PIONEER is a next-generation rare pion decay experiment, accepted as a high priority experiment at the Paul Scherrer Institute (PSI) in Switzerland.
In the first phase of the experiment, PIONEER aims at improving the measurement of the ratio between pions (\(\pi^+\)) decaying into positrons (\(e^+\)) and those decaying into muons (\(\mu^+\)):
\(R_{e/\mu}=\frac{\pi^+\rightarrow e^+ \nu_e (\gamma)}{\pi^+\rightarrow \mu^+ \nu_{\mu} (\gamma)}\).
This measurement, highly sensitive to "new physics" at high mass scales, has broad implications for the universality of lepton interactions. Using state-of-the-art instrumentation - learning from the previous generation PEN and PiENu measurements - and a new high-intensity beam, measurements of the pion decay to electrons vs. muons and pion beta decay will improve on previous studies by an order of magnitude to the 10-4 precision level. A disagreement with the theoretical Standard Model (SM) prediction, which has a remarkable precision at the same level, would unambigously imply new physics beyond the SM. PIONEER will also be sensitive to a wide range of new physics scenarios, 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 PSI and state-of-the-art detector technologies: an active target using Low Gain Avalanche Detectors (LGAD) combined with a calorimeter using liquid noble gas (Xenon) or LYSO crystals.
For more information about the experiment, see the Overview page at the experiment's main website.
PIONEER is an international endeavor 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.
References to the experiment are arxiv:2203.01981 and arxiv:2203.05505. A more uptodate set of slides was presented at the 2024 Open CHRISP Users Meeting BVR55.