With the advent of detectors with sub-keV sensitivities, atomic ionization has been identified as a promising avenue to probe possible neutrino electromagnetic properties. The interaction crosssections induced by millicharged neutrinos are evaluated with the ab-initio multi-configuration relativistic random-phase approximation. There is significant enhancement at atomic binding energies compared to that when the electrons are taken as free particles. Positive signals would distinctly manifest as peaks at specific energies with known intensity ratios. Selected reactor neutrino data with germanium detectors at analysis threshold as low as 300 eV are studied. No such signatures are observed, and a combined limit on the neutrino charge fraction of |δQ| < 1.0 × 10 −12 at 90% confidence level is derived.PACS numbers: 14.60. Lm, 13.15.+g, 13.40.Gp The physical origin and experimental consequences of finite neutrino masses and mixings [1] are not fully understood. Investigations on anomalous neutrino properties and interactions [2] are crucial to address these fundamental questions and may provide hints or constraints to new physics beyond the Standard Model (SM). An avenue is on the studies of possible neutrino electromagnetic interactions [2-4] which, in addition, offer the potentials to differentiate between Majorana and Dirac neutrinos. The neutrino electromagnetic form factors in C, P and T-conserving theories can be formulated as:, and (2)γ µ and σ µν are the standard QED matrices, e 0 and m e are the electron charge and mass, respectively, q = (q 0 , q) is the four-momentum transfer, while the neutrino properties are parametrized by the neutrino fractional charge relative to the electron (δ Q − commonly referred to as "neutrino millicharge" in the literature), the neutrino charge radius ( r 2 ν ), and the anomalous neutrino magnetic moment (µ ν ) [3,4] in units of the Bohr magneton µ B . The F 1 and F 2 terms characterize neutrino interactions without and with a change of the helicity states, respectively. The studies of δ Q and r 2 ν should in general * Corresponding Author: lakhwinder@phys.sinica.edu.tw † Corresponding Author: htwong@phys.sinica.edu.tw ‡ Corresponding Author: b97b02002@ntu.edu.tw be coupled to those due to SM-electroweak interactions to account for the possible interference effects among them. For completeness, we note that two additional form factors are possible [4]: the electric dipole moments in theories violating both P-and T-symmetries, and the anapole moments in P-violating theories.The theme of this article is to report a new direct laboratory limit on |δ Q |. The searches are based onν e emitted from the nuclear power reactor via atomic ionization [5], an interaction channel considered for the first time in this process. The cross-section is derived using the Multi-Configuration Relativistic Random-Phase Approximation (MCRRPA) theory [6,7]. As will be demonstrated in Figure 1b, the bounds on event rates from δ Qinduced atomic interactions [ν e -A(δ Q )]:to be probed in this work (...

Low-energy electronic recoil caused by solar neutrinos in multi-ton xenon detectors is an important subject not only because it is a source of the irreducible background for direct searches of weakly-interacting massive particles (WIMPs), but also because it provides a viable way to measure the solar $pp$ and $^{7}\textrm{Be}$ neutrinos at the precision level of current standard solar model predictions. In this work we perform $\textit{ab initio}$ many-body calculations for the structure, photoionization, and neutrino-ionization of xenon. It is found that the atomic binding effect yields a sizable suppression to the neutrino-electron scattering cross section at low recoil energies. Compared with the previous calculation based on the free electron picture, our calculated event rate of electronic recoil in the same detector configuration is reduced by about $25\%$. We present in this paper the electronic recoil rate spectrum in the energy window of 100 eV - 30 keV with the standard per ton per year normalization for xenon detectors, and discuss its implication for low energy solar neutrino detection (as the signal) and WIMP search (as a source of background).Comment: 12 pages, 3 figure

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The electromagnetic properties of neutrinos, which are either trivial or negligible in the context of the Standard Model, can probe new physics and have significant implications in astrophysics and cosmology. The current best direct limits on the neutrino millicharges and magnetic moments are both derived from data taken with germanium detectors with low thresholds at keV levels. In this paper, we discuss in detail a robust, ab initio method: the multiconfiguration relativistic random phase approximation, that enables us to reliably understand the germanium detector response at the sub-keV level, where atomic many-body physics matters. Using existing data with sub-keV thresholds, limits on reactor antineutrino's millicharge, magnetic moment, and charge radius squared are derived. The projected sensitivities for next generation experiments are also given and discussed