Jul 6 – 13, 2022
Bologna, Italy
Europe/Rome timezone

Precision measurements of rare pion decays with the PIONEER experiment

Jul 7, 2022, 5:00 PM
Room 9 (Indaco)

Room 9 (Indaco)

Parallel Talk Quark and Lepton Flavour Physics Quark and Lepton Flavour Physics


Dieter Ries (Institute of Nuclear Chemistry, Johannes Gutenberg University Mainz, Germany)


The proposal for a next-generation rare pion decay experiment, PIONEER, has recently been approved to the Paul Scherrer Institute (PSI) ring cyclotron.
PIONEER is strongly motivated by several inconsistencies between Standard Model (SM) predictions and data pointing towards the potential violation of lepton flavor universality. It will probe non-SM explanations of these anomalies through sensitivity to quantum effects of new particles even if their masses are at very high scales.

The measurement of the charged pion branching ratio
$R_{e/\mu} = \Gamma(\pi^+\rightarrow e^+\nu(\gamma))/\Gamma(\pi^+\rightarrow \mu^+\nu(\gamma))$ for pion decays to positrons relative to muons is extremely sensitive to a wide variety of new physics effects. At present, the SM prediction for $R_{e/\mu}$ is known to the order of $10^{-4}$, which is 15 times more precise than the current experimental result. An experiment reaching the theoretical accuracy will test lepton flavor universality at an unprecedented level, probing mass scales up to the PeV range.

The measurement of the rare process of pion beta decay, $\pi^+\to \pi^0 e^+ \nu (\gamma)$, with an improvement in sensitivity by a factor of 3-10, will determine ${\left|V_{ud}\right|}$ in a theoretically pristine manner and test CKM unitarity, which is very important in light of the recently emerged tensions. In addition, various exotic rare decays involving sterile neutrinos and axions will be searched for with unprecedented sensitivity.

The experiment design benefits from experience with the PIENU and PEN experiments at TRIUMF and at PSI. Excellent energy and time resolutions, greatly increased calorimeter depth, high-speed detector and electronics response, large solid angle coverage, and complete event reconstruction are all critical aspects of the approach.
In the PIONEER experiment design, an intense pion beam is brought to rest in a segmented, instrumented (active) target (ATAR). The proposed technology for the ATAR is based on low-gain avalanche detectors (LGADs), which can provide precise spatial and temporal resolution for particle tracks and thus separate even very closely spaced decays and decay products. The proposed detector will also include a 3$\pi$ sr, 25 radiation length ($X_0$) electromagnetic calorimeter. A cylindrical tracker surrounding the ATAR is used to link the locations of pions stopping in the target to showers in the calorimeter.

This presentation will introduce the theoretical motivations for PIONEER, discuss the experiment design, and present recent results from simulations and a first testing campaign at the PSI P-5 charged pion beamline.

In-person participation No

Primary authors

Jennifer Ott (University of California, Santa Cruz (US)) Dieter Ries (Institute of Nuclear Chemistry, Johannes Gutenberg University Mainz, Germany)

Presentation materials