Speaker
Dinko Pocanic
(University of Virginia)
Description
Thanks to the simple underlying dynamics of pion and muon decays,
small numbers of the available decay channels, and extremely well
controlled radiative and loop corrections, these decays offer a
uniquely sensitive means to explore details and limits of the
underlying symmetries. For example the anomalously suppressed decay
of the charged pion to the electron, $\pi^+ \to e^+\nu$ (labeled
$\pi_{e2}$), provided an early signal of the V-A Lorentz structure of
the weak interaction. Today, the $\pi_{e2}$ decay still offers the
most sensitive test of lepton universality: the equality of the lepton
couplings to the weak boson regardless of the lepton family
(generation). Hence, $\pi_{e2}$ decay is highly sensitive to
non-(V-A) terms manifested through pseudoscalar contributions.
Radiative decays of the muon, $\mu^+ \to e^+ \nu \bar{\nu} \gamma$,
and pion, $\pi^+ \to e^+\nu\gamma$, or $\pi_{e2\gamma}$, are sensitive
in different ways to departures from the basic V-A dynamics of the
weak interaction. Currently the experimental precision of all these
processes lags significantly, i.e., by an order of magnitude or more,
behind the precision of their theoretical description.
We report on new, unpublished results on the radiative decay of the
muon (RMD) from a comprehensive program of precise measurements of the
pion and muon rare decays at the Paul Scherrer Institute, Switzerland,
the PIBETA and PEN experiments. Compared to previous experiments, our
recently completed data analysis improves the uncertainty of the RMD
branching ratio by a factor of almost 30, and that of the extracted
value for the Michel parameter $\bar{\eta}$ by a factor of 4. Both
are in good agreement with the SM expectations.
We also update the current status of the data analyses of $\pi_{e2}$
(implications on limits of lepton universality, as well as non-$V-A$
extensions to the SM) and $\pi_{e2\gamma}$ decays (pion form factors,
additions to $V-A$ interaction terms). This work is highly
complementary to the Frascati kaon decay measurement program, with
both providing key new information.
Primary author
Dinko Pocanic
(University of Virginia)
