Proc. Neutrino Astrophysics - MPP Theory Group

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Events/day/kton/bin
0.3
0.25
0.2
0.15
0.1
0.05
SK 374day 6.5-20MeV 22.5kt ALL
0
-1 -0.5 0 0.5 1
cosθ sun
Figure 2: (a) Angular distribution to the solar direction and (b) heliograph for 374.2 days
data.
The energy spectrum of observed neutrinos is shown in Fig. 3(a). In this figure, expected
spectra of MSW small angle parameter and just-so parameter are also shown. At first sight,
small angle solution has better fit than flat (no oscillation) and just-so solution, but it is not
significant within an experimental error. The day and night flux difference is obtained by;
D − N
D + N
= −0.031 ± 0.024(stat.) ± 0.014(syst.).
If night data are divided into five bins, those differences are shown in Fig. 3(b). In this figure,
typical day/night flux variation of the large angle and the small angle solutions are also shown.
There is no significant difference in day/night fluxes in present observation. Also Fig. 3(c)
shows the seasonal variation of solar neutrino fluxes. Each season is pile up among different
years. Solid line corresponds to the expected variation from an eccentricity of the Sun orbit.
Within experimental error, there is no seasonal variation in present analysis. Those results
are also same ones from Kamiokande.
Two flavor neutrino oscillation
For astrophysical solution, it is generally difficult to explain the solar neutrino problem with
the modification of SSM including the observations from helioseismology. On the other hands,
the elementary particle solution using MSW neutrino oscillation [5] seems to be an excellent
for explanation of the solar neutrino problem, because it can distort the spectra of solar
neutrinos. Also MSW oscillation can give a effect in the day/night fluxes variation. As
obtained by Fig. 3, our observed spectrum can be seen slightly as distorted one, but not
seen in variance between day/night fluxes. Using these results, we can obtained the excluded
region at 95% C.L. in MS diagram as shown in Fig. 4(a).
If we take the constraint of measured 8 B solar neutrino flux, allowed region of 95%, 90%
and 68% C.L. as shown in Fig. 4(b). From these figures, our results and the allowed region
given by Ref. [4] are consistent in MSW oscillation analysis.