Within the context of a unified gauge theory of integer-change quarks and leptons, it is shown that quarks may decay purely leptonically as well as semi-leptonically, with the following selection rules: (1) Yellow and blue quarks decay predominantly into mesons +
neutrinos
with lifetimes ≈ 10
-11
to 10
-12
s (for
m
q
≈ 2GeV). (2) Red quark can decay leptonically or into mesons + neutrinos or mesons + charged leptons (e
-
, μ
-
). They are long-lived ( ≈ 10
-8
s) if no coloured gluon states exist with masses lower than red quarks. (3) If such gluons do exist, the lifetime of red quarks may be of the same order as that for yellow and blue quarks. In this event and if
m
q
≲ 2 GeV, SLAC (μe) events may possibly be interpreted as the chain decay e
+
+ e
-
→ q̅
R
+ q
R
→ gluon + v ↓ 10
-14
s gluon + v̅ ↓ 10
-14
↓ μ + v̅ e + v (4) In lepto-production experiment, production of colour is suppressed to a 10-15% level in the present kinematic region, because of the gauge character of coloured gluons. There should be no such suppression in N-N production of colour.
The World as Quarks, Leptons and Bosons