Background: The rate lambda(pp mu) characterizes the formation of pp mu molecules in collisions of muonic p mu atoms with hydrogen. In measurements of the basic weak muon capture reaction on the proton to determine the pseudoscalar coupling g(P), capture occurs from both atomic and molecular states. Thus knowledge of lambda(pp mu) is required for a correct interpretation of these experiments.Purpose: Recently the MuCap experiment has measured the capture rate Lambda(S) from the singlet p mu atom, employing a low-density active target to suppress pp mu formation [V. Andreev et al. (MuCap Collaboration), Phys. Rev. Lett. 110, 012504 (2013)]. Nevertheless, given the unprecedented precision of this experiment, the existing experimental knowledge in lambda(pp mu) had to be improved.Method: The MuCap experiment derived the weak capture rate from the muon disappearance rate in ultrapure hydrogen. By doping the hydrogen with 20 ppm of argon, a competing process to pp mu formation was introduced, which allowed the extraction of pp mu from the observed time distribution of decay electrons.Results: The pp mu formation rate was measured as lambda(pp mu) = (2.01 +/- 0.06(stat) +/- 0.03(sys)) x 10(6) s(-1). This result updates the pp mu value used in the abovementioned MuCap publication.Conclusions: The 2.5x higher precision compared to earlier experiments, and the fact that the measurement was performed under nearly identical conditions as the main data taking, reduces the uncertainty induced by lambda(pp mu) to a minor contribution to the overall uncertainty of Lambda(S) and g(P), as determined in the MuCap experiment. Our final value for lambda(pp mu) shifts Lambda(S) and g(P) by less than one-tenth of their respective uncertainties compared to our results published earlier.

The Michel parameter xi" has been determined from a measurement of the longitudinal polarization of positrons emitted in the decay of polarized and depolarized muons. The result, xi" = 0.981 +- 0.045stat +- 0.003syst, is consistent with the Standard Model prediction of unity, and provides an order of magnitude improvement in the relative precision of this parameter. This value sets new constraints on exotic couplings beyond the dominant V-A description of the leptonic weak interaction.

The MuCap experiment at the Paul Scherrer Institute has measured the rate Λ(S) of muon capture from the singlet state of the muonic hydrogen atom to a precision of 1%. A muon beam was stopped in a time projection chamber filled with 10-bar, ultrapure hydrogen gas. Cylindrical wire chambers and a segmented scintillator barrel detected electrons from muon decay. Λ(S) is determined from the difference between the μ(-) disappearance rate in hydrogen and the free muon decay rate. The result is based on the analysis of 1.2 × 10(10) μ(-) decays, from which we extract the capture rate Λ(S) = (714.9 ± 5.4(stat) ± 5.1(syst)) s(-1) and derive the proton's pseudoscalar coupling g(P)(q(0)(2) = -0.88 m(μ)(2)) = 8.06 ± 0.55.

We propose to measure the rate for muon capture on the deuteron to better than 1.5 process on a nucleus that can both be calculated and measured to a high degree of precision. The measurement will provide a benchmark result, far more precise than any current experimental information on weak interaction processes in the two-nucleon system. Moreover, it can impact our understanding of fundamental reactions of astrophysical interest, like solar pp fusion and the ν+d reactions observed by the Sudbury Neutrino Observatory. Recent effective field theory calculations have demonstrated, that all these reactions are related by one axial two-body current term, parameterized by a single low-energy constant. Muon capture on the deuteron is a clean and accurate way to determine this constant. Once it is known, the above mentioned astrophysical, as well as other important two-nucleon reactions, will be determined in a model independent way at the same precision as the measured muon capture reaction.

The rate of nuclear muon capture by the proton has been measured using a new technique based on a time projection chamber operating in ultraclean, deuterium-depleted hydrogen gas, which is key to avoiding uncertainties from muonic molecule formation. The capture rate from the hyperfine singlet ground state of the microp atom was obtained from the difference between the micro(-) disappearance rate in hydrogen and the world average for the micro(+) decay rate, yielding Lambda(S)=725.0+/-17.4 s(-1), from which the induced pseudoscalar coupling of the nucleon, g(P)(q(2)=-0.88m(2)(micro))=7.3+/-1.1, is extracted.

The rate of nuclear muon capture by the proton has been measured using a new technique based on a time projection chamber operating in ultraclean, deuterium-depleted hydrogen gas, which is key to avoiding uncertainties from muonic molecule formation.The capture rate from the hyperfine singlet ground state of the p atom was obtained from the difference between the ÿ disappearance rate in hydrogen and the world average for the decay rate, yielding S 725:0 17:4 s ÿ1 , from which the induced pseudoscalar coupling of the nucleon, g P q 2 ÿ0:88m 2 7:3 1:1, is extracted.

The positrons and coincident gamma-rays following the pure Fermi 0+ → 0+ branch of β+-decay in 14O were measured. The Doppler shift of the γ-quantum observed is sensitive to the beta-neutrino correlation coefficient, which is unity (α = 1) in the absence of scalar coupling. The interatomic interaction of recoiling daughters was noticed, which has prevented to obtain an improved result for scalar coupling despite of the excellent statistical accuracy of the measurement. At the same time, it was shown that the method described can be used as a very sensitive and flexible tool to probe the interatomic interactions in matter at the hundred eV energy level. Perspectives in this direction as well as the future angular-correlation experiments aimed to search for scalar coupling in beta-decay are discussed.

This paper reports a precision measurement of the ratio of the exclusive capture rates from the two hyperfine levels of the muonic B-11 atom leading to the excited level of Be-11 at 320 keV: lambda(+)/lambda(-) = 0.028 +/- 0.021 (stat.) +/-0.003 (syst.). Using the most up-to-date halo wave functions of the nuclei involved, the induced pseudoscalar to axialvector form factor ratio is obtained as g(P)/g(A)=4.3(-4.3)(+2.8)(stat.) +/-0.5 (syst.). This value agrees with the partial conservation of the axial current prediction and does not reproduce the anomaly observed in radiative muon capture on hydrogen. The hyperfine conversion rate between the two hyperfine levels is also determined, R = [18 +/- 16(stat.) +/-1 (syst.)] x 10(3) s(-1). The total capture rate from the lower hyperfine level and its difference compared to the upper one were also determined: lambda(C)(-) =[23.53 +/- 0.72(stat.) +/-0.2.5(syst.)] x 10(3) s(-1) and Deltalambda=[-13.2+/-1.7(stat.) +/-0.7(syst.)] x 10(3) s(-1).

A precision measurement of the 277 keV gamma ray produced by capturing muons in gaseous oxygen was performed using high-resolution HPGe detectors. The Doppler-broadened shape of this line is sensitive to the possible admixture into muon capture of genuine scalar interaction. This experiment complements, in the muon sector, in principle, similar ones undertaken recently in nuclear beta decay.Using a procedure discussed in earlier papers, a fit to the experimental line-shape allowed us to obtain for the recoil-gamma correlation coefficient the value of a(2)(1) = 0.096 +/- 0.041 (95% CL). The available evaluations of the contributing nuclear matrix elements, performed in the impulse approximation in the absence of scalar interaction, produce correlation coefficients in disagreement with our result. These evaluations are independent of the induced pseudoscalar coupling and so of the validity of the PCAC hypothesis, but their reliability and model dependence remain to be investigated. (C) 2002 Elsevier Science B.V. All fights reserved.

The β–ν angular correlation in the β + decay of 18 Ne and 14 O has been investigated using a new experimental technique. The technique is based on the precise measurement of the energy Doppler shift of a γ-quantum following the β ± decay. The measurement with the 18 Ne isotope gives the β–ν angular correlation coefficient α = +1.06 ±0.19 which corresponds to a constraint on the scalar interaction of √{|C_S| 2 +|C'_S| 2 } ≤ 0.29| C V |. An inter-atomic interaction between the 14 O daughter atom ( 14 N) and a CO complex has been studied in a measurement with the 14 O isotope.

The WITCH (Weak Interaction Trap for CHarged particles) experiment is a retardation spectrometer coupled to a Penning trap and measures the beta-neutrino angular correlation via the shape of the recoil energy spectrum. The present form of the Standard Model describes weak processes in terms of vector and axial-vector type interactions, but the possible presence of scalar and tensor interactions is not yet ruled out. The main aim of this experiment is a test of the Standard Model for possible admixture of scalar and tensor currents.

This work was carried out as part of a project aiming at a greatly improved measurement of the muon capture rate from the singlet state of the μp atom. The experiment will be performed at the intense muon beam of PSI using a new experimental method allowing high precision measurements of the lifetime of muons stopped in ultra-pure deuterium-depleted hydrogen (protium). The basic element of the detector is a time projection chamber operating in hydrogen gas at 10bar pressure. The arrival times and trajectories of the incoming muons and the outgoing decay electrons are measured with this device providing effective suppression of background. The system of chambers and electronics is designed for the large muon stop rates required for attaining high statistical accuracy. During four beam periods at PSI, data were taken. Also, various studies of the MWPC performance in hydrogen were made including ageing studies of the chambers under irradiation with stopped muons and with alpha and beta sources. It was demonstrated that the MWPCs can operate in pure hydrogen under 10 bar pressure with gas gains up to 5000, which is sufficient for the detection of relativistic electrons.

The angular correlations were investigated measuring with high-precision HPGe detectors γ-lines following the ordinary muon capture (OMC) on 28 Si and 16 O. The Doppler-broadened shapes of these lines are sensitive to the weak couplings as well as to the nuclear matrix elements (NME) of OMC transitions. A considerable quenching of the induced pseudoscalar form-factor g P and a nonzero value of sum of scalar couplings C S + g S are reported contradictory to the postulates of the Standard Model. However, the questions of reliability of the NME used to extract the weak couplings from the experimentally measured correlation parameters are still open. Future steps in order to solve this problem are discussed.

The study of pure leptonic electroweak processes, such as muon decay, is appropriate to test the Standard Model of the electroweak interactions as well as some of its extensions since the radiative corrections are negligible (and understood) at this level of precision. Unlike most of the Michel parameters used to describe muon decay, the uncertainty on the experimental value of xi " that fits the Standard Model V-A interaction, is large (xi " = 0.65 +/-0.36) [1]. In order to improve the accuracy of this value, we have measured the longitudinal polarisation, FL, of positrons emitted in the decay of polarized muons as a function of, the positron energy. In fact, the value of P-L, at the positron endpoint energy equal to unity in the Standard Model, decreases for high energy positrons emitted antiparallel to the muon spin if the combination of Michel parameters xi "/xi xi' - 1 deviates from the Standard Model value, i.e. from zero. We plan to improve the precision on xi " to 0.5%. Preliminary results are described.

A new experiment is under preparation with the aim to improve considerably the present knowledge of the rate Λ s , which should be measured on a level of 1% or better, for the basic electroweak capture reaction of a negative muon on the free proton μ p 1 s → n + ν μ . The capture rate will be determined by measuring the lifetime of μ − stopped in ultra pure hydrogen at 10 bar pressure and comparing it with the lifetime of the unbound μ + . A new experimental method was developed for this project which should allow measuring the μ − lifetime with at least 10 ppm precision. The basic element of the detector is operating in the hydrogen gas time projection chamber (TPC) surrounded by multi-wire proportional chambers (MWPCs) and scintillator counters. The arrival times and trajectories of the incoming muons and the outgoing decay electrons are measured with this device providing effective suppression of background. Using the TPC as an active target, we can monitor on-line the protium contamination by impurities with a sensitivity better than 10 −8 . This can be done by detecting the charged products of the muon capture reaction on these impurities. It was demonstrated that the TPC and MWPCs can operate in pure hydrogen under 10 bar pressure providing gas gain up to 10 000.

Two experiments to search for new physics beyond the standard model for electroweak interactions by measuring correlations between different spin and momentum vectors in nuclear β-decay are discussed. In the first experiment the correlation between the emission asymmetry and the longitudinal polarisation of positrons emitted by polarised nuclei is determined. This type of measurement is sensitive to the presence of right-handed currents but also to possible scalar and tensor-type currents in the weak interaction. The aim of the second experiment is to determine the βν-correlation in β-decay by measuring the energy spectrum of the recoil ions, using a Penning trap and a retardation spectrometer. In this case the focus is on the search for scalar currents in the weak interaction. The results of the experiments presented here are complementary to results from experiments in muon decay and at high-energy colliders.

Muon decay allows for the most precise experiments in testing the Standard Model of electroweak interactions. An experiment aimed to measure one of the Michel parameters with an improvement of 70 is described. A production run is foreseen for summer 2000.

We report on a recent experiment searching for right-handed currents in the Sb-118 nuclear beta -decay. The correlation between the spin polarization and the beta -emission asymmetry of the positrons from the Sb-118 decay is sensitive to the helicity structure of the weak interaction. A precision measurement of this correlation improves the limit on right-handed currents in beta -decay.

We describe an improved follow-up of our previous spin-neutrino correlation experiment, where the 1229 and 2171 keV gamma-rays emitted after the 28Si(μ,ν)28Al(1+, 2202 keV) reaction were observed by high-resolution HPGe detectors at different angles with respect to the muon spin. In the experiment described here, a magnetic field was used both to select events according to the spin-gamma angle and to measure the residual muon polarization by the muSR method.