Measurement of the neutrino asymmetry parameter B in neutron decay

A new measurement of the neutrino asymmetry parameter B in neutron decay, the angular correlation between neutron spin and antineutrino momentum, is presented. The result, B=0.9802(50), confirms earlier measurements but features considerably smaller corrections. It agrees with the standard model expectation and permits updated tests on "new physics" in neutron decay.     

Energy dependence of the beta asymmetry in the decay of 8Li

The energy dependence of the asymmetry in the beta decay of ⁸ Li to the first excited state of ⁸Be has been measured. Expectations based on the conserved-vector-current hypothesis and the spherical shell model are in good agreement with the measured asymmetry for beta energies above ∼5 MeV. No second class currents are needed to explain the data but the present experiment is not a very sensitive test.     

Measurement of the Michel parameter rho in muon decay

The TWIST Collaboration has measured the Michel parameter rho in normal muon decay, mu(+)--> e(+)nu(e)nu (mu). In the standard model, rho = 3/4. Deviations from this value imply mixing of left- and right-handed muon and electron couplings. We find rho=0.750 80+/-0.000 32(stat) +/- 0.000 97(syst) +/- 0.000 23, where the last uncertainty represents the dependence of rho on the Michel parameter eta. This result sets new limits on the W(L)-W(R) mixing angle in left-right symmetric models.     

Measurement of the neutron lifetime using a proton trap

We report a new measurement of the neutron decay lifetime by the absolute counting of in-beam neutrons and their decay protons. Protons were confined in a quasi-Penning trap and counted with a silicon detector. The neutron beam fluence was measured by capture in a thin 6LiF foil detector with known absolute efficiency. The combination of these simultaneous measurements gives the neutron lifetime: tau(n)=(886.8+/-1.2[stat]+/-3.2[syst]) s. The systematic uncertainty is dominated by uncertainties in the mass of the 6LiF deposit and the 6Li(n,t) cross section. This is the most precise measurement of the neutron lifetime to date using an in-beam method.     

Constraints on the decay period of a free neutron from the characteristics of tritium beta decay

The reduced decay period of the triton, (ft _1/2)_T=(1129.6±3)s, and the free-neutron decay period, (t _1/2)_n=(616.7±;2.7±1.3) s, are determined from the experimental and theoretical values of the chemical shifts of atomic and molecular tritium.     

Gamma-ray emission in competition with neutron emission in the beta decay of Br

Evidence is presented for the existence of a γ-ray at 5594.7±0.6 keV following beta decay of ⁸⁷Br. This observation indicates population of neutron unbound levels in ⁸⁷Kr from which photon emission competes successfully with neutron emission.     

Structure of weak nucleon currents and charge asymmetry in nuclear beta decay

Structure of weak nucleon currents is studied from the beta decays in the A=12 system. We have investigated the experimental data on the beta-ray angular distribution in the aligned ¹²B and ¹²N recently given by the Osaka group with our formulas to find a new limit for the induced tensor current. A possible difference of the ratio y of the axial charge to Gamow-Teller matrix elements between the beta decays of ¹²B and ¹²N is calculated theoretically in connection with the ft-value asymmetry, since it affects the evaluation of the strength of the induced tensor current.     

Measurement of the time-dependent CP asymmetry in the B0-->phiK0 decay

We present a measurement of the time-dependent CP asymmetry for the neutral B-meson decay B0-->phiK0. We use a sample of approximately 114 x 10(6) B-meson pairs taken at the Upsilon(4S) resonance with the BABAR detector at the PEP-II B-meson factory at SLAC. We reconstruct the CP eigenstates phiK0S and phiK0L, where phi-->K+K-, K0S-->pi+pi-, and K0L is observed via its hadronic interactions. The other B meson in the event is tagged as either a B0 or Bbar0 from its decay products. The values of the CP-violation parameters are SphiK=0.47+/-0.34(stat)+0.08-0.06(syst) and CphiK=0.01+/-0.33(stat)+/-0.10(syst).