536
10 Particle Physics – II
Table 10.1
Mass
Common
Particle
(MeV /c
2 ) τ (s)
decay mode
Hadrons
⎧
⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨
⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩
Mesons
⎧
⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨
⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩
Pions
π
− , π
+
139
2.5 × 10
−8
μν
π
0
135
1.8 × 10
−16
γ γ
Kaons
⎧
⎪ ⎪ ⎪ ⎪ ⎪ ⎨
⎪ ⎪ ⎪ ⎪ ⎪ ⎩
K
− , K
+
494
1.2 × 10
−8
μν
K
0
498
π
± π
0
Mixture of K 1 , K 2
K 1
0.89 × 10
−10 π
+ π
−
π
0 π
0
K 2
5.18 × 10
−8
π
0 π
0 π
0
π
+ π
− π
0
πμν
πμe ¯
ν
η
550
10
−18
γ + γ
π
+ + π
− + π
0
Baryons
⎧
⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨
⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩
Nucleons
p
938.2
> 10
38
stable
n
939.5
10
3
pe
−
ν
Hyperons
⎧
⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨
⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩
Λ
1,115
2.6 × 10
−10
pπ
− , nπ
0
Σ
+
1,189
0.8 × 10
−10
pπ
0
, nπ
+
Σ
0
1,192
10
−20
Λγ
Σ
−
1,197
1.6 × 10
−10
n π
−
Ξ
0
1,314
3 × 10
−10
Λπ
0
Ξ
−
1,321
1.8 × 10
−10
Λπ
−
Ω
−
1,675
1.3 × 10
−10
Ξπ
ΛK
−
Photon
γ
0
∞
Stable
Leptons
⎧
⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨
⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩
τ
−
1,784
3.4 × 10
−13
Electrons
and neutrinos
μ
−
105
2 × 10
−6
eν¯ ν
e
−
0.51
∞
Stable
ν e
0
∞
Stable
ν μ
< 0.5
S t a b l e
ν τ
< 164
∞
Stable
Graviton
?
0
–
S t a b l e
Here we will be concerned with only the first three types. Table 10.2 summarizes
the characteristics of the interactions.
Coupling constant: Particles interact through strong electromagnetic or weak charges.
The square of the charge is known as the coupling constant. It enters the interaction
matrix which determines the cross-sections and decay rates. Strictly speaking, the
coupling constants are not constant but vary very gradually with the particle energy.
They are called running constants.
QED (quantum electrodynamics) is the quantum field theory of the electro magnetic interaction whose predictions have been verified to a precision of one part in a
billion.
QCD (quantum chromo dynamics) is the field theory of the strong color interaction
between quarks.