Elementary Particles
375
Strangeness
The weak interaction plays an important role in the behaviour of what are known
as strange particles.
The K-mesons and Λ, Σ, Ξ baryons were discovered in the cosmic rays
(which are rays of generally high energy particles originating from the outer
space) in the early fifties. After the construction of high energy accelerators,
they could be produced and studied in a controlled manner. They are produced
in reactions of the type
π
–
+ P → K
0
+ Λ
(10.26)
The rate of their production is typical of strongly interacting particles (e.g
comparable to the production of π
0
N). However, the decay of Λ,
Λ → π
–
+ P or π
0
+ N
(10.27)
is very slow. The lifetime of strange particles is generally of the order of 10
–8
to
10
–10
s (except for Σ
0
which decays into Λ + γ in less than 10
–14
s) whereas the
typical lifetimes of decays of strongly interacting particles are of the order of
10
–22
s. The unusual behaviour of these particles, as strongly interacting particles
in production and as weakly interacting particles in decays, brought them the
name of strange particles.
It was observed that the strange particles are produced in pairs [K
0
and Λ in
Eq. (10.26)], called associated production, whereas the decay processes involve
individual strange particles. This is reminiscent of a neutral system (e.g. radiation)
producing a pair of oppositely charged particles (e.g. e and e
+
) but a charged
particle being forbidden to decay into a neutral system by charge conservation.
Using this analogy, Gell-Mann and Nishijima introduced a new quantum number
S called the strangeness which is conserved in strong interaction. Thus K
0
is
assigned strangeness 1 while Λ is assigned strangeness –1, and π
–
and p are
assigned strangeness zero. Thus, the total strangeness is conserved in the reaction
given in Eq. (10.26) (being zero both before and after the reaction). However, it
is not conserved in the decay process given in Eq. (10.27) and hence the decay
would be forbidden by strong interaction. Strangeness is conserved in
electomagnetic interactions as well, so that the decay in Eq. (10.27) proceeds
via the weak interaction which does not conserve strangeness. This would explain
the long lifetime of Λ. Indeed the strength of the interaction for the decay in
Eq. (10.27) is of the same order as the strength of the interaction which leads to
the β-decay of the neutron in Eq. (10.22), ones the dependence of the decay on
the masses is separated out. Thus, it is the weak interaction which governs the
strangeness-changing processes, e.g. decay of Λ.
The strangeness of a particle is given by the relation
Q =
1
2
(S + B) + I z
(10.28)
375
Strangeness
The weak interaction plays an important role in the behaviour of what are known
as strange particles.
The K-mesons and Λ, Σ, Ξ baryons were discovered in the cosmic rays
(which are rays of generally high energy particles originating from the outer
space) in the early fifties. After the construction of high energy accelerators,
they could be produced and studied in a controlled manner. They are produced
in reactions of the type
π
–
+ P → K
0
+ Λ
(10.26)
The rate of their production is typical of strongly interacting particles (e.g
comparable to the production of π
0
N). However, the decay of Λ,
Λ → π
–
+ P or π
0
+ N
(10.27)
is very slow. The lifetime of strange particles is generally of the order of 10
–8
to
10
–10
s (except for Σ
0
which decays into Λ + γ in less than 10
–14
s) whereas the
typical lifetimes of decays of strongly interacting particles are of the order of
10
–22
s. The unusual behaviour of these particles, as strongly interacting particles
in production and as weakly interacting particles in decays, brought them the
name of strange particles.
It was observed that the strange particles are produced in pairs [K
0
and Λ in
Eq. (10.26)], called associated production, whereas the decay processes involve
individual strange particles. This is reminiscent of a neutral system (e.g. radiation)
producing a pair of oppositely charged particles (e.g. e and e
+
) but a charged
particle being forbidden to decay into a neutral system by charge conservation.
Using this analogy, Gell-Mann and Nishijima introduced a new quantum number
S called the strangeness which is conserved in strong interaction. Thus K
0
is
assigned strangeness 1 while Λ is assigned strangeness –1, and π
–
and p are
assigned strangeness zero. Thus, the total strangeness is conserved in the reaction
given in Eq. (10.26) (being zero both before and after the reaction). However, it
is not conserved in the decay process given in Eq. (10.27) and hence the decay
would be forbidden by strong interaction. Strangeness is conserved in
electomagnetic interactions as well, so that the decay in Eq. (10.27) proceeds
via the weak interaction which does not conserve strangeness. This would explain
the long lifetime of Λ. Indeed the strength of the interaction for the decay in
Eq. (10.27) is of the same order as the strength of the interaction which leads to
the β-decay of the neutron in Eq. (10.22), ones the dependence of the decay on
the masses is separated out. Thus, it is the weak interaction which governs the
strangeness-changing processes, e.g. decay of Λ.
The strangeness of a particle is given by the relation
Q =
1
2
(S + B) + I z
(10.28)