The long-lived neutral kaon decays semileptonically through the rule: and
PHYSICS · Multi-part · Fermi University Physics · Particle Physics & Quantum Fields · Flavour, neutrinos and CP violation · Flavour, neutrinos and CP violation
Symmetry and Flavour, Ch Neutral kaons: mixing, strangeness oscillation and CP violation
Problem
The long-lived neutral kaon KL decays semileptonically through the ΔS=ΔQ rule: K0→π−ℓ+νℓ and Kˉ0→π+ℓ−νˉℓ, so whichever charge the outgoing lepton carries, the pion emitted with it always carries the opposite charge. Earth's measurement of the K0-Kˉ0 mixture inside KL shows that the channel producing a positively charged lepton (together with a π−) occurs more often than the channel producing a negatively charged lepton (together with a π+), by a fractional excess δL=(3.32±0.06)×10−3. Radio contact is established with a civilization in a distant galaxy. Nothing about Earth's conventions for 'positive' and 'negative' charge, or for 'matter' and 'antimatter', has been exchanged; only physics can be sent by radio, no sample of matter.
((a)) State the single measurement to instruct the civilization to perform on their own neutral, strange meson-antimeson system, and the result you need them to report back, so that the two civilizations can agree on which of their two charges is the same physical charge as Earth's positive charge, with no prior convention shared.
((b)) The civilization performs the measurement and replies: the longest-lived member of their neutral, strange meson-antimeson pair decays semileptonically to a pion they call type P together with a lepton they call type A more often than to a pion of type Q with a lepton of type B, by a fractional excess (3.4±0.2)×10−3. State which of their lepton types, A or B, is the same physical charge as Earth's positron, and justify the identification using the ΔS=ΔQ rule and the fact that δL is a fixed, sign-definite number set by nature. Comment on whether the size of their reported excess is consistent with the Earth measurement.