5.1. BASIC KNOWLEDGE OF PARTICLE PHYSICS 259
take three valuesL=± 1 ,0. For all non-lepton particles, their lepton number is zero, and
Le=
1 fore−,νe,
−1 fore+,νe,
0 others,Lμ=
1 forμ−,νμ,
−1 forμ+,νμ,
0 others,Lτ=
1 forτ−,ντ,
−1 forτ+,ντ,
0 others.The lepton numbers are additive and conservative quantities.
6.Baryon number B.The hadrons are classified into baryons and mesons. Baryons are
fermions and mesons are bosons. Also, baryons have an additive and conservative quantum
number: the baryon numberB, defined by
B=
1 for a baryon,
−1 for an antibaryon,
0 for all other particles.- Parityπ. Due to the Noether Theorem2.38, each symmetry is associated with a
conservation law. The parity is a conservative quantum number corresponding to the spatial
reflection symmetry:
x→ −x (x∈R^3 ).
Parityπonly takes two values:
π=± 1.
The weak interaction violates parity; see (Lee and Yang, 1956 ;Wu, Ambler, Hayward, Hoppes and Hudson,
1957 ). The paritiesπof most particles are obtained by two methods: the experimental and
the artificial means.
The parity is a multiplication quantum number. For example,for anNparticles system:
A=A 1 +···+AN,its tatol parityπAis given by
πA= (− 1 )lπA 1 ···πAN,
wherelis the sum of orbital quantum numbers of all particles, andπAkis the parity ofAk
particle.
The parity conservation is defined as follows. For a particlereaction:
(5.1.5) A 1 +···+AN−→B 1 +···+BK,