542 10 Particle Physics – II
wheremis the mass of the exchanged boson. The quantity (q^2 +m^2 )−^1 is known
as the propagator term. For low momentum transfers the propagator is insensitive to
q, but in high energy collisions for large momentum transfer,f(q^2 ) decreases with
increasingq.
Spurionis a hypothetical particle which is introduced into the initial state to convert
the weak decay into a strong interaction.
Weak interaction- characteristics
Weak decays have long lifetimes (> 10 −^13 s) and small interaction cross-sections,
typically∼ 10 −^39 cm^2. Charged leptons experience both weak and em interaction
while neutrinos only weak interaction. Strangeness and parity are not conserved.
Depending on the extent to which leptons are involved, the weak decays are
divided into three classes.
(1)Leptonic decaysin which the decay products are leptons only, as in the decay
μ−→e−+νe+νμ
(2)Semi leptonic decayswhich involve both hadrons and leptons. Examples are
(a)n→p+e−+νeforΔS= 0
(b)K+→π^0 +e++νefor|ΔS|= 1(3)Non-leptonic decayswhich do not involve leptons. Parity or strangeness are not
conserved, the selection rules beingΔS=±1 andΔI=±^12 as in the decay,
Λ→p+π−
In weak decays the flavors of the quark changes in contrast with strong or electro-
magnetic decays where the flavour is conserved. For example the decay of neutron,
n→p+e−+νeis represented byudd→udu+e−+νein which a d-quark is
converted into a u-quark,d→u+e−+νe.
Charge current weak interactionis mediated by massive bosonsW±.TheW±
exchange results in the change of the lepton charge as in the anti neutrino absorption
νe+p→n+e+.Neutral current weak interactionis mediated by the massive bosonZ^0. The exchange
ofZ^0 does not cause the change of lepton as inνμ+e−→νμ+e−.
Electro – Weak interaction
The electro magnetic interaction and weak interaction are two aspects of a single
interaction called electro-weak interaction. The corresponding charges are related
byθwthe Weinberg angle