PIONEER

A next-generation rare pion decay experiment

external page PIONEER is a next-generation rare pion decay experiment, accepted as a high priority experiment at the external page Paul Scherrer Institute (PSI) in Switzerland. In the first phase of the experiment, PIONEER aims at improving the measurement of the ratio between pions (𝜋⁺) decaying into positrons (𝑒⁺) and those decaying into muons (𝜇⁺):

𝑅_𝑒/𝜇= 𝜋⁺→𝑒⁺𝜈_𝑒(𝛾) / 𝜋⁺→𝜇⁺𝜈_𝜇(𝛾).

This measurement, highly sensitive to "new physics" at high mass scales, has broad implications for the universality of lepton interactions. PIONEER aims at improving on previous studies by more than 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. In the second and third phase of the experiment, PIONEER will also measure the rare pion beta decay, which - in combination with other experiments - can be used to test the charge-parity (CP) symmetry of the quark sector. Furthermore, PIONEER will be sensitive to a wide range of new physics scenarios, well beyond what can be achieved with high energy colliders today.

Pioneer setup crystal calorimeter — Figure (a): Sketch of the full PIONEER detector setup. The high intensity pion beam is entering from the left and stopping in a highly segmented active target (ATAR), shown in the zoom in. The decay positrons escape the target and are measured in a LYSO crystal calorimeter. Figure (b) and (c): Physics goals of the PIONEER experiment in Phase-I and Phase-II/III respectively.

In order to achieve this ambitious goals, PIONEER will be using the world’s most intense pion beam at PSI and state-of-the-art detector technologies, learning from the previous generation PEN and PiENu experiments. An active target using Low Gain Avalanche Detectors (LGAD) will be combined with a fast, high resolution calorimeter using LYSO crystals or liquid Xenon.

For more information about the experiment, see the external page Overviewpage 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 external page Collaboration page for more details.