T. Kozlowski

Test of lepton-flavour conservation in μ → e conversion on titanium☆

Abstract A search for μ → e conversion in muonic atoms is being performed at PSI with the SINDRUM II spectrometer. A first measurement on Ti gives upper limits on the branching ratios for the ground-state transitions of Γ(μ − Ti → e − Ti g.s. ) Γ(μ − Ti capture ) −12 and Γ(μ − Ti → e + Ca g.s. ) Γ(μ − Ti capture ) −12 (90% confidence ) . With the assumption of a giant resonance excitation of the Ca nucleaus the limit on the total rate for μ − → e + conversion is Γ(μ − Ti → e + Ca ∗ ) Γ(μ − Ti capture ) −11 .

Measurement of the decay π+→e+νee+e− and search for a light Higgs boson

Abstract The radiative pion decay π + →e + ν e e + e − has been measured and values for the weak form factors F V , F A and R have been obtained. The measured branching ratio is Λ ( π + →e + ν e e + e − )/ Λ ( π + → μ + ν μ )=(3.2±0.5)×10 −9 . A light Higgs boson h with standard couplings to quarks is excluded in the mass range 10 MeV/ c 2 ⩽ m h ⩽110 MeV/ c 2 at 90% CL.

First observation of the decay π+ → e+νe+e− and a determination of the form factors FV, FA, R☆

Abstract The radiative pion decay π+ → e+νe+e- has been observed for the first time. Information on the form factors was obtained from the kinematical distribution of 79 events. The vector form factor has the same sign as the pion decay constant fπ. The ratios of axial-vector form factors to the vector form factor are γ = F A F V = 0.7 ± 0.5 and ξ = R F V = 2.3 ± 0.6. The value for γ is in agreement with the small positive value (0.53 ± 0.06) and excludes the large negative value (−2.49 ± 0.06) of γ obtained in π+ → e+νγ experiments.

Energy spectra and asymmetries of neutrons emitted after muon capture

Abstract Inclusive energy spectra and the energy dependence of the asymmetry parameter have been measured for neutrons emitted after muon capture in O, Si, Ca and Pb. In addition a neutron-neutron coincidence measurement has been performed for Ca. The results of these measurements confirm the exponential shape of the energy spectra and the positive asymmetry observed by Sundelin and Edelstein. The observation of neutrons with energies above 50 MeV and the observation of simultaneous emission of two high-energy neutrons suggest the importance of short-range nucleonnucleon correlations in this process.

Limits for short-lived neutral particles emitted in μ+ or π+ decay

Abstract In the data taken for the μ + → e + e + e - and π + → e + ν e + e - experiments a search was made for neutral particles φ produced in the decay μ + → e + φ or π + → e + νφ with subsequent decay φ → e + e - . No candidates were found, Upper limits for the branching ratios are given as a function of their masses and lifetimes. For lifetimes below 10 −10 s limits down to 2 × 10 −12 are obtained.

Observation of particle unstable 4H in pion absorption in 7Li

Abstract A particle unstable state of 4H has been identified in the reaction π − + 7 Li → 4 H+t , 4 H → t +π by observing the two emitted tritons. A resonance energy EΓ=(2.7±0.6) MeV and a reduced width γ2=(2.3±0.6) MeV were deduced, in agreement with the parameters of the ground-state level (Jπ=2−) found in the p-wave of n-t-scattering. The yield per stopped pion is (5.6±1.8)×10−4. Previous measurements in pion absorption are in disagreement with our results.

New Precise Measurement of the Pion Weak Form Factors in the Pi+ -> e+ nu gamma Decay

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.

Coincident emission of neutrons and charged particles after π− absorption in 6Li, 7Li, 12C, 59Co and 197Au

Abstract Energy spectra d 3 W /d E 1 d E 2 d cos ϑ of neutron-neutron and neutron-charged particle pairs (nn, np, nd, nt) emitted after the absorption of stopped negative pions in 6 Li, 7 Li, 12 C, 59 Co and 197 Au have been measured for opening angles between 50° and 180°. Particle pairs emitted from light nuclei without energy loss due to final-state interactions were identified by the energy balance. Pion absorption on two nucleons (quasi-deuteron mechanism) was found to be the main process. The yields of nucleon pairs from quasi-free absorption on a quasi-deuteron are 0.52 ± 0.09 per π-stop, 0.57 ± 0.10 per π-stop and 0.19 ± 0.04 per π-stop for 6 Li, 7 Li and 12 C, respectively. The ratio R of np to pp pairs that absorb the pion was determined for quasi-free absorption on p-shell nucleons of 12 C: R (p-shell) = 6.3 ± 1.4. For pion absorption on s-shell nucleons of lithium lower limits R (s-shell) > 4.4 for 6 Li and R (s-shell) > 7.8 for 7 Li were determined. These values are larger than the statistical ratios R stat = 2 N /( Z − 1) of np to pp pairs in the nucleus. Lower limits for the probability for pion absorption on heavier clusters ( 3 He, α) are W (hc) > 0.16 per π-stop for 6 Li, W (hc) > 0.11 per π-stop for 7 Li and W ( α ) > 0.01 per π-stop for 12 C. The shape of the energy spectra of pairs emitted directly after quasi-free absorption on 2 or 4 nucleons could be explained by a simple model for the momentum distribution of the absorbing nucleons. The results are in agreement with the parameters of the momentum distributions known from other reactions. Energy spectra and opening-angle distributions were compared with the intranuclear cascade calculations of Chiang, Hufner and Hachenberg. For 59 Co and 197 Au the spectra are strongly influenced by the final-state interactions; lower limits of R > 1.8 for 59 Co and R > 2.5 for 197 Au were determined.

Precise Measurement of the π+ → e+v Branching Ratio

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.

New Precise Measurement of the Pion Weak Form Factors in π+ → e+νγ Decay

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.