Experimental results on pion decays obtained with the PIBETA spectrometer at the Paul Scherrer Institute (PSI) are reviewed. For pion beta decay π+ → π0е+ν (πβ), a precision measurement of relative probability yields Г(πβ) = [1.036 ± 0.004(stat) ± 0.004(syst) ± 0.003(π+→е+ν)] × 10–8, which implies Vud = 0.9728(30) for the corresponding element of the Cabibbo–Kobayashi–Maskawa mixing matrix. Using a sample of 65 × 103 events, relative probability of the π+→е+νγ radiative pion decay (RPD) in the kinematic region of Eγ > 10 MeV and θeγ > 40° is measured as Bexp = 73.86(54) × 10–8. A statistical analysis of measured Ee+ and Eγ distributions for this decay yield the values FV = 0.0258(17) and FA = 0.0117(17) for the pion weak formfactors. Assuming that FV linearly depends on the е+ν invariant mass q2 as FV(q2) = FV(0)(1 + aq2), the slope parameter is extracted as а = 0.10(6). The pion polarizability and neutral-pion lifetime are estimated as αE = 2.78(10) × 10–4 fm3 and τ(π0) = (8.5 ± 1.1) × 10–17 s, respectively. The data for decays π+→ е+ ν and \({\mu ^ + } \to {e^ + }v\bar v\gamma \) have been collected and are being processed. The follow-up PEN experiment aims at reducing the uncertainty on the π+ → е+ ν relative probability by almost an order of magnitude (to 5 × 10–4).

With few open channels and uncomplicated theoretical description, charged pion decays are uniquely sensitive to certain standard model (SM) symmetries, the universality of weak fermion couplings, and to aspects of pion structure and chiral dynamics. We review the current knowledge of the pion electronic decay π^+ → e^+ ν_e(γ), or π_e2(γ), and the resulting limits on non-SM processes. Focusing on the PEN experiment at the Paul Scherrer Institute (PSI), Switzerland, we examine the prospects for further improvement in the near term.

Allowed charged $\pi$ 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 $\pi^+ \to e^+\nu_e(\gamma)$, or $\pi_{e2(\gamma)}$, where the $(\gamma)$ 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 $\pi_{e2(\gamma)}$ decays. Focusing on the PEN experiment at the Paul Scherrer Institute (PSI), Switzerland, we examine the prospects for further improvement in the near term.

Simple dynamics, few available decay channels, and highly controlled radiative and loop corrections, make pion and muon decays a sensitive means of exploring details of the underlying symmetries. We review the current status of the rare decays: π + → e + ν (π e2 ), π + → e + νγ (π e2γ ), π + → π 0 e + ν (π e3 ), and [Formula: see text]. For the latter we report new preliminary values for the branching ratio B(E γ > 10 MeV , θ eγ > 30°) = 4.365 (9) stat (42) syst × 10 -3 , and the decay parameter [Formula: see text], both in excellent agreement with standard model predictions. We review recent measurements, particularly by the PIBETA and PEN experiments, and near-term prospects for improvement. These and other similar precise low energy studies complement modern collider results materially.

Building on the rare pion and muon decay results of the PIBETA experiment, the PEN collaboration has undertaken a precise measurement of B_{\pi e2} = R^\pi_{e/\mu}, the \pi^+ -> e^+\nu(\gamma) decay branching ratio, at the Paul Scherrer Institute, to reduce the present 40\times experimental precision lag behind theory to ~ 6-7\times. Because of large helicity suppression, R^\pi_{e/\mu} is uniquely sensitive to contributions from non-(V-A) physics, making this decay a particularly suitable subject of study. Even at current precision, the experimental value of B_{\pi e2} provides the most accurate test of lepton universality available. During runs in 2008-10, PEN has accumulated over 2\times 10^7 \pi_{e2} events; a comprehensive maximum-likelihood analysis is currently under way. The new data will also lead to improved precision of the earlier PIBETA results on radiative \pi and \mu decays.

The PEN collaboration carries out a precision measurement of the π+ → e +ν decay branching ratio at the Paul Scherrer Institute (PSI, Switzerland). A special mini time-projection chamber (mTPC) has been developed for registration of particles in the pion beam. The chamber contains 14 × 10−3 g/cm2 of matter on the particle path. The chamber was successfully used during one-year-long run. Design and characteristics of the mTPC are described.

We have measured the pi+-->e+ nugamma branching ratio over a wide region of phase space, based on a total of 65 460 events acquired using the PIBETA detector. Minimum-chi2 fits to the measured (E(e+), E(gamma) energy distributions result in the weak form factor value of F(A)=0.0119(1) with a fixed value of F(V)=0.0259. An unconstrained fit yields F(V)=0.0258(17) and F(A)=0.0117(17). In addition, we have measured a=0.10(6) for the dependence of F(V) on q2, the e+ nu pair invariant mass squared, parametrized as F(V)(q2)=F(V)(0)(1+aq(2)). The branching ratio for the kinematic region E(gamma)>10 MeV and theta(e(+)gamma)>40 degrees is measured to be B(expt)=73.86(54)x10(-8). Earlier deviations we reported in the high-E(gamma)-low-E(e+) kinematic region are resolved without a tensor term. We also derive new values for the pion polarizability alpha(E)=2.78(10)x10(-4) fm3 and neutral pion lifetime tau(pi0)=(8.5+/-1.1)x10(-17) s.

The PEN Collaboration is conducting a new measurement of the π+ → e + vL branching ratio at the Paul Scherrer Institute, with the goal uncertainty of δB/B πe2 = 5 × 10−4 or lower. At present, the combined accuracy of all published πe2 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.

A new measurement of Be 2, the + ! e + ( ) decay branching ratio, is currently under way at the Paul Scherrer Institute. The present experimental result on Be 2 constitutes the most accurate test of lepton universality available. The accuracy, however, still lags behind the theoretical precision by over an order of magnitude. Thanks to the large helicity suppression of e2 decay, the branching ratio is susceptible to significant contributions from new physics, making this decay a particularly suitable subject of study.

A new measurement of Bπe2, the π+→e+ν(γ) decay branching ratio, is currently under way at the Paul Scherrer Institute. The present experimental result on Bπe2 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 πe2 decay, its branching ratio is susceptible to significant contributions from new physics, making this decay a particularly suitable subject of study.

M. Bychkov, ∗ D. Počanić, † B. A. VanDevender, ‡ V. A. Baranov, W. Bertl, Yu. M. Bystritsky, E. Frlež, V. A. Kalinnikov, N. V. Khomutov, A. S. Korenchenko, S. M. Korenchenko, M. Korolija, T. Kozlowski, N. P. Kravchuk, N. A. Kuchinsky, W. Li, § D. Mekterović, D. Mzhavia, 6 S. Ritt, P. Robmann, O. A. Rondon-Aramayo, A. M. Rozhdestvensky, T. Sakhelashvili, S. Scheu, U. Straumann, I. Supek, Z. Tsamalaidze, 6 A. van der Schaaf, E. P. Velicheva, V. P. Volnykh, Y. Wang, ¶ and H.-P. Wirtz ∗∗ Department of Physics, University of Virginia, Charlottesville, VA 22904-4714, USA Joint Institute for Nuclear Research, RU-141980 Dubna, Russia Paul Scherrer Institute, Villigen PSI, CH-5232, Switzerland Rudjer Bošković Institute, HR-10000 Zagreb, Croatia Institute for Nuclear Studies, PL-05-400 Swierk, Poland Institute for High Energy Physics, Tbilisi State University, GUS-380086 Tbilisi, Georgia Physik Institut der Universität Zürich, CH-8057 Zürich, Switzerland (Dated: 9 Apr 2008)

A precise investigation of radiative pion decay (π+ → e+νγ) in a pion beam from the meson factory of the Paul Scherrer Institute (Switzerland) was performed by the PIBETA Collaboration with the aid of the PIBETA detector. This resulted in finding 41 601 events of radiative pion decay in three kinematical regions. The absolute values of the branching ratio for radiative pion decay were determined in each of these regions. To a precision approximately four times higher than that known previously, the ratio of the axial-vector to the vector form factor was found to be γ ≡ F A /F V = 0.443 (15), the latter being fixed at F V = 0.0259. The number of events found in the kinematical region specified by photon energies of Eγ > 55.6 MeV, positron energies of E e > 20.0 MeV, and angles of θγ, e > 40° between the momenta of the corresponding particles (B region) was 5233. In region B, the measured branching ratio for radiative pion decay, Rπ→evγ(expt) = 11.6(3) × 10−8, proved to be smaller by eight standard deviations than that which follows from the Standard Model, Rπ→evγ (theor) = 14.34(1) × 10−8.

The construction and characteristics of the PIBETA spectrometer are described. This spectrometer is designed to implement a program of precise measurement of pion β decay π+ → π0 + e + + νe at the Paul Scherrer Institute (Switzerland). A spherical calorimeter, consisting of 240 crystals of pure CsI scintillator and embracing a solid angle of ∼3π, is the main detector of the setup. In addition, the spectrometer is composed of an active collimator (which also acts as a beam degrader), a segmented active plastic target, two multiwire cylindrical proportional chambers, a 20-element cylindrical plastic hodoscope, and veto counters of cosmic muons.

We describe the design, construction and performance of the PIBETA detector built for the precise measurement of the branching ratio of pion beta decay, π+→π0e+νe, at the Paul Scherrer Institute. The central part of the detector is a 240-module spherical pure CsI calorimeter covering ∼3πsr solid angle. The calorimeter is supplemented with an active collimator/beam degrader system, an active segmented plastic target, a pair of low-mass cylindrical wire chambers and a 20-element cylindrical plastic scintillator hodoscope. The whole detector system is housed inside a temperature-controlled lead brick enclosure, which in turn is lined with cosmic muon plastic veto counters. Commissioning and calibration data were taken during two 3-month beam periods in 1999/2000 with π+ stopping rates between 1.3·103π+/s and 1.3·106π+/s. We examine the timing, energy and angular detector resolution for photons, positrons and protons in the energy range of 5–150MeV, as well as the response of the detector to cosmic muons. We illustrate the detector signatures for the assorted rare pion and muon decays and their associated backgrounds.

Using a large acceptance calorimeter and a stopped pion beam we have made a precise measurement of the rare Pi+ -> Pi0 e+ Nu,(pi_beta) decay branching ratio. We have evaluated the branching ratio by normalizing the number of observed pi_beta decays to the number of observed Pi+ -> e+ Nu, (pi_e2) decays. We find the value of Gamma(Pi+ -> Pi0 e+ Nu)/Gamma(total) = [1.036 +/- 0.004(stat.) +/- 0.004(syst.) +/- 0.003(pi_e2)] x 10^-8, where the first uncertainty is statistical, the second systematic, and the third is the pi_e2 branching ratio uncertainty. Our result agrees well with the Standard Model prediction.

We have studied radiative pion decays pi(+)-->e(+)nugamma in three broad kinematic regions using the PIBETA detector and a stopped pion beam. Based on Dalitz distributions of 41 601 events we have evaluated absolute pi-->enugamma branching ratios in the three regions. Minimum chi(2) fits to the integral and differential (E(e(+)),E(gamma)) distributions result in the axial-to-vector weak form factor ratio of gamma identical with F(A)/F(V)=0.443(15), or F(A)=0.0115(4) with F(V)=0.0259. However, deviations from standard model predictions in the high-E(gamma)-low-E(e(+)) kinematic region indicate the need for further theoretical and experimental work.

E. Frlež, ∗ D. Počanić, V. A. Baranov, W. Bertl, M. Bychkov, N. V. Khomutov, A. S. Korenchenko, S. M. Korenchenko, T. Kozlowski, N. P. Kravchuk, N. A. Kuchinsky, W. Li, R. C. Minehart, D. Mzhavia, B. G. Ritchie, S. Ritt, A. M. Rozhdestvensky, V. V. Sidorkin, L. C. Smith, I. Supek, Z. Tsamalaidze, B. A. VanDevender, E. P. Velicheva, Y. Wang, H.-P. Wirtz, † and K. O. H. Ziock Department of Physics, University of Virginia, Charlottesville, VA 22904-4714, USA Paul Scherrer Institut, Villigen PSI, CH-5232, Switzerland Joint Institute for Nuclear Research, RU-141980 Dubna, Russia Institute for Nuclear Studies, PL-05-400 Swierk, Poland Institute for High Energy Physics, Tbilisi State University, GUS-380086 Tbilisi, Georgia Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287, USA Rudjer Bošković Institute, HR-10000 Zagreb, Croatia (Dated: 9 December 2003)

A special feature of the system is the integration of the amplifying section, signal delay unit, and digital section for the data outputting to computer into a common АДД-32 module (CAMAC). The 1-M-size АДД-32 module contains the electronics for 32 wires, which makes it possible to place up to 672 data channels in the CAMAC crate with a standard bus. To amplify signals, Ampl 8.3 eight-channel microcircuits are used. The output logic signals are delayed by 9-channel IDT72421 FIFO microcircuits enabling us to obtain a total signal delay of up to hundreds of microseconds and longer. The digital section of the system is based on ALTERA programmable logic arrays. At present, this system with a total of 576-channels is used in the PIBETA experiment to study rare pion decays on the PSI accelerator (Switzerland).