33 research outputs found
PEN: a low energy test of lepton universality
Allowed charged meson decays are characterized by simple dynamics, few
available decay channels, mainly into leptons, and extremely well controlled
radiative and loop corrections. In that sense, pion decays represent a
veritable triumph of the standard model (SM) of elementary particles and
interactions. This relative theoretical simplicity makes charged pion decays a
sensitive means for testing the underlying symmetries and the universality of
weak fermion couplings, as well as for studying pion structure and chiral
dynamics. Even after considerable recent improvements, experimental precision
is lagging far behind that of the theoretical description for pion decays. We
review the current state of experimental study of the pion electronic decay
, or , where the
indicates inclusion and explicit treatment of radiative decay events. We
briefly review the limits on non-SM processes arising from the present level of
experimental precision in decays. Focusing on the PEN
experiment at the Paul Scherrer Institute (PSI), Switzerland, we examine the
prospects for further improvement in the near term.Comment: 11 pages, 5 figures; paper presented at the XIII International
Conference on Heavy Quarks and Leptons, 22-27 May 2016, Blacksburg, Virginia,
US
PEN experiment: a precise measurement of the pi+ -> e+ nu decay branching fraction
A new measurement of , the decay
branching ratio, is currently under way at the Paul Scherrer Institute. The
present experimental result on constitutes the most accurate test
of lepton universality available. The accuracy, however, still lags behind the
theoretical precision by over an order of magnitude. Because of the large
helicity suppression of the decay, its branching ratio is
susceptible to significant contributions from new physics, making this decay a
particularly suitable subject of study.Comment: 4 pages, 3 figures, talk given at the Tenth Conference on the
Intersections of Particle and Nuclear Physics (CIPANP 2009), La Jolla/San
Diego, CA, 26-31 May 2009; to appear in Proceedings to be published by the
American Institute of Physic
Precise Measurement of pi+ -> e+ nu Branching Ratio
The PEN Collaboration is conducting a new measurement of the pi+ -> e+ nu
branching ratio at the Paul Scherrer Institute, with the goal uncertainty of
delta B/B(pie2)=5E-4 or lower. At present, the combined accuracy of all
published pie2 decay measurements lags behind the theoretical calculation by a
factor of 40. In this contribution we report on the PEN detector configuration
and its performance during two development runs done in 2007 and 2008.Comment: pdflatex, 11 pages, 5 figures, to be published in "Progress in
High-Energy Physics and Nuclear Safety", NATO Science for Peace Series: B -
Physics and Biophysic
Muon capture by 3He nuclei followed by proton and deuteron production
The paper describes an experiment aimed at studying muon capture by
nuclei in pure and mixtures at various densities. Energy distributions of
protons and deuterons produced via and are measured for the
energy intervals MeV and MeV, respectively. Muon capture
rates, and are obtained using two different analysis methods. The
least--squares methods gives , . The Bayes theorem
gives ,
. The experimental
differential capture rates, and , are compared with theoretical
calculations performed using the plane--wave impulse approximation (PWIA) with
the realistic NN interaction Bonn B potential. Extrapolation to the full energy
range yields total proton and deuteron capture rates in good agreement with
former results.Comment: 17 pages, 13 figures, accepted for publication in PR
New Precise Measurement of the Pion Weak Form Factors in the Pi+ -> e+ nu gamma Decay
We have measured the branching ratio over a
wide region of phase space, based on a total of 65,460 events acquired using
the PIBETA detector. Minimum- fits to the measured
energy distributions result in the weak form factor value of
with a fixed value of . An unconstrained fit yields
and . In addition, we have measured
for the dependence of on , the pair
invariant mass squared, parametrized as . The
branching ratio for the kinematic region MeV and is measured to be . Earlier deviations we reported in the high-/low- kinematic region are resolved, and we find full compatibility with CVC
and standard calculations without a tensor term. We also derive new
values for the pion polarizability, , and neutral pion lifetime, s.Comment: 4 pages, 2 PDF figure