J.-L. Vuilleumier

Limits on cold dark matter from the Gotthard Ge experiment

Abstract Data from a high purity Ge detector operated in the Gotthard underground laboratory, corresponding to 51.6 kg day, were used to set limits on cold dark matter. In particular, Dirac neutrinos with masses between 10 and 2400 GeV are ruled out. A new lower limit on the half-life for the decay of the electron into weakly interacting particles of 1.9 × 1023 yr at 68% CL was also derived.

Search for ββ decay in 136Xe: new results from the Gotthard experiment

Abstract The Gotthard Xe experiment has taken an additional 6013 hours of data with an upgraded readout system. Lower limits for the half-life of the double beta decay in 136 Xe are T 1/2 0 ν >4.4×10 23 yr (including 6830 hours taken in an earlier phase), T 1/2 0 νχ 0 >0.72×10 22 yr and T 1/2 2 ν >3.6×10 20 yr at 90% confidence level. Alpha spectroscopy, as well as βα and αα coincidence techniques lead to a determination of the activity of the gas of 10 −12 g/g effective of 232 Th and 238 U. Evaluations of background strongly suggest that an appreciable fraction of the ββ -candidates comes from the allowed 2 ν decay.

A search for neutrino oscillations by measurement of the νe spectra at two distances from a nuclear reactor

Abstract The positron spectrum from the reaction ν e + p → e + + n has been measured at L = 45.9 m from the core of the Gosgen power reactor. The data, combined with those of our previous measurement at 37.9 m, are analysed in terms of two-neutrino oscillations of the type ν e ↔ x . This analysis is essentially independent of external data on the spectral distribution of reactor neutrinos. No evidence for neutrino oscillations was found. A comprehensive analysis including data on the spectral distribution yields upper limits for the oscillation parameters of Δm 2 ⩽ 0.016 eV 2 (90% CL for full mixing, and sin 2 2 θ ⩽ 0.16 (90% CL) for large Δm 2 .

New limits on oscillation parameters for electron antineutrinos

Abstract The positron spectrum from the reaction ν e + p → e + + n has been measured at L=37.9 from the core of the Gosgen power reactor to search for neutrino oscillations of the type ν e ↔ x in the low mass parameter range. The results are consistent with the no neutrino oscillation hypothesis. Upper limits of δm 2 ⪕ 0.016 eV 2 (90% c.l..) for full mixing, and sin 2 2θ⪕0.17 (90% c.l.) in the limit of large mass parameters were determined. For the ratio of the integrated experimental yield to that predicted for the case of no oscillations a value of 1.05 ± 0.02 (statistical) ± 0.05 (systematic; 68% c.l.) was obtained.

Experimental study of neutrino oscillations at a fission reactor

Abstract The electron-antineutrino spectrum has been measured at a position 8.75 m from the “point-like” core of the ILL 235 U fission reactor, using the reaction ν e + p → e + + n. Positrons and neutrons were detected in coincidence by means of a low-background liquid scintillator and a 3 He detector system. The observed neutron correlated positron spectrum is consistent with theoretical predictions assuming no neutrino oscillations. Upper limits for the oscillation parameter are presented.

Absolute measurement of the beta spectrum from 235U fission as a basis for reactor antineutrino experiments

The beta spectrum from fission products in thermal-neutron induced fission of 235U has been measured on line at the ILL reactor with a magnetic spectrometer. In the energy range from Eβ = 2.0 to 7.5 MeV the spectrum was determined per fission with an accuracy of 5%. The experimental electron spectrum was converted to the corresponding electron antineutrino spectrum to form a basis for the study of antineutrino induced reactions at reactors.

A search for double beta decay in 76Ge

This thesis describes a search for the double beta (ββ) decay of ⁷⁶Ge using an narray of eight high purity, high resolution germanium detectors containing a total nof 3.9 x 10²⁴ ⁷⁶Ge nuclei. There are three modes of ββ decay: neutrinoless (0ѵ)ββ ndecay, ββ decay with Majoron emission (x⁰ββ decay) and two neutrino (2ѵ)ββ ndecay. The first two modes violate lepton number conservation, while the third is nallowed by the Standard Model. 0ѵββ decay may take place to the ground state of nthe daughter nucleus (0⁺ → 0⁺ transition) or to an excited state of the daughter nnucleus (0⁺ → 2⁺ transition). The detector was operated for a total time of 2033 h n(0.23 y) which translates to 1.30 kg-y. The background at the ββ decay transition nenergy was 0.53 counts keV⁻¹ y⁻¹ (10²³ ⁷⁶Ge nuclei)⁻¹. No evidence for double beta ndecay of any sort was found and the half life limits are T0ѵ½(0⁺ → 0⁺) > 1.2 x 10²³ y, nT0ѵ½(0⁺ → 2⁺) > 2 x 10²² y, Tχo½ > 8 x 10²⁰ y, and T2ѵ½ > 2 x 10²⁰ y, all at 90% c.l. nThe limit for the neutrinoless mode translates to an upper limit of between 16 and n1.6 eV on the Majorana mass of the neutrino, depending on the nuclear matrix nelement used.

An experimental limit on production of axions in a fission reactor

Abstract We report on an experiment conducted at the ILL reactor aimed at searching for coincident gamma-ray events from axion decay. No such events are seen and our results are interpreted, using the calculations by Donnelly et al., as an upper limit of the axion mass of 280 keV.

Search for Axions in Specific Nuclear

Abstract A search for axions was performed at a 2.8 GWth light-water power reactor by investigating in particular their production in specific nuclear transitions. Limits for the product (axion coupling) · (decay width) are derived, ruling out axions as described in the standard model.

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Abstract A search for majoron emission accompanying neutrinoles double beta decay in 76 Ge was conducted with a low background germanium detector in the St. Gotthard underground laboratory. No evidence for neutrinoless double beta decay with or without majoron emission was found, with a lower limit for the half life of the majoron branch of t 1 2 > 1.2 × 10 21 yr at 90% confidence level.