Kamiokande results on atmospheric neutrinos and solar neutrinos

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Nuclear PhysicsB (Proc. Suppl.) 35 (1994) 407--411 North-Holland

K a m i o k a n d e R e s u l t s o n A t m o s p h e r i c N e u t r i n o s a n d Solar Neulrinos Y. Suzuki Institute for Cosmic Ray Research, University of Tokyo, 3-2-1, Midoricho, Tanashi, Tokyo, 188 Japan

The recent results on the atmospheric neutrinos and solar neutrinos are summarized. The double ratio, (~t/e)data/(g/e)MC, of the fully contained atmospheric neutrino events for 6.18ktonyr is (0.59-0.60)!-0.06~0.05. The solar neutrino measurement extended till the end of July in 1993 gives almost entire coverage of the solar maximum in solar cycle 22. The observation gives the almost constant solar SB-neutrino flux of Data/SSM = 0.50-~.04:L-0.06 on an average for the Kamiokande II and III data.

1. ATMOSPHERIC

NEUTRINOS

The Kamiokande detector was explained at many occasions, therefore we give here only a few lines of the description. The detector is a 3000 ton water Cherenkov detector of which 1040 tons are used for the study of the atmospheric neutrinos and most inner 680 tons is used for the solar neutrino analysis. It is located at 2700m (w.e.) underground. The inner detector volume is viewed by 948 20-inch photomultiplier tubes(PMT). The 20% of the inner surface is covered by the photo-cathode. The inner volume is surrounded by the outer anti-counter with the thickness ranging -70 to 150 cm. One hundred twenty three PMT are used for the anti-layer. The atmospheric neutrinos are produced up in the atmosphere through the interactions of primary cosmic rays, mainly protons with the atmosphere and subsequent meson decays. The energy of these neutrinos ranges several tens of MeV to very high energy beyond multi-TeV. The distance to the production points from the surface of the earth ranges several tens of km to 13000 km for the case that the neutrinos arrive across the earth. The atmospheric neutrinos interact in the detector and produce electrons and muons through the charged current interactions, since the cross sections of these interactions become small below several hundreds MeV (therefore harder to be detected) the flux in the high energy region decreases rapidly, the atmospheric neutrino events around 1 GeV dominate in underground neutrino detectors. Those events are major backgrounds for the search for proton decay, although the most suitable events to search for neutrino oscillations in the range, Am2-1-10-3eV 2

(-E/L). The atmospheric neutrinos also interact with the rock surrounding the detector and produce muons which traverse or stop in the detector. There are four data samples from the Kamiokande experiment to study atmospheric neutrinos; (1) fully contained events with visible energy less than 1.33 GeV; (2) only vertex contained events (mainly muon events) and fully contained events with visible energy greater than 1.33 GeV (mainly electron events); (3) upward going through-going events (muons with energy greater than 7 GeV); and (4) upward going stopping muon events. The incident neutrino energy and the distance to the production point from the detector responsible to those different types of events are different, and the sensitivity to the neutrino oscillation parameters of those data samples are Am2-10"l~10"4eV2, 1-10-3eV 2, -10-2eV 2, -10 .3 for samples (1), (2), (3) and (4), respectively. The upward going stopping muon events are now under study, and we are preparing a draft for the sample (2). The result for the upward going muons were already published. Here we report on updated results of the fully contained events. The first result on the atmospheric neutrinos in Kamiokande was published in 1988[1], and 277 fully contained events were observed for 2.77 ktonyears. The observed flux of the electron neutrinos was consistent with the expectation (~datave/~MCve = 105+10%), whereas about half of the muon neutrinos (t~-datavp/~MCv~t=59+7%) was found. It was pointed out soon after the publication that the effect of the muon polarization in the calculation of the processes of the production of the atmospheric

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Y. Suzuki/Kamiokande results on atmospheric neutrinos and solar neutrinos

Table 1. The results of the ix e separation for the data from 6.18 kton~ exposure. DATA Bartol[7] I L'K[8] Honda[10] B-NIl l] Single-Ring 389 528 ] 413 471 348 l.t-like 191 325 256 292 214 e-like 198 203 157 179 133 }He 0.96 1.60 1.63 1.63 1.61

neutrinos was not taken into account. But it turned out that the effect was only several percents as far as the ratio of the ve/vlx was concerned. The updated results based on the measurement of 3.43 kton+0 09 years, (~t/e)data/(la/e)MC = 0.61+.0.~ 8 :t0.05, was reported in 1990. Other experiments at that time, Frejus[2] and Nusex[3], either support nor reject our result, since their statistics are not sufficient to make a conclusion. The second paper on the atmospheric neutrinos from the kamiokande collaboration was published in 1992 for the data taken from 4.92ktonyr exposure[4] with the result

on the ratio, (I.t/e)data/(ix/e)MC = 0.60:l:_+Ou.OU~:L-'O.05. The new results from IMB III[5] and Suodan II[6] give similar results to the Kamiokande results. The data is now accumulated for 6.18ktonyr. We have obtained 557 fully contained events out of which 389 events are single ring events subjecting to the Ix-e separation (see [1],[2] for the details of the data analysis). The results of the Ix-e separation are shown in table 1 with the expectations from the four different flux calculations [7][8][9][10]. Although the prediction of the number of events varies a lot, the la/e ratio stays within a couple of percent. The double ratios, (~t/e)data/(ix/e)MC, for those different flux calculations are listed in table 2. It shows that the systematic errors due to the flux calculations are small as long as we take the ratio of the e-like and Ix-like events. Fable 2. The double ratio of (ix/e)data/($t/e)MC for four different flux calculations.

2. S O L A R N E U T R I N O S The solar neutrino measurements in the Kamiokande detector[ 11] started in January 1987 are now extended to almost 7 years, but with the 8 month break between April 1990 and December 1990 due to the replacement of dead photo multiplier tubes and installation of new electronics. About seventeen handled days of data are analyzed until July 1993 which cover the entire solar maximum periods in solar cycle 22112]. Therefore we are able to study not only on the solar SB neutrino flux with high precision, but also to make a detailed study on a correlation of the flux with the solar activity. The increased statistics also enable us to make much sophisticated study like a search for Hep neutrinos[13]. The Hep neutrinos come from the process 3He+p. Only the Hep and aB neutrinos can be detected by Kamiokande.

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scattering, the incident direction of neutrinos is well kept in the direction of the recoil electrons within E0~<2me although the energy of the electron is lower than the neutrino energy, ranging 0 to Ev. This directionality is especially useful to extract solar neutrino signals. The angular resolution of A0~~28° at 10 MeV, however, is determined mainly by the multiple Coulomb scattering of electrons in water. Note that the cross section of v , + e ~ v ~ + e scattering is 6~8 times higher than v+e--->v+e scattering of other kinds of (anti)neutrinos. The data taking periods are divided into three different terms due to the different detector conditions as shown in Fig. 2. After December 1990 we called the data taking period as Kamiokande III. The event reductions and reconstruction are the same as those of KAM II and the details can be found in ref [ 11 ].

[15]. The expected energy spectra of low energy neutrinos, such as solar (anti)neutrinos and supernova neutrinos, are given in Fig. 1 together with those from other origins. The energy threshold of the analysis has been 7.0 MeV since December 1991 and the trigger threshold is now 5 MeV. Low energy (anti)neutrinos less than -30 MeV are mainly detected through v+e--->v+e elastic scattering in water(H~O) except that anti-electronneutrinos(~) are detected through f~+p--->e++n interaction with -20 times larger cross sections than the v+e--->v+e scattering. In the v+e--->v+e

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reactions maintain the neutrino direction. It is expected that excess events along the ecliptic may be seen by the solar neutrino events. But it turned out to be that there are not sufficient events to make it prominent. Then we take a celestial coordinate such that the sun always sits at the center of the coordinate. In Fig. 4 is shown the direction of the final events in this coordinate system. We now can see clearly the excess at the center of the coordinate above the uniform background. This is the image of the sun taken by neulrinos-"NEUTRINO HELIOGRAPH". The spread of the image corresponds to the angular resolution of the detector. In order to give a slightly different impression, the data are divided into 4°x4 ° bins and a box is plotted, of which the size is weighted by the number of events in each bin as shown in Fig. 5. This demonstrates clearly we are seeing the sun by means of neutrinos well above the background. The Kamiokande detector is mainly sensitive to the SB-neutrinos since the minimum energy threshold of the analysis is 7.0 MeV. The coS0sun distribution of the events above 7.0MeV (7.5 MeV for a part of the data) is plotted in Fig. 8. The peak

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Fig. 8. The sun spot number for the same time period in Fig. 7. toward the sun is prominent. The expected distribution from the standard solar model[16] is also shown in the solid line. The averaged flux of DATA/SSM=0.541-0.06(stat.)+0.06(syst.) £or KAM III, is obtained. Assuming the spectrum shape of the SB flux, the observed flux is 3.19+0.41(stat.)+0.35(syst.) cm-2sec -l. If we combine the result with that from KAM II Ill, then the averaged flux becomes 0.50+0.04(stat.)+0.06(syst.). In order to see the time dependence of the solar neutrino flux, the data are divided into 8 terms that each consists approximately 200 days of data. It is clearly seen in Fig. 7 that there is no strong time variations of the solar neutrino flux observed in the Kamiokande detector. The sun spot number of the same time intervals [12] are shown in Fig. 8. The

K. S. Hirata et al., Phys. Lett. B 2 0 5 (1988)416. [2] Ch. Berger et al., Phys. Lett.B245(1990)305. [3] M. Aglietta et al., Europhys. 80989)611. [4] K.S. Hiram et al., Phys. Lett. B280 (1992]146. [5] R. Becker-Szendy et al., Phys. Rev. D46(1992)3720. [6] M. Goodman, Talk given at the atmospheric neuuino workshop, Louisiana(1993). [7] G. Barr, T.K. Gaisser and T. Stanev, Phys. Rev. D39(1989)3532. [8] H. Lee and Y.S.Koh, Nuovo Cimento 105B(1990)883. [9] M. Honda, K.Kasahara, K.Hidaka and S. Midorikawa, Phys. Lett. B248(1990)193. [10] E.V. Bugaev and V.A. Naumov, Phys. Lett. B232(1989)391. [ll] K. S. Hirata,et al., Phys. Rev. Lett. 63, 6 0989); K. S. Hirata et al., Phys. Rev. Lett. 65, 1297 0990); K. S. Hirata et al., Phys. Rev. Lett. 65, 1301 (1990); K. S. Hirata et al., Phys. Rev. Lett. 66, 9 (1991); K. S. Hirata et al., Phys. Rev. D44, 2241 (1991). [12] Preliminary Report and Forecast of Solar Geophysical Data, U. S. Department of Commerce, National Oceanic and Atmospheric Administration, Space Environment Laboratory. [13] Y.Suzuki, in the Proc. of the International Symposium on Neutrino Astrophysics, p61, ed by Y. Suzuki and K. Nakamura, 1992. [14] C. S. Lira and W. J. Marciano, Phys. Rev. D37, 1368 (1988); E. Kh. Akhemedov, Soy. Phys. JETP 68, 690 0989): H. Minakata and H. Nunokawa, Phys. Rev. Lett. 63, 121; Phys. Rev. D43, R297 (1991); R. Barbieri et al., Phys. Lett. B259, l l 9 (1991); C. S. Lim, in these proceedings. [15] Y.Suzuki, in the Proc. of the International Symposium on Neuu'ino Asu'ophysics, p61, ed by Y. Suzuki and K. Nakamura, 1992. [16] We used the SSM of Bahcall (J. N. Bahcall and R. K. Ulrich, Rev. Mod. Phys. 60, 297 (1988)) in this paper.