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10 Particle Physics – II
(c) Decay via strong or electromagnetic interaction is forbidden because of
violation of cp invariance. For ρ
0
, cp = −1. But c η = c π 0 = +1 because
both decay to two gamma rays.
(d) The decay is allowed via electromagnetic interaction.
10.13 Parity of deuteron π d = π p π n . (−1)
l . As π p = π n = +1 for s or d state
l = 0 or 2.
π d = +1
The intrinsic parity of quarks is assumed to be positive because the intrinsic
parity of a nucleon (+) comes from the parities of three quarks and l = 0.
10.14 Decay (a) is forbidden by the ΔS = ΔQ rule for the semi – leptonic decays
and (b) is forbidden by the ΔS = 0, ±1 rule for the hadronic weak decays.
(c) and (d) are allowed and both have been experimentally observed.
10.15 Since strange particles are always produced in pairs, as in the reaction π
−
+
p → Λ + K
0 , the intrinsic parity of a strange particle can only be determined
relative to that of another. Thus, for example, one can determine the kaon
parity relative to that of Λ, which by convention is assigned a positive parity.
Consider the reaction
K
−
+ He
4
→ Λ He
4
+ π
−
→ Λ H
4
+ π
0
These reactions are known to occur in a helium bubble chamber. Now, the
Λ is bound in an s-state relative to the nuclear core He
3 or H
3 which have
positive parity. Furthermore, all the participants in the reaction are spinless.
L initial = L final . The orbital angular momentum does not contribute to the
parity because of s-state. The only relevant parities in the above reaction are
P K = P Λ . P π − = −P Λ , as P π − = −1
The validity of the argument obviously hinges on the hyper-nuclei having
zero spin. If the spin were 1, for example, angular momentum conservation
would require l = 1 in the final state, thus reversing the conclusion. The
spin of Λ H
4 has also been experimentally determined to be indeed zero. It is
concluded that the relative parity of K
− is negative.
10.16 (a) (i) Hadron is an elementary particle which participates in strong interactions, examples being neutron and pion.
(ii) Lepton participates in weak interaction and if charged in em interaction as well, examples being electron and muon. Lepton number is
universally conserved.
(iii) Baryon comprises nucleons and hyperons which participate in strong
interaction, examples being proton and Σ-hyperon. Baryons are
fermions and baryon number is universally conserved.
(iv) Mesons are the carrier of strong forces, examples being pion and kaon.
10 Particle Physics – II
(c) Decay via strong or electromagnetic interaction is forbidden because of
violation of cp invariance. For ρ
0
, cp = −1. But c η = c π 0 = +1 because
both decay to two gamma rays.
(d) The decay is allowed via electromagnetic interaction.
10.13 Parity of deuteron π d = π p π n . (−1)
l . As π p = π n = +1 for s or d state
l = 0 or 2.
π d = +1
The intrinsic parity of quarks is assumed to be positive because the intrinsic
parity of a nucleon (+) comes from the parities of three quarks and l = 0.
10.14 Decay (a) is forbidden by the ΔS = ΔQ rule for the semi – leptonic decays
and (b) is forbidden by the ΔS = 0, ±1 rule for the hadronic weak decays.
(c) and (d) are allowed and both have been experimentally observed.
10.15 Since strange particles are always produced in pairs, as in the reaction π
−
+
p → Λ + K
0 , the intrinsic parity of a strange particle can only be determined
relative to that of another. Thus, for example, one can determine the kaon
parity relative to that of Λ, which by convention is assigned a positive parity.
Consider the reaction
K
−
+ He
4
→ Λ He
4
+ π
−
→ Λ H
4
+ π
0
These reactions are known to occur in a helium bubble chamber. Now, the
Λ is bound in an s-state relative to the nuclear core He
3 or H
3 which have
positive parity. Furthermore, all the participants in the reaction are spinless.
L initial = L final . The orbital angular momentum does not contribute to the
parity because of s-state. The only relevant parities in the above reaction are
P K = P Λ . P π − = −P Λ , as P π − = −1
The validity of the argument obviously hinges on the hyper-nuclei having
zero spin. If the spin were 1, for example, angular momentum conservation
would require l = 1 in the final state, thus reversing the conclusion. The
spin of Λ H
4 has also been experimentally determined to be indeed zero. It is
concluded that the relative parity of K
− is negative.
10.16 (a) (i) Hadron is an elementary particle which participates in strong interactions, examples being neutron and pion.
(ii) Lepton participates in weak interaction and if charged in em interaction as well, examples being electron and muon. Lepton number is
universally conserved.
(iii) Baryon comprises nucleons and hyperons which participate in strong
interaction, examples being proton and Σ-hyperon. Baryons are
fermions and baryon number is universally conserved.
(iv) Mesons are the carrier of strong forces, examples being pion and kaon.