W. Hampel

GALLEX solar neutrino observations: Results for GALLEX IV

Abstract We report the GALLEX solar neutrino results for the measuring period GALLEX III, the period from 12 October 1994-4 October 1995. Counting for these runs was completed on 29 March 1996. The GALLEX III result (14 runs) is [53.9 ± 10.6(stat.) ± 3.1 (syst.)] SNU (1σ). This is 15.8 SNU below but statistically compatible with the new combined result for GALLEX (I+II+III) (53 runs), which is [69.7 ± 6.7(stat.) −4.5 +3.9 (syst.)] SNU (1σ) or (69.7 −8.1 +7.8 ) SNU with errors quadratically added. We also give the preliminary result from our second 51 Cr-source experiment: the measured detector response is 83±10% of expectation. The combined result from both GALLEX 51 Cr-source experiments is 92±8% of expectation.

Solar neutrinos observed by GALLEX at Gran Sasso.

We have measured the rate of production of 71Ge from 71Ga by solar neutrinos. The target consists of 30.3 t of gallium in the form of 8.13 M aqueous gallium chloride solution (101 t), shielded by ≈ 3300 m water equivalent of standard rock in the Gran Sasso Underground Laboratory (Italy). In nearly one year of operation, 14 measurements of the production rate of 71Ge were carried out to give, after corrections for side reactions and other backgrounds, an average value of 83 + 19 (stat.) ± 8 (syst.) SNU (1σ) due to solar neutrinos. This conclusion constitutes the first observation of solar pp neutrinos. Our result is consistent with the presence of the full pp neutrino flux expected according to the “standard solar model” together with a reduced flux of 8B + 7Be neutrinos as observed in the Homestake and Kamiokande experiments. Astrophysical reasons remain as a possible explanation of the solar neutrino problem. On the other hand, if the result is to be interpreted in terms of the MSW effect, it would fix neutrino masses and mixing angles within a very restricted range.

Limits on spin-dependent WIMP-nucleon cross sections from 225 live days of XENON100 data

We present new experimental constraints on the elastic, spin-dependent WIMP-nucleon cross section using recent data from the XENON100 experiment, operated in the Laboratori Nazionali del Gran Sasso in Italy. An analysis of 224.6 live days×34 kg of exposure acquired during 2011 and 2012 revealed no excess signal due to axial-vector WIMP interactions with 129Xe and 131Xe nuclei. This leads to the most stringent upper limits on WIMP-neutron cross sections for WIMP masses above 6 GeV/c², with a minimum cross section of 3.5×10(-40) cm² at a WIMP mass of 45 GeV/c², at 90% confidence level.

Reanalysis of the Gallex solar neutrino flux and source experiments

Abstract After the completion of the gallium solar neutrino experiments at the Laboratori Nazionali del Gran Sasso ( Gallex : 1991–1997; GNO: 1998–2003) we have retrospectively updated the Gallex results with the help of new technical data that were impossible to acquire for principle reasons before the completion of the low rate measurement phase (that is, before the end of the GNO solar runs). Subsequent high rate experiments have allowed the calibration of absolute internal counter efficiencies and of an advanced pulse shape analysis for counter background discrimination. The updated overall result for Gallex (only) is 73.4 − 7.3 + 7.1 SNU . This is 5.3% below the old value of 77.5 − 7.8 + 7.5 SNU ( Gallex Collaboration, W. Hampel et al., 1999 [1] ), with a substantially reduced error. A similar reduction is obtained from the reanalysis of the 51Cr neutrino source experiments of 1994/1995.

First results from the 51Cr neutrino source experiment with the GALLEX detector

Abstract The radiochemical GALLEX experiment, which has been measuring the solar neutrino flux since May 1991, has performed an investigation with an intense man-made 51 Cr neutrino source (61.9 ± 1.2 PBq). The source, produced via neutron irradiation of ≈ 36 kg of chromium enriched in 50 Cr, primarily emits 746 keV neutrinos. It was placed for a period of 3.5 months in the reentrant tube in the GALLEX tank, to expose the gallium chloride target to a known neutrino flux. This experiment provides the ratio, R , of the production rate of Cr-produced 71 Ge measured in these source exposures to the rate expected from the known source activity: R = 1.04 ± 0.12. This result not only constitutes the first observation of low-energy neutrinos from a terrestrial source, but also (a) provides an overall check of GALLEX, indicating that there are no significant experimental artifacts or unknown errors at the 10% level that are comparable to the 40% deficit in observed solar neutrino signal, and (b) directly demonstrates for the first time, using a man-made neutrino source, the validity of the basic principles of radiochemical methods used to detect rare events (at the level of 10 atoms or less). Because of the close similarity in neutrino energy spectra from 51 Cr and from the solar 7 Be branch, this source experiment also shows that the gallium detector is sensitive to 7 Be neutrinos with full efficiency.

GALLEX results from the first 30 solar neutrino runs

Abstract We report new GALLEX solar neutrino results from 15 runs covering 406 days (live time) within the exposure period 19 August 1992–13 October 1993 (“GALLEX II”). With counting data considered until 4 January 1994, the new result is [78±13 (stat.) ±5 (stat.)] SNU (1σ). It confirms our previous result for the 15 initial runs (“GALLEX I”) of [81±17( stat .)±9( syst .)] SNU. After two years of recording the solar neutrino flux with the GALLEX detector the combined result from 30 solar runs (GALLEX I + GALLEX II) is [79±10( stat .)±6( syst .)] SNU (1 σ ). In addition, 19 “blank” runs gave the expected null result. GALLEX neutrino experiments are continuing.

Implications of the GALLEX determination of the solar neutrino flux

Abstract The GALLEX result 83 ± 19 (stat.) ± 8 (syst.) SNU is two standard deviations below the predictions of stellar model calculations (124–132 SNU). To fit this result together with those of the chlorine and Kamiokande experiments requires severe stretching of solar models but does not rule out such a procedure, leaving the possibility of massless neutrinos. It clearly implies that the pp neutrinos have been detected. The Mikheyev-Smirnov-Wolfenstein (MSW) mechanism provides a good fit, and the GALLEX result fixes the Δm 2 and sin 2 2 θ parameters in two very confined ranges (around Δm 2 = 6 × 10 −6 eV 2 and sin 2 2 θ = 7 × 10 −3 and around Δm 2 = 8 × 10 −6 eV 2 and sin 2 2 θ = 0.6). Explanations of the solar neutrino problems based on the decay or magnetic interactions of neutrinos are disfavoured.

GALLEX solar neutrino observations: Complete results for GALLEX II

Abstract We report the solar neutrino results from the complete set of runs in the exposure period, GALLEX II, from 19 August 1992 - 23 June 1994. Counting for these runs was completed on 10 December 1994. The GALLEX II result (24 runs) is [75.2±9.7 (stat) −4.6+4.1 (syst)] SNU (1 σ). After three years of recording the solar neutrino flux with the GALLEX detector, the combined result from the 39 completed solar runs (GALLEX I+II) is [77.1±8.5 (stat) −5.4+4.4 (syst) SNU (1 σ) or 77.1 −10.1+9.6 SNU with errors combined in quadrature. The combined error (± 13%) has now approached a level where the limits on the derived contribution of 7Be neutrinos to the GALLEX signal confront the predictions of solar models.

First Axion Results from the XENON100 Experiment

We present the first results of searches for axions and axionlike particles with the XENON100 experiment. The axion-electron coupling constant, g Ae , has been probed by exploiting the axioelectric effect in liquid xenon. A profile likelihood analysis of 224.6 live days × 34-kg exposure has shown no evidence for a signal. By rejecting g Ae larger than 7.7×10 −12 (90% C.L.) in the solar axion search, we set the best limit to date on this coupling. In the frame of the DFSZ and KSVZ models, we exclude QCD axions heavier than 0.3 and 80  eV/c 2 , respectively. For axionlike particles, under the assumption that they constitute the whole abundance of dark matter in our galaxy, we constrain g Ae to be lower than 1×10 −12 (90% C.L.) for masses between 5 and 10  keV/c 2 .

Matter effects for solar neutrino oscillations

Possible solar neutrino oscillations are reviewed in the two-neutrino case taking into account the effect of coherent forward scattering when neutrinos travel through the sun and earth. As recently pointed out by Mikheyev and Smirnov this effect can induce a large suppression of the solar νe flux for values of Δm2 around 10−4–10−8 eV2 even for small values of the mixing angle. It also may cause substantial modifications of the solar neutrino spectrum shape. All this may be used for determining Δm2 and sin2 2θ in a large domain from the experimental results of the chlorine, gallium, indium and heavy water detectors.