KamLAND bounds on solar antineutrinos and neutrino transition magnetic moments

We investigate the possibility of detecting solar electron antineutrinos with the KamLAND experiment. These electron antineutrinos are predicted by spin-flavor oscillations at a significant rate even if this mechanism is not the leading solution to the SNP. KamLAND is sensitive to antineutrinos originated from solar B-8 neutrinos. From KamLAND negative results after 145 days of data taking, we obtain model independent limits on the total flux of solar electron antineutrinos Phi (B-8) < 1.1-3.5 x 10(4) cm(-2) s(-1), more than one order of magnitude smaller than existing limits, and on their appearance probability P < 0.15% (95% CL). 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 muB < 2.3 x 10(-21) MeV 95% CL (for LMA (Delta m(2), tan(2) theta) values). Limits on neutrino transition moments are also obtained. For realistic values of other astrophysical solar parameters these upper limits would imply that the neutrino magnetic moment is constrained to be, in the most conservative case, mu <= 3.9 x 10(-12) mu(B) (95% CL) for a relatively small field B = 50 kG. For higher values of the magnetic field we obtain: mu <= 9.0 x 10(-13) mu(B) for field B = 200 kG and mu <= 2.0 x 10(-13) mu(B) for field B = 1000 kG at the same statistical significance.