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Precise measurement of rare pion decay

University of British Columbia

The PIENU experiment at TRIUMF aims to measure the pion decay branching ratio, defined as the relative rate of decay of pions into electrons over muons including associated neutrinos and radiative components (denoted R_π) to a precision level of O(0.1%). This Standard Model (SM) observable provides a sensitive test of lepton universality, where weak coupling strengths are assumed to be equal for all leptons (g=g_e=g_μ= g_τ). Comparing the measured experimental (R_π^exp) and calculated SM (R_π^SM) ratios, the ratio of the coupling constants can be extracted and compared with the SM expectation g_e/g_μ=1 as follows g_e /g_μ = (R_π^exp/R^π^SM )¹/². The current theoretical calculation of the SM prediction R_π^SM=1.2352±0.0002)×10-⁴ with a precision of 0.016% is more precise than the measurements of previous generation experiments by a factor of 30; thus, there is scope for significant improvement. If the measurement is consistent with the SM, new constraints could be set on new physics scenarios for SM extensions, such as R-parity-violating super-symmetry, leptoquarks, and heavy neutrinos lighter than the pion. Most remarkably, a deviation from the SM could result from a new pseudo-scalar interaction with an energy scale of up to O(1000TeV) which would enhance the branching ratio by O(0.1%). In some instances, these constraints can far exceed the reach of direct searches at colliders. Between 2009 and 2012 around 6.5 million π+→e+ν_e events were gathered. The analysis of a subset of the 2010 data with 0.4 million events was published in 2015, giving R_π^exp=(1.2344±0.0023(stat.)±0.0019(syst.))×10-⁴, with a precision of 0.24%. This is in agreement with the SM, representing a 0.12% measurement of lepton universality at g_e/g_μ=0.9996±0.0012. The analysis presented in this thesis is blinded but includes the highest quality data portion available, around 3 million π⁺→e⁺ν_e events. For this work, major experimental systematic problems have been solved allowing for increased precision up to 0.12% for R_π^exp and up to 0.06% for lepton universality.

Text

2019-04-24

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/69938

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/