕ 56 ⊕ 20.
The classification scheme corresponding to (5.2.73) does not seem to be realistic. The irre-
ducible representation ofSU(N)is only a phenomenological theory in elementary particle
physics.
5.3 Weakton Model of Elementary Particles
5.3.1 Decay means the interior structure
As we addressed in Section5.1.3, all charged leptons, quarks and mediators can undergo
decay as follows:
1) Charged lepton radiation and decay:e−→e−+γ,
(5.3.1) μ−→e−+νe+νμ,
τ−→μ−+νμ+ντ.2) Quark decay:d→u+e−+νe,
(5.3.2) s→d+γ,
c→d+s+u.