Neutrino Electromagnetic Interactions

Neutrino oscillation experiments provide compelling evidence on finite neutrino masses and mixings. It is expected that these would lead to anomalous couplings between the neutrinos and the photons (Figure 1). The studies of neutrino electromagnetic interactions [1] are promising avenues to probe these possibilities.

Figure 1. Studies of neutrino electromagnetic processes probe the possible existence of couplings between the neutrino and the photon. Neutrino magnetic moments parametrize the case when the incoming and outgoing neutrinos have opposite helicities (as depicted), while neutrino milli-charge and charge radius describe one where the spin states are conserved.

Intrinsic neutrino properties such as magnetic moments (μ_ν) [2], milli-charge (q_ν) [3] and charge radius (<r_ν²>) are the realizations of anomalous neutrino electromagnetic effects. Their experimental manifestations are the deviations of the integral and differential cross-sections in neutrino-atom interactions:

ν + A → ν + A⁺ + e^-

relative to those due to Standard Model electroweak processes. In particular, both ν_ν and q_ν would provide cross-section enhancement as well as distinct spectral features at low (<10 keV) energy transfer or equivalently the measureable energy in an interaction (Figure 1). These distinct features have also be adopted in placing constraints on sterile neutrinos as dark matter [4].

Germanium detectors, with their low threshold and excellent energy resolution, are optimal in the studies of neutrino electromagnetic effects [5]. Some of the most sensitive studies have been obtained with germanium detectors using reactor neutrinos at the Kuo-Sheng Reactor Neutrino Laboratory in Taiwan by the TEXONO group, a collaborating partner or the PIRE-GEMADARC research program. The expected improvement of the germanium detector technologies in detection threshold, energy resolution and intrinsic background in the course of the PIRE-GEMADARC program will further extend the sensitivity reach in these investigations.

Figure 2: Schematics of neutrino electromagnetic interaction with matter. Interactions with changes in the neutrino helicity states probe neutrino magnetic moments (as depicted) while those without changes are due to neutrino milli-charge and charge radius. The observable signatures are derived from the measurements of the final-state photons and electrons.

Figure 3: Measureable differential spectra for various neutrino interactions with matter using reactor neutrinos at typical configurations – black and blue lines correspond to the Standard Model neutrino-electron and –nucleus elastic scatterings; red, magenta add green lines denote those due to neutrino magnetic moments, milli-charge and charge radius at the specified values, respectively.

References:

1. C. Giunti, A. Studenikin, Rev. Mod. Phys. 87 (2015) 531 (and references therein).
2. H.T. Wong, et al. (TEXONO Collaboration),Phys. Rev. D75 (2007) 012001; A.G. Beda, et al. (GEMMA Collaboration), Phys. Part. Nucl. Lett. 10 (2013) 139.
3. J.W. Chen, et al., Phys. Rev. D90 (2014) 011301(R).
4. J.W. Chen, et al., Phys. Rev. D.93 (2016) 093012.
5. J.W. Chen, et al., Phys. Rev. D.91 (2015) 013005; A.K. Soma et al. (TEXONO Collaboration), Nucl. Instrum. Meth. A836 (2016) 67.