J. Kleinfeller

Upper limits for neutrino oscillations ν ¯ μ → ν ¯ e from muon decay at rest

The KARMEN experiment at the spallation neutron source ISIS used \numub from \mup--decay at rest in the search for neutrino oscillations \numubnueb in the appearance mode, with p(\nueb,e+)n as detection reaction of \nueb. In total, 15 candidates fulfill all conditions for the \nueb signature, in agreement with the background expectation of 15.8+-0.5 events, yielding no indication for oscillations. A single event based likelihood analysis leads to upper limits on the oscillation parameters: sin^2(2theta) 100 eV^2 and Dm^2<0.055 eV^2 for sin^2(2theta)=1 at 90% confidence. Thus, KARMEN does not confirm the LSND experiment and restricts significantly its favored parameter region for \numubnueb.

The high resolution neutrino calorimeter KARMEN

Abstract KARMEN is a 56 t scintillation calorimeter designed for beam dump neutrino experiments at the neutron spallation facility ISIS of the Rutherford Appleton Laboratory. The calorimetric properties are demonstrated by cosmic muons and laser calibration. The measured energy resolution of the detector is σ E / E ≈ 11.5%/√ E [MeV], the position resolution σ x = 5 cm and the timing resolution σ t ≈ 350 ps.

Upper limits for neutrino oscillations muon-anti-neutrino ---> electron-anti-neutrino from muon decay at rest

The KARMEN experiment at the spallation neutron source ISIS used \numub from \mup--decay at rest in the search for neutrino oscillations \numubnueb in the appearance mode, with p(\nueb,e+)n as detection reaction of \nueb. In total, 15 candidates fulfill all conditions for the \nueb signature, in agreement with the background expectation of 15.8+-0.5 events, yielding no indication for oscillations. A single event based likelihood analysis leads to upper limits on the oscillation parameters: sin^2(2theta) 100 eV^2 and Dm^2<0.055 eV^2 for sin^2(2theta)=1 at 90% confidence. Thus, KARMEN does not confirm the LSND experiment and restricts significantly its favored parameter region for \numubnueb.

Anomaly in the time distribution of neutrinos from a pulsed beam stop source

Abstract Analysis of the charged and neutral current reactions 12 C (ν e , e − ) 12 N and 12 C (ν, ν′) 12 C ∗ induced by neutrinos from π+- and μ+-decays at rest reveals an anomaly in the time distribution after all π+ have decayed: the measured time constant for subsequent events differs substantially from the value of 2.2 μs corresponding to the μ+ lifetime. This anomaly cannot currently be explained by background processes or errors in the experimental set-up. A satisfactory description of the time spectrum is achieved by assuming it has two components, one exponential with a 2.2 μs time constant, the other a Gaussian signal of 83±28 events at 3.6 μs after beam-on-target. A speculative explanation, but one fully consistent with all the data, is that these delayed events originate from the decay of a slowly moving (β∼0.02) massive neutral particle produced in the beam stop. Further measurements to improve statistical significance are necessary.

First observation of the neutral current nuclear excitation 12C(v,v′)12C∗(1+, 1)

Abstract The neutral current nuclear excitation 12 (v,v′) 12 C ∗ (1 + , 1; 15.1 MeV) has been observed for the first time. For v e and v μ from μ + -decay at rest the flux averaged cross section was determined to be NC (v e + v μ ) > = [10.8±5.1 (stat.) ±1.1 (syst.)]×10 −42 cm 2 .

Cross section of the charged current reaction 12C(ve, e−)12Ng.s.

Abstract The charged current nuclear transition 12 C(v e , e − ) 12 N g.s. has been observed in the KARMEN experiment. The flux average cross section for ve from μ+ decay at rest is determined to be 〈σ〉 = [8.1±0.9(stat.)±0.75 (syst.)]×10−42cm2. For the first time also the energy dependence of the cross section has been measured for neutrino energies up to 50 MeV.

Cross-section of the charged current reaction C-12 (electron-neutrino e-) N-12 (g.s.).

Abstract The charged current nuclear transition 12 C(v e , e − ) 12 N g.s. has been observed in the KARMEN experiment. The flux average cross section for ve from μ+ decay at rest is determined to be 〈σ〉 = [8.1±0.9(stat.)±0.75 (syst.)]×10−42cm2. For the first time also the energy dependence of the cross section has been measured for neutrino energies up to 50 MeV.

Improved limits on nu(e) emission from mu+ decay.

We investigated mu(+) decays at rest produced at the ISIS beam stop target. Lepton flavor (LF) conservation has been tested by searching for nu(e) via the detection reaction p(nu(e),e(+))n. No nu(e) signal from LF violating mu(+) decays was identified. We extract upper limits of the branching ratio (BR) for the LF violating decay mu(+)-->e(+)+nu(e)+nu(-) compared to the standard model (SM) mu(+)-->e(+)+nu(e)+nu(mu) decay: BR<0.9(1.7) x 10(-3) (90% C.L.) depending on the spectral distribution of nu(e) characterized by the Michel parameter rho=0.75(0.0). These results improve earlier limits by one order of magnitude and restrict extensions of the SM in which nu(e) emission from mu(+) decay is allowed with considerable strength. The decay mu(+)-->e(+)+nu(e)+nu(mu) often proposed as a potential source for the nu(e) signal observed in the LSND experiment can be excluded.

Measurement of the weak neutral current excitation 12C(νμνμ′)12C∗(1+,1;15.1MeV) at Eνμ=29.8 MeV

Abstract The weak neutral current reaction 12 C( ν μ , ν μ ′ ) 12 C ∗ (1 + ,1; 15.1MeV) has been observed for the first time in the KARMEN experiment. Neutrino events were separated from background using two different analysis methods. The measured cross section σ NC =(3.2 ± 0.5 stat. ± 0.4 syst. ) × 10 −42 cm 2 for monoenergetic ν μ from π + -decay at rest is in good agreement with the standard model, the isovector-axialvector coupling constant of weak hadronic current deduced from this experiment is | β |=1.11±0.13.

MEASUREMENT OF THE ENERGY SPECTRUM OF VE FROM MUON DECAY AND IMPLICATIONS FOR THE LORENTZ STRUCTURE OF THE WEAK INTERACTION

The KARMEN experiment uses the reaction 12C(\nu_e,e-)12N to measure the energy distribution of \nu_e emitted in muon decay at rest. The \nu_e analog \omega_l of the famous Michel parameter \rho has been derived from a maximum-likelihood analysis of events near the kinematic end point, E_max. The result, \omega_l = (2.7^{+3.8}_{-3.3}\pm 3.1) * 10^{-2}, is in good agreement with the standard model prediction \omega_l= 0. We deduce a 90% confidence upper limit of \omega_l <= 0.113, which corresponds to a limit of |g^S_RL + 2 g^T_RL| <= 0.78 on the interference term between scalar and tensor coupling constants.