Two families of experiment measure the neutron lifetime by different methods
PHYSICS · Multi-part · Fermi University Physics · Scientific Practice & Experimental Methods · Data analysis and model fitting in the laboratory · Records, communication and the epistemology of evidence
Measurement and Method, Ch Models, idealisation and what a measurement is evidence for
Problem
Two families of experiment measure the neutron lifetime τn by different methods. "Bottle" experiments store ultracold neutrons and count how many survive; "beam" experiments count the protons produced by neutron decay in a beam of known flux. The averages Chapter 7 compared areτbottleτbeam=(879.6±0.6)s,=(888.0±2.0)s,with both uncertainties standard uncertainties (k=1).
((a)) Compute the consistency ratio z between the two values.
((b)) A large ∣z∣ shows that something in the two experiments' chains of assumptions is inconsistent. Explain why it does not, by itself, tell anyone which chain is at fault, or that new neutron physics has been found.
((c)) One hypothesis floated to explain the gap was an unseen decay channel (the neutron decaying to some product other than a proton, electron and antineutrino, at roughly the size of the discrepancy). Other experiments then searched directly for the particles this channel would produce. Explain, in terms of the criteria of theory choice, why the hypothesis lost support, without being ruled out, once that search was reported.
((d)) State what the field actually did next (bottle and beam experiments continued, and a newer beam method was built), and say what this response shows about how theory choice proceeds when no single result is decisive.