Session LM: Electroweak Interactions

Parity-violating electron-nucleus scattering at Mainz

Parity-violating (PV) elastic scattering of longitudinally polarized electrons by an unpolarized nucleus is a powerful tool for precision tests of the Standard Model (SM) and for studies of the nuclear structure. The respective left-right asymmetry ($A_{PV}$) probes one of the fundamental parameters of the SM - the weak mixing angle. Besides precision tests of the SM, parity-violating electron-nucleus scattering can be employed for a determination of the spatial distribution of neutrons within the nucleus and thus enables one to deduce the thickness of the neutron skin. Knowledge of the neutron skin can have a strong impact in many areas of physics. In my talk I will review the parity violating program at the MESA facility in Mainz and discuss our recent study of theoretical uncertainties for a measurement of $A_{PV}$ on a ${}^{12}$C target. Our calculation is performed in a connection with the prospective measurement at MESA.   

Uncertainty estimates for muon capture on the deuteron

Muon capture on the deuteron is a two-body electroweak process that can provide information on the properties of the electroweak current in nuclear effective field theory but also the three-body force in the nuclear Hamiltonian. I will discuss recent progress on the calculation of the rate for this process and corresponding uncertainty estimates.   

Spin Tracking for the Fermilab E989 Muon $g-2$ Experiment

The E989 Muon $g-2$ Experiment at Fermilab studies the anomalous magnetic moment $a_\mu$ of the muon with a precision goal of 140 ppb. Previous measurements at Brookhaven National Laboratory estimated a discrepancy greater than 3 sigma with respect to the Standard Model prediction. With 20 times more statistics, the E989 experiment aims to evaluate this discrepancy. The measurement of $a_\mu$ depends on highly precise measurements of the magnetic field and the difference between spin precession and cyclotron frequencies, $\omega_a$. By design, muons of ``magic momentum'' (3.094 GeV/$c$) are stored in the 1.45 T magnetic storage ring. This momentum is chosen to minimize distortions of $\omega_a$ caused by the electrostatic quadrupoles used to vertically focus the beam. Deviations from the magic momentum and vertical oscillations induced by the quadrupoles result in sub-ppm corrections that must be calculated by precise reconstruction of the beam's motion, both in data and simulation. This talk will present an approach to evaluate these corrections via development of a spin tracking simulation. This simulation will be used to study the beam-related systematic errors to $\omega_a$ and other studies such as beam tracking or convolution of the beam with the magnetic field.   

The Stopped-Kaon Decay Experiment TREK/E36 at J-PARC

The TREK/E36 experiment conducted at J-PARC in Japan aims to test lepton universality in the ratio of decay widths, $R_{K} = \Gamma(K_{e2})/\Gamma(K_{\mu2})$, by utilizing a superconducting toroidal spectrometer, a scintillating fiber target, particle identification systems in combination with a highly segmented CsI(Tl) photon calorimeter covering $75\%$ of $4\pi$ and charged-particle tracking detectors. Additionally the set-up of the E36 detector system facilitates searches for light $U(1)$ gauge bosons below 300 MeV/$c^2$. These bosons could be associated with dark matter or explain the established muon-related anomalies such as the muon $g-2$ value, and the proton radius puzzle. The status and approach of the analysis will be presented.   

Study of radiative pion and muon decays in the PEN experiment

Radiative decays of the muon: $\mu^{+} \to e^{+} + \nu_{e} + \bar{\nu}_{\mu} + \gamma$, and pion: $\pi^{+} \to e^{+} + \nu_{e} + \gamma$, or $\pi_{e2\gamma}$, where the photon carries away an appreciable portion of the available energy, are sensitive in different ways to departures from the basic $V-A$ dynamics of the weak interaction. Like other pion and muon decay channels, they are described with extreme precision in the standard model (SM) of elementary particles and fields. Hence, both processes are effective in studies of the limits on certain non-SM interactions. Additionally, the radiative pion electronic decay offers direct information on the weak form factors of the pion. The ratio of the axial-vector and vector pion form factors, $F_{A} /F_{V}$ provides critical basic input for low energy chiral lagrangians, such as chiral perturbation theory. We discuss the physics reach, and present an analysis update, of the pion and muon radiative decay measurements in the PEN experiment.   

The Systematics of PEN: A Precision Measurement of the Pion Electronic Decay Branching Ratio

The goal of the PEN collaboration, an international collaboration led by the University of Virginia, is to obtain the pion electronic decay branching ratio $\Gamma(\pi \rightarrow \text{e}\nu (\gamma))/\Gamma(\pi \rightarrow \mu \nu (\gamma))$ with a relative uncertainty of 5$\times 10^{-4}$ or better. This measurement provides the best test for electron muon universality . The PEN experiment was performed using stopped pions at the Paul Scherrer Institude in Zurich. The detector was comprised of active plastic beam counters and stopping target, a mini-time projection chamber, two cylindrical mult-wire proportional chambers, twenty plastic stave hodoscopes, and a 240 module pure CsI calorimeter with a solid angle coverage of 3$\pi$ steradians. An ultra realistic Monte Carlo simulation was constructed by the PEN collaboration to obtain the acceptances and study the low energy tail in the CsI calorimeter. Branching ratio extraction requires precise descriptions of hard radiative decays, decays in flight, and corrections from the chamber efficiencies, timings, and the the CsI low energy tail, the most difficult systematic in the analysis due to muon decay background, energy leakage and photonuclear effects. A detailed analysis of the uncertainty budget will be presented.