Nuclear Muon Capture in Hydrogen and its Interplay with Muon Atomic Physics

TL;DR

The MuCap experiment measured muon capture in hydrogen to determine the pseudoscalar form factor, testing QCD symmetries, and plans to extend this research to deuteron capture for insights into two-nucleon axial currents.

Contribution

This work introduces a novel experimental technique using a time projection chamber with ultra-pure hydrogen to measure muon capture and determine the pseudoscalar form factor, advancing understanding of weak interactions.

Findings

Measured $g_P$ consistent with theoretical predictions

Developed advanced techniques for muon capture measurement

Planned future experiments on muon capture in deuteron

Abstract

The singlet capture rate $\Lambda_S$ for the semileptonic weak process $\mu+p \to n+\nu_\mu$ has been measured in the MuCap experiment. The novel experimental technique is based on stopping muons in an active target, consisting of a time projection chamber operating with ultra-pure hydrogen. This allows the unambiguous determination of the pseudoscalar form factor $g_P$ of the charged electroweak current of the nucleon. Our first result $g_P(q^2=-0.88 m^2_\mu) = 7.3 \pm 1.1 $ is consistent with accurate theoretical predictions and constitutes an important test of QCD symmetries. Additional data are being collected with the aim of a three-fold reduction of the experimental uncertainties. Building on the developed advanced techniques, the new MuSun experiment is being planned to measure the muon capture rate on the deuteron to 1.5% precision. This would provide the by far most accurate experimental information on the axial current interacting with the two-nucleon system and determine the low energy constant $L_{1A}$ relevant for solar neutrino reactions. Muon induced atomic and molecular processes represent challenges as well as opportunities for this science program, and their interplay with the main nuclear and weak-interaction physics aspects will be discussed.