Musun: measuring the rate of muon capture in dueterium
Phillips, Justin Daniel
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The MuSun experiment is designed to measure the rate of nuclear capture of muons on deuterium. This is accomplished at the Paul Scherrer Institute (PSI), where we stop 4 MeV negative muons in an ultra-pure, cryogenic deuterium gas, and measure their disappearance rate using the resulting decay electrons. In particular, we employ the lifetime method, constructing our experiment such that the muons can either capture on a deuterium nucleus or freely decay in our target. The final result can then be obtained by finding the difference between our measured disappearance rate in deuterium and the free muon decay rate. The two best measurements of the capture rate were performed over twenty years ago, yielding results of 470±29 Hz and 409±40 Hz. Improving the precision of the measured capture rate (Ad) is of particular interest today. Various theoretical approaches predict Ad to be in the range of 380 Hz to 420 Hz, with many claiming uncertainties of only 1%. The relative error in the earlier experiments is nearly an order of magnitude larger, and the more precise of the two differs from these new calculations by over two standard deviations. The goal of MuSun is to measure Ad to 6 Hz, improving the overall experimental uncertainty to 1.5%. Besides helping to determine the validity of the techniques used in the new theory calculations, increasing the precision on the measurement of Ad immediately improves the accuracy on the strength of other related interactions, such as the first stage of solar fusion and neutrino scattering in the SNO experiment. While muon capture in heavier nuclei has been measured to greater precision, the level of uncertainty in theoretical calculations involving atoms with three or more nucleons is comparatively much greater. Muon capture on the deuteron provides a unique opportunity for the study of weak interactions in complex nuclei: readily measured in experiment and readily calculated in theory.
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