Limiting neutrino magnetic moments with Borexino Phase-II solar neutrino data

A search for the solar neutrino effective magnetic moment has been performed using data from 1291.5 days exposure during the second phase of the Borexino experiment. No significant deviations from the expected shape of the electron recoil spectrum from solar neutrinos have been found, and a new upper limit on the effective neutrino magnetic moment of ${\mu }_{\nu }^{\mathrm{eff}}<{2.8\times 10}^{-11}$ ${\mu }_B$ at 90% C.L. has been set using constraints on the sum of the solar neutrino fluxes implied by the radiochemical gallium experiments. Using the limit for the effective neutrino moment, new limits for the magnetic moments of the neutrino flavor states, and for the elements of the neutrino magnetic moments matrix for Dirac and Majorana neutrinos, are derived.

Figure 1

Spectral fit with the neutrino effective moment fixed at ${\mu }_{\nu }^{\mathrm{eff}}=2.8\times {10}^{-11}{\mu }_B$ (note the scale is double logarithmic to underline the contributions at lower energies). The fitting curves for ${\mu }_{\nu }^{\mathrm{eff}}=2.8\times {10}^{-11}{\mu }_B$ and ${\mu }_{\nu }^{\mathrm{eff}}=0$ are visually indistinguishable.

Figure 2

Resulting weighted likelihood profile used to estimate the limit on the neutrino magnetic moment. The profile does not follow the Gaussian distribution as it is flatter initially and goes to 0 faster than the normal distribution. The limit corresponds to 90% of the total area under the curve. Note that unphysical values of ${\mu }_{\nu }^{\mathrm{eff}}<0$ are not considered.