We have studied the muon neutrino and antineutrino quasi-elastic (QEL) scattering reactions (ν μ n → μ − p andν μ p → μ + n) using a set of experimental data collected by the NOMAD Collaboration. We have performed measurements of the cross-section of these processes on a nuclear target (mainly carbon) normalizing it to the total ν μ (ν μ ) charged-current cross section. The results for the flux-averaged QEL cross sections in the (anti)neutrino energy interval 3-100 GeV are σ qel ν μ = (0.92 ± 0.02(stat) ± 0.06(syst)) × 10 −38 cm 2 and σ qel ν μ = (0.81 ± 0.05(stat) ± 0.09(syst)) × 10 −38 cm 2 for neutrino and antineutrino, respectively. The axial mass parameter M A was extracted from the measured quasi-elastic neutrino cross section. The corresponding result is M A = 1.05±0.02(stat)±0.06(syst) GeV. It is consistent with the axial mass values recalculated from the antineutrino cross section and extracted from the pure Q 2 shape analysis of the high purity sample of ν μ quasielastic 2-track events, but has smaller systematic error and should be quoted as the main result of this work. Our measured M A is found to be in good agreement with the world average value obtained in previous deuterium filled bubble chamber experiments. The NOMAD measurement of M A is lower than those recently published by K2K and MiniBooNE Collaborations. However, within the large errors quoted by these experiments on M A , these results are compatible with the more precise NOMAD value.PACS 13.15.+g · 25.30.Pt

We present the results of a search for ν µ → ν e oscillations in the NOMAD experiment at CERN. The experiment looked for the appearance of ν e in a predominantly ν µ wide-band neutrino beam at the CERN SPS. No evidence for oscillations was found. The 90% confidence limits obtained are m 2 < 0.4 eV 2 for maximal mixing and sin 2 (2θ) < 1.4 × 10 −3 for large m 2. This result excludes the LSND allowed region of oscillation parameters with m 2 10 eV 2 .

No abstract

We report on a search for heavy neutrinos (ν 4 ) produced in the decay D s → τ ν 4 at the SPS proton target followed by the decay ν 4 → ν τ e + e − in the NOMAD detector. Both decays are expected to occur if ν 4 is a component of ν τ . From the analysis of the data collected during the 1996-1998 runs with 4.1 × 10 19 protons on target, a single candidate event consistent with background expectations was found. This allows to derive an upper limit on the mixing strength between the heavy neutrino and the tau neutrino in the ν 4 mass range from 10 to 190 MeV. Windows between the SN1987a and Big Bang Nucleosynthesis lower limits and our result are still open for future experimental searches. The results obtained are used to constrain an interpretation of the time anomaly observed in the KARMEN1 detector.Key words: neutrino mixing, neutrino decay IntroductionIn the Standard Model all fundamental fermions have a right-handed component that transforms as an isosinglet under the SU(2) L gauge group except neutrinos, which are observed only in left-handed form. However, heavy neutrinos which are decoupled from W and Z bosons and hence are mostly isosinglet (sterile) arise in many models that attempt to unify the presently known interactions into a single gauge scheme, such as Grand Unified Theories or Superstrings inspired models [1]. They are also predicted in models trying to solve the problem of baryo-or leptogenesis in the Universe, in many extended electroweak models, such as left-right symmetric and see-saw models [1]. Their masses are predicted to be within the GeV − TeV range. The existence of a light ( eV or ≪ eV) sterile neutrino is expected in schemes that attempt to solve the presently observed indication from atmospheric, solar and LSND experiments that neutrinos are massive, see e.g. [2] and references therein. More generally one can also look for an isosinglet neutrino with intermediate mass such as in the keV − MeV range. For instance, such neutrinos with masses in the range 1 -40 keV were recently considered as a candidate for warm dark matter [3].If heavy neutrinos exist, many crucial questions arise. For example, for massive neutrinos the flavour eigenstates (ν e , ν µ , ν τ , ...) need not coincide with the mass eigenstates (ν 1 , ν 2 , ν 3 , ν 4 ...), but would, in general, be related through a unitary transformation. Such a generalised mixing:could result in neutrino oscillations when the mass differences are small, and in decays of heavy neutrinos when the mass differences are large. The motivation and purpose of this work is to search for a neutral heavy lepton ν 4 which is dominantly associated with the third generation of light neutrinos, ν τ , via the mixing term |U τ 4 | 2 . If such a particle exists it might be produced in the decay D s → τ ν 4 at the SPS proton target followed by the decay ν 4 → ν τ e + e − in the NOMAD detector as is illustrated in Figure 1 (see also Section 3). The experimental signature of these events is clean and they can be selected with small background due t...