Limit on the Electron Neutrino Magnetic Moment from the Kuo-Sheng Reactor Neutrino Experiment

A search of neutrino magnetic moment was carried out at the Kuo-Sheng Nuclear Power Station at a distance of 28 m from the 2.9 GW reactor core. With a high purity germanium detector of mass 1.06 kg surrounded by scintillating NaI(Tl) and CsI(Tl) crystals as anti-Compton detectors, a detection threshold of 5 keV and a background level of $1\mathrm{k}{\mathrm{g}}^{\mathrm{-}\mathrm{1}}\mathrm{k}\mathrm{e}{\mathrm{V}}^{\mathrm{-}\mathrm{1}}\mathrm{d}\mathrm{a}{\mathrm{y}}^{\mathrm{-}\mathrm{1}}$ at 12–60 keV were achieved. Based on 4712 and 1250 h of reactor ON and OFF data, respectively, the limit on the neutrino magnetic moment of ${\mu }_{{\overline{\nu }}_e}<1.3\times {10}^{-10}{\mu }_{\mathrm{B}}$ at $90\%$ confidence level was derived. An indirect bound of the ${\overline{\nu }}_e$ radiative lifetime of $m_{\nu }^3{\tau }_{\nu }>2.8\times {10}^{18}\mathrm{e}{\mathrm{V}}^{\mathrm{3}}\mathrm{s}$ can be inferred.

Figure 1

Schematic layout of the HPGe with its anti-Compton detectors as well as inner shielding and radon purge system.

Figure 2

(a) The energy spectra for lone events, and (b) the residual of the ON spectrum over the OFF background, from $4712/1250$ live time hours of reactor ON/OFF data taking.

Figure 3

Summary of the results in (a) the searches of neutrino magnetic moments with reactor neutrinos, and (b) the bounds of neutrino radiative decay lifetime. See text for explanations.