A new upper limit for the probability of spontaneous muonium to antimuonium conversion was established at P MM ≤ 8.2 · 10 −11 (90%C.L.) in 0.1 T magnetic field, which implies consequences for speculative extensions to the standard model. Coupling parameters in R-parity violating supersymmetry and the mass of a flavour diagonal bileptonic gauge boson can be significantly restricted. A Z 8 model with radiative mass generation through heavy lepton seed and the minimal version of 331-GUT models are ruled out.

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.

Using a large acceptance calorimeter and a stopped pion beam we have made a precise measurement of the rare pi(+)-->pi(0)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(+)-->pi(0)e(+)nu)/Gamma(total)=[1.036+/-0.004(stat)+/-0.004(syst)+/-0.003(pi(e2))]x10(-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 π + → e + νγ in three broad kinematic regions using the PIBETA detector and a stopped pion beam. Based on Dalitz distributions of 42,209 events we have evaluated absolute π → eνγ branching ratios in the three regions. Minimum χ 2 fits to the integral and differential (E e + , Eγ ) distributions result in the axial-to-vector weak form factor ratio of γ ≡ FA/FV = 0.443(15), or FA = 0.0115(4) with FV = 0.0259. However, deviations from Standard Model predictions in the high-Eγ/low-E e + kinematic region indicate the need for further theoretical and experimental work.PACS numbers: 11.30. Rd, 13.20.Cz, 14.40.Aq In the Standard Model description of radiative pion decay π + → e + νγ, where γ is a real or virtual photon (e + e − pair), the decay amplitude M depends on the vector V and axial vector A weak hadronic currents [1]. Both currents contribute to the structure-dependent terms SD V and SD A associated with virtual hadronic states, while only the axial-vector current contributes to the inner bremsstrahlung process IB. Thus, it is convenient to write the decay amplitude as a sum:where p, k and q are the pion, electron and photon fourmomenta, respectively, e and m e are the electron charge and mass, G F is the Fermi coupling constant, V ud is the CKM quark mixing matrix element, while f π is the pion decay constant. The structure-dependent amplitude is parameterized by the vector and axial vector form factors, F V and F A :The conserved vector current (CVC) hypothesis [4,5] relates F V to the π 0 lifetime yielding F V = 0.0259(5) [6], which agrees with the relativistic quark model and chiral perturbation theory [7]. Chiral symmetry calculations [7,8,9] yield F A in the range 0.010-0.014.The combined π → eνγ event count of all previously published experiments is less than 1,200 events, while the overall uncertainties of the parameter γ ≡ F A /F V extracted from data range from 12 % to 56 % [10,11,12,13,14,15].In this Letter we present a first analysis of the π + → e + νγ (πe2γ) events recorded with the PIBETA detector in the course of a new measurement of the π + → π 0 e + ν (πβ) branching ratio [16,17] from 1999 to 2001.The measurements were performed in the πE1 channel at the Paul Scherrer Institute (PSI), Villigen, Switzerland. The π + beam with p ≃ 113 MeV/c, ∆p/p ≤ 1.3 %, and 24 mr horizontal and vertical divergence, had an average intensity of 6.8 · 10 5 π + /s, and produced a σ x,y = 9 mm beam spot on target. A total of 2.2 · 10 13 π + stops were recorded during the running period.The beam particles were first registered in a 3 mm thick beam counter (BC) placed ∼395 cm upstream of the detector center. The pions were slowed in a 30 mm thick active degrader (AD) and stopped in a 50 mm long segmented active target detector (AT) positioned in the center of the PIBETA detector. The BC, AD and AT detectors are all made of plastic scintillation material.The e + and µ + beam contaminations determined by the time-of-flight method were small, 0.4 % and 0.2 %, respectively. The bac...

We describe the design, construction and performance of the PI-BETA detector built for the precise measurement of the branching ratio of pion beta decay, π + → π 0 e + ν e , at the Paul Scherrer Institute. The central part of the detector is a 240-module spherical pure CsI calorimeter covering ∼3π sr 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 temperaturecontrolled lead brick enclosure which in turn is lined with cosmic muon plastic veto counters. Commissioning and calibration data were taken during two three-month beam periods in 1999/2000 with π + stopping rates between 1.3·10 3 π + /s and 1.3·10 6 π + /s. We examine the timing, energy and angular detector resolution for photons, positrons and protons in the energy range of 5-150 MeV, 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.

We have measured scintillation properties of pure CsI crystals used in the shower calorimeter built for a precise determination of the π + →π 0 e + ν e decay rate at the Paul Scherrer Institute (PSI). All 240 individual crystals painted with a special wavelength-shifting solution were examined in a custom-build detection apparatus (RASTA-radioactive source tomography apparatus) that uses a 137 Cs radioactive gamma source, cosmic muons and a light emitting diode as complementary probes of the scintillator light response. We have extracted the total light output, axial light collection nonuniformities and timing responses of the individual CsI crystals. These results predict improved performance of the 3π sr PIBETA calorimeter due to the painted lateral surfaces of 240 CsI crystals. The wavelength-shifting paint treatment did not affect appreciably the total light output and timing resolution of our crystal sample. The predicted energy resolution for positrons and photons in the energy range of 10-100 MeV was nevertheless improved due to the more favorable axial light collection probability variation. We have compared simulated calorimeter ADC spectra due to 70 MeV positrons and photons with a Monte Carlo calculation of an ideal detector light response.

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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 + π 0 + e + + ν e at the Paul Scherrer Institute (Switzerland). A spherical calorimeter, consisting of 240 crystals of pure CsI scintillator and embracing a solid angle of ~3 π , 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. z ˆ c ˆ cś ˆ c´