René Prieels

Measurement of the Rate of Muon Capture in Hydrogen Gas and Determination of the Proton's Pseudoscalar Coupling g P

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.

Measurement of muon capture on the proton to 1% precision and determination of the pseudoscalar coupling gP

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 protons pseudoscalar coupling g(P)(q(0)(2) = -0.88 m(μ)(2)) = 8.06 ± 0.55.

Measurement of the induced pseudoscalar form factor in the capture of polarized muons by Si nuclei

Abstract The observation with high-resolution Ge detectors of the 1229 and 2171 keV gamma-rays emitted after the 28 Si(μ, ν) 28 Al(1 + , 2202 keV) reaction allowed us to determine the nuclear-amplitude ratio x ≡ M(2) M(−1) characteristic of this muon-capture reaction. Using the nuclear-structure dependent evaluation of various correction terms (model of Ciechanowicz), our result x = +0.254±0.034 provides for the induced pseudoscalar coupling g p the ratio g p g a (0.848m μ 2 ) = 3.4 ± 1.0 , to be compared to the PCAC prediction of g p g a = 7 . The model dependence of this apparent quenching remains to be assessed.

Is there an indication for the existence of right-handed weak currents in nuclear beta decay?

The authors examine recent claims that experiments on neutron-decay are at variance with the standard V-A helicity structure of the standard model. The analysis presented here combines the neutron data with other recent experimental results and theoretical elaborations in the field of nuclear beta decay. If interpreted in the framework of left-right symmetric models, the data favour the existence of a right-handed weak gauge boson with mass between 207 and 369 GeV c2 at 95% CL. The fit using the standard V-A helicity structure has a chi 2 of 8.52 for four degrees of freedom and excludes the V-A solution at 2.53 standard deviations. This exclusion is due mainly to the new neutron asymmetry data and is in disagreement with the conclusions drawn from muon decay and from the KL-KS mass difference in the framework of manifest left-right symmetric models.

Positron polarization in the decay of polarized N-12: a precision test of the Standard Model

The longitudinal polarization of positrons emitted along and opposite to the nuclear spin direction has been measured in the decay of polarized N-12. The results are consistent with the Standard Model prediction. In manifest left-right symmetric models this measurement provides a lower bound of 310 GeV/c(2) at 90% CL on the mass of a possible right-handed gauge boson contributing to the electroweak interaction. In generalized versions of this model our measurement provides constraints complementary to those set by high-energy experiments

Searches for admixture of massive neutrinos into the electron flavour

The authors have searched for the admixture of massive neutrinos in the beta-spectrum of /sup 20/F measured in this laboratory. No indication was found for such an admixture; the corresponding constraints are compared to those deduced from their analysis of recently updated rates of superallowed Fermi decays and other relevant information.

Search for right-handed weak currents in the beta-asymmetry-polarization correlation from N-12 decay

Abstract The polarization of positrons emitted along and opposite to the nuclear spin direction has been measured in the decay of polarized 12 N. The results are consistent with the Standard Model prediction. In manifest left-right symmetric models this measurement provides a lower limit of 220 GeV/c 2 at 90% CL on the mass of an eventual right-handed gauge boson contributing to the electroweak interaction.

The Threshold Photoproduction of pi0 on Nucleons and on Few Nucleon Systems

Abstract The absolute value of the π 0 photoproduction cross section on the proton recently measured near threshold enables to reanalyze previous data collected on 2 H, 3 He, and 4 He relatively to the proton. Absolute cross sections are presented for these nuclei in the energy region extending up to 10 MeV above threshold. The threshold s-wave amplitudes for 2 H and 3 He thus obtained are discussed in relation with the neutron threshold amplitude E ( nπ 0 ) 0+ value.

MEASUREMENT OF THE F-20 BETA SPECTRUM: A LOW-ENERGY TEST OF THE STANDARD ELECTROWEAK GAUGE MODEL

Abstract We measured the energy spectrum of the beta-rays emitted in the disintegration of the nucleus 20 F in order to compare it to the prediction of the conserved isovector current theory of weak interaction (CVC). The improvements over existing measurements concern the target-material, the spectrometer, the detector-design and the normalization-procedure. Denoting the spectra shape factor as: C ( E ) = 1 + C 1 E + C 2 E 2 + C 3 / e , where E is the total energy expressed in units of the electron mass m 0 , we obtained: C 1 = (5.7±0.4)10 −3 / m 0 c 2 , to be compared to the CVC prediction of: C 1,cvc = (6.0±0.3)10 −3 / m 0 C 2 .

Fundamental weak interaction studies using polarised nuclei and ion traps

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.