Subatomic Physics

11.4. A Variety of Weak Processes 345
Table 11.3: Strangeness-Changing Semileptonic Decays.
Spin-parity Sequence t 1/2 † Emax(e) ft 1/2
Decay (of hadron) (sec) (MeV) (sec)
K + → π0e + νe 0− → 0− 1.8 × 10−7 358 1 × 106 Λ0 → pe−¯νe 1 + 1 +
→ 2 2 2.2 × 10−7 177 2 × 104 Σ− → ne−¯νe 1 + 1 +
→ 2 2 1.0 × 10−7 257 1 × 105 † Partial half-life.
Hadronic Processes Examples of weak decays in which only hadrons are involved
are
and
K + −→ π + π 0
−→ π + π + π −
−→ π + π 0 π 0
Λ 0 −→ pπ −
−→ nπ 0
(11.31)
(11.32)
Other weak decays involving only hadrons can be found in the tables of PDG. All
of these obey the strangeness selection rule
|∆S| =1.
The absence of observed ∆S = 0 transitions is easily explained: transitions
without change of strangeness can proceed by hadronic or electromagnetic decays,
and the weak branch is hidden.
Why are all the processes listed in the present section called weak, regardless of
whether they involve leptons, hadrons, or both? The justification comes from the
fact that the strength of the interaction responsible for the various processes appears
to be the same. Additional support comes from considerations of selection rules
and from the observation that all processes that are weak according to the strength
classification also show violations of parity and charge conjugation invariance.
The strength of the interaction responsible for a decay expresses itself in the lifetime,
other things being equal. The decays in Table 11.2 are of the type A → Beν.
While the decay energies vary by about a factor of 100 and the density-of-states
factors by a factor of 10 10 ,theft values are approximately the same. It is therefore
likely that the three very different decays in Table 11.2 are caused by the same force.
A discrepancy appears when the ft values in Table 11.2 and 11.3 are compared.
While the decays appear to be similar, the ft values for hypercharge-changing decays
are between one and three orders of magnitude larger than the corresponding