KamLAND, solar antineutrinos and their magnetic moment

We investigate the possibility of detecting solar antineutrinos with the KamLAND experiment. These antineutrinos are predicted by spin-flavor oscillations at a significant rate even if this mechanism is not the leading solution to the Solar Neutrino Problem. The recent evidence from SNO shows that a) the neutrino oscillates, only around 34% of the initial solar neutrinos arrive at the Earth as electron neutrinos and b) the conversion is mainly into active neutrinos, however a non e,μ,τ component is allowed: the fraction of oscillation into non-μ−τ neutrinos is found to be cos 2α = 0.08+0.20−0.40. This residual flux could include sterile neutrinos and/or the antineutrinos of the active flavors. KamLAND is potentially sensitive to antineutrinos derived from solar 8B neutrinos. In case of negative results, we find that KamLAND could put strict limits on the flux of solar antineutrinos, Φ(8B) < 1.0 × 104 cm−2s−1, more than one order of magnitude smaller than existing limits, and on their appearance probability P < 0.20−0.15% (95% CL) after 1-3 years of operation. Assuming a concrete model for antineutrino production by spin-flavor precession, this upper bound implies an upper limit on the product of the intrinsic neutrino magnetic moment and the value of the solar magnetic field μB < 10−21 MeV (95% CL). For B ~ 10−100 kG, we would have μ < 10−11−10−12 μB (95% CL). In the opposite case, if spin-flavor precession is indeed at work even at a non-leading rate, the additional flux of antineutrinos could strongly distort the signal spectrum seen at KamLAND at energies above 4−5 MeV and their contribution should properly be taken into account.