Prospects for PIONEER

Abstract

Pion $\beta$ decay, $\pi^+ \to\pi^0 e^+ \nu_e$, offers a pristine way to measure the CKM matrix element $V_{ud}$ in a purely mesonic system, with excellent control over the hadronic matrix elements. We review the physics goals and current status of the PIONEER experiment, which aims at major improvements in the branching fractions for the $\pi^+\to e^+ \nu_e$ decay in Phase I and for pion $\beta$ decay in Phases II and III of its experimental program, potentially leading to a measurement of $V_{ud}$ competitive with determinations from $\beta$ decays involving nucleons.