Kamland Results

The LMA solution of the solar neutrino problem has been explored with the 1,000 ton liquid scintillator detector, KamLAND. It utilizes nuclear power reactors distributing effectively ∼180 km from the experimental site. Comparing observed neutrino rate with the calculation of reactor operation histories, an evidence for reactor neutrino disappearance has been obtained from 162 ton·year exposure data. This deficit is only compatible with the LMA solution and the other solutions in the two neutrino oscillation hypothesis are excluded at 99.95% confidence level. In spite of the deficits observed in all solar neutrino experiments and improvement of their precisions, the solar neutrino problem has lasted for more than 30 years. A recent revolutionary neutral-current-measurement by SNO experiment 1 has provided an evidence of neutrino flavor transformation and proved that the problem is really on neutrino characteristic. We also know the evidence of atmospheric neutrino oscillation from SK 2 , and it is natural to invest neutrino oscillation hypothesis to explain the solar neutrino problem. In the two neutrino oscillation hypothesis, the large mixing angle (LMA) solution is the most preferable solution. However, another solution also appears at 99% confidence level and even completely different models such as neutrino magnetic moment 3 and neutrino decay 4 can explain all the experimental data as well as the LMA solution. It is necessary to use a well-understood artificial neutrino source to meet a breakthrough in the solar neutrino study. Anti-electron-neutrinos from nuclear power plants are such candidates. Total power generation of worldwide reactors amounts to ∼1.1 TW and it corresponds to one third mole ¯ ν e creation per second. Fortunate characteristic of KamLAND site is that 70 GW (7% of world total) distributes at distances from 130 to 240 km and consists 80% of neutrino flux at Kamioka site, ∼ 5 × 10 6 /cm 2 /sec. This long baseline, effectively 180 km, is sensitive to the LMA oscillation parameters as shown in Fig. 1. Thermal power output of rector cores directly relates to the neutrino flux. Those of all 54 Japanese commercial reactor cores are measured with strictly better than 2% accuracy. In more detail, contributions from 4 different fissile nuclei, 235 U, 238 U, 239 Pu and 241 Pu, have to be known for 1% precision. Since the chemical composition of nuclear fuel changes in time as a burn-up effect, we have to know the initial 235 U enrichment and history …