Investigating Lepton Universality via a Measurement of the Positronic Pion Decay Branching Ratio

The PEN experiment at the Paul Scherrer Institute in Switzerland aims to measure the positronic pion decay branching ratio to an unprecedented relative precision of 0.05%. The measurement tests the existence of lepton universality and puts constraints on several theories beyond the Standard Model of particle physics. This dissertation will first describe the experimental configuration and data collection during the PEN experiment. Focus is placed on the development of data analysis tools including calibrations, event reconstruction algorithms, and a maximum likelihood analysis framework designed specifically for this experiment. The 8×10 5 π + → e + ν e events observed in 2008 were used in this study to obtain R πe2 =[1.112±0.002(stat.)]×10 −4 , where the central value is still intentionally blinded with an unknown multiplicative random number. Using only a small fraction of the PEN data, our statistical uncertainty in R πe2 is already smaller than the combined (statistical and systematic) uncertainty in the experimental world average. Including the estimated 20×10 6 additional π + → e + ν e events from 2009 and 2010 will further reduce the uncertainty on this measurement.

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