The baryon (

PHYSICS · Multi-part · Fermi University Physics · Particle Physics & Quantum Fields · Symmetry, groups and the quark model · Symmetry, groups and the quark model

Particle Physics, Ch Parity, charge conjugation, time reversal and CPT

Problem

The Λ\Lambda baryon (JP=12+J^P = \tfrac12^+, mass 1115.7 MeV/c21115.7\,\mathrm{MeV}/c^2, strangeness S=−1S=-1) decays with lifetime τ=2.6×10−10 s\tau = 2.6\times10^{-10}\,\mathrm{s}, and about 64%64\% of the time via Λ→p π−\Lambda \to p\,\pi^- (proton P=+1P=+1, pion P=−1P=-1, both non-strange). A worked solution reads: "The decay proceeds with the products in a relative L=0L=0 state, so Pfinal=P(p) P(π−) (−1)0=(+1)(−1)(1)=−1P_{\rm final} = P(p)\,P(\pi^-)\,(-1)^0 = (+1)(-1)(1) = -1. Since P(Λ)=+1P(\Lambda) = +1, parity is not conserved by this assignment, so by the multiplicative rule the decay Λ→p π−\Lambda \to p\,\pi^- is forbidden and should never occur." ((a)) Find the error in this reasoning. ((b)) Explain, using the strangeness of the initial and final states and the measured lifetime, why the decay is in fact observed at the rate quoted.

Browse the problem library