Solar neutrinos: Experiments

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Experimental results

The experimental research on solar neutrinos started on late sixties with the experiment at Homestake. It employs a radiochemical method, where a neutrino may induce a nuclear reaction
Cl-Ar
and the resulting argon nuclei are counted by their radioactive decay.

The three gallium experiments, SAGE, Gallex and GNO use a similar technique, using the reaction
Ga-Ge

Kamiokande and SuperKamiokande are water Cerenkov detectors looking mainly for the elastic scattering process. The charged lepton is detected by its Cerenkov radiation. These detectors can measure the energy and direction of the neutrino at real time. Because of the angular resolution they can distinguish neutrinos coming from the Sun from the background.
SNO is also a Cerenkov detector, filled with heavy water. It can distinguish charged current and neutral current reactions. Below CC refers to charged current reaction, NC to neutral current reaction and ES is elastic scattering, as in SK.

Experiment measured flux ratio exp/BP98 threshold energy Years of
running
Homestake 2.56 ± 0.16 ± 0.16 0.33 ± 0.03 ± 0.05 0.814 MeV 1970-1995
Kamiokande 2.80 ± 0.19 ± 0.33 0.54 ± 0.08 +0.10-0.07 7.5 MeV 1986-1995
SAGE 75 ± 7 ± 3 0.58 ± 0.06 ± 0.03 0.233 MeV 1990-2006
Gallex 78 ± 6 ± 5 0.60 ± 0.06 ± 0.04 0.233 MeV 1991-1996
Super-
Kamiokande		 2.35 ± 0.02 ± 0.08 0.465 ± 0.005 +0.016-0.015 (BP00) 5.5 (6.5) MeV 1996-
GNO 66 ± 10 ± 3 0.51 ± 0.08 ± 0.03 0.233 MeV 1998-
SNO 1.68 ± 0.06 ± +0.08 -0.09 (CC)
2.35 ± 0.22 ± 0.15 (ES)
4.94 ± 0.21 +0.38-0.34 (NC) 		6.75 MeV 1999-
  • The values for Chlorine and Gallium experiments are given in SNU.
  • The values for Cerenkov experiments are given in units of 1010 counts/m2 s.
  • The errors for the relative values correspond to experimental and theoretical errors, respectively, with the statistical and systematic errors added quadratically. Some of the relative values are based on my own calculation from the published results.
  • BP98 Refers to Bahcall and Pinsonneault model of 1998.

SuperKamiokande has provided additional data:

  • Day night asymmetry (2000)
    D-N
    ------- = - 0.034 ± 0.022 ± 0.013
    D+N
    See the plot (1999):
    SK: day-night spectrum
  • Seasonal variation: No visible distortion beyond the eccentricity of the orbit. See the plot:
    SK: seasonal spectrum

Future projects

Experiment Place method sensitivity threshold energy scheduled
to start
Borexino Gran Sasso Liquid Scintillator charged and
neutral currents		 0.25 MeV
or less		 200?
ICARUS Gran Sasso liquid argon
image chamber		 all neutrinos 5.9 MeV 2005 (?)
HELLAZ Gran Sasso Helium gas in
time projection chamber		 mostly electron
neutrinos		 0.2 MeV > 200?
Iodine Homestake radiochemical electron neutrinos    
HERON   phonon excitation
in superfluid helium 		mostly electron neutrinos (0.5 keV?) >> 2005
CLEAN   Liquid helium electron neutrinos low still dream
LENA   scintillator >> 2005
UNO   Megaton Cherenkov still dream
MEMPHYS   Megaton Cherenkov still dream
HYPER-K   Megaton Cherenkov still dream

Also other proposals exist.

Solutions

Solution Status Requirement Time dependence Other features Original references
Resonant conversion to muon or tau neutrinos a good fit, well motivated mixing of neutrinos, mass O(10-3 eV), see plot. day night effect for large mixing large angle solution best fit Mikheyev and Smirnov (86), Wolfenstein (78).
Resonant conversion to sterile neutrinos disallowed by SNO data mixing of neutrinos, mass O(10-3 eV), see plot. day night effect for large mixing    
vacuum oscillation not a good fit
inconsistent with SNO		 mixing of neutrinos, mass O(10-5 eV), see plot. annual variations   Pontecorvo (67)
helicity flip bad fit magnetic moment O(10-11 µB). anticorrelation with sunspots solar magnetic field unknown Voloshin, Vysotsky and Okun (86)
resonant spin-flavor conversion still alive
(hard to kill) unmotivated		 magnetic moment O(10-11 µB), mass O(10-3 eV). anticorrelation with sunspots taking the solar magnetic field as free parameter fits all the results Akhmedov (88)
Lim and Marciano (88)
neutrino decay inconsistent lifetime < 8 min static constrained by SN1987A Bahcall, Cabibbo and Yahil (72) Pakvasa and Tennakone (72)
solar astrophysics desperate new physics inside the sun model dependent Conflicts helioseismology  

The solar neutrino problem can be considered as an almost solved case.

  • The results from SNO clearly indicate that the total neutrino flux from the sun is in accordance with the solar models of Bahcall and Pisconneault.

  • The Kamland reactor neutrino experiment has measured the electron antineutrino flux from nearby reactors and the results are consistent with the solar neutrino oscillations.

  • The results of all solar neutrino data (SNO, SK, ...) and the Kamland experiment point to the values:
    delta m122 ≈ 8 10-5 eV2, sin2 2 t12 ≈ 0.8

    for the oscillation parameters.

    Go to Solar neutrinos: Theory page.

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    • Last modified 10.4.2005 (webmaster)