PIONEER is a new rare pion decay experiment, accepted as a high priority experiment at the Paul Scherrer Institute in Switzerland (link to the PSI PIONEER website).
PIONEER will address several tensions in the flavour sector. The first phase of the experiment aims at improving the measurement of the ratio between pions (\(\pi^+\)) decaying into positrons (\(e^+\)) and those decaying into muons (\(\mu^+\)): \(R_{e/\mu}=\frac{\pi^+\rightarrow e^+ \nu_e (\gamma)}{\pi^+\rightarrow \mu^+ \nu_{\mu} (\gamma)}\).
The currently most precise measurement of \(R_{e/\mu}\) is provided by the PIENU experiment at TRIUMF \(\text{(}R_{e/\mu}= 1.2344(23)(19)\times 10^{-4}\)). PIONEER aims at improving that measurement by more than an order of magnitude, aligning it with the precision level achieved by theory (\(\sim 0.01\%\)). At this precision, PIONEER will provide the best test of the hypothesis that the electron-type particles (e.g. electrons and muons known as leptons) have identical weak interaction strengths. This is a feature of the Standard Model of particle physics (SM) known as lepton flavour universality (LFU). Recent “anomalies” hint at a potential violation of LFU, which would be a sign of new physics, strongly motivating new precision measurements.
PIONEER will also be sensitive to a wide range of new physics scenario including at very high energies, well beyond what can be achieved with high energy colliders. In order to achieve this ambitious goal PIONEER will be using the world’s most intense pion beam at the Paul Scherrer Institute (PSI) in Switzerland, state-of-the-art silicon (Low Gain Avalanche Detectors) and noble liquid (liquid xenon) technologies in an optimized geometry. R&D is also pursued on alternative technologies.
For more information about the experiment, see the Overview page.
PIONEER is an international endeavour with involvement from 80 researchers from 25 universities and national laboratories across Canada, China, Japan, Switzerland and USA. See the Collaboration page for more details!
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