Baryon meson
What we knew about Weak interaction in the beginning: β Decay
Weak Interaction β Decay 衰變的交互作用： 這個作用中中子不帶電，所以不是電磁力。強度又較核力弱很多，比重力強很多，所以只能是一個新的交互作用，稱為弱交互作用。
e ν p ? n
P318
The electron energy spectrum shows the interaction has “no structure”! ν ν p p ? n n The spectrum can be produced by phase space alone! M only need to be a constant!
π μ e π 與 μ是透過弱交互作用來衰變？
Muon decay
Positron energy spectrum in muon decay
νμ νe e μ This Fermi theory describes Weak decays very well! But it can’t be the final theory. It’s non-renormalizable! It has to change when energy gets higher. But how?
弱交互作用也可以想像如核力一樣由一個媒介子來傳播! p e ν W - n 弱交互作用的媒介粒子：W± boson
p e ν e ν p W - n n
νμ νe e μ νμ νe e μ W
The weak vertices of leptons coupling with W μ νμ -ig W
The weak vertices of quarks coupling with W d u s c -ig W
The weak vertices of leptons coupling with W μ νμ -ig W
For a toy ABC model Three scalar particle with masses mA, mB ,mC Factors External Lines 1 Internal Lines C Vertex -ig A B
The weak vertices of leptons coupling with W μ νμ -ig
The weak vertices of leptons coupling with W μ νμ -ig
2 × 2 matrices
W e νe μ νμ -ig
A e νe Z
p n p p p
質子中子都是由夸克組成，因此基本上，β衰變是夸克的衰變：
交互作用不只改變粒子的動量，也可能改變粒子的身分！ n p p n