Abstract
We propose an experimental setup to observe coherent elastic neutrino-atom scattering ($\mathrm{CE}\ensuremath{\nu}\mathrm{AS}$) using electron antineutrinos from tritium decay and a liquid helium target. In this scattering process with the whole atom, that has not been observed so far, the electrons tend to screen the weak charge of the nucleus as seen by the electron antineutrino probe. The interference between the nucleus and the electron cloud produces a sharp dip in the recoil spectrum at atomic recoil energies of about 9 meV, reducing sizably the number of expected events with respect to the coherent elastic neutrino-nucleus scattering case. We estimate that with a 60 g tritium source surrounded by 500 kg of liquid helium in a cylindrical tank, one could observe the existence of $\mathrm{CE}\ensuremath{\nu}\mathrm{AS}$ processes at $3\ensuremath{\sigma}$ in 5 yr of data taking. Keeping the same amount of helium and the same data-taking period, we test the sensitivity to the Weinberg angle and a possible neutrino magnetic moment for three different scenarios: 60, 160, and 500 g of tritium. In the latter scenario, the Standard Model (SM) value of the Weinberg angle can be measured with a statistical uncertainty of ${\mathrm{sin}}^{2}{{\ensuremath{\vartheta}}_{W}^{\mathrm{SM}}}_{\ensuremath{-}0.016}^{+0.015}$. This would represent the lowest-energy measurement of ${\mathrm{sin}}^{2}{\ensuremath{\vartheta}}_{W}$, with the advantage of being not affected by the uncertainties on the neutron form factor of the nucleus as the current lowest-energy determination. Finally, we study the sensitivity of this apparatus to a possible electron neutrino magnetic moment and we find that using 60 g of tritium it is possible to set an upper limit of about $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\ensuremath{\mu}}_{B}$ at 90% C.L., that is more than one order of magnitude smaller than the current experimental limit.
Highlights
In this paper we proposed an experimental setup to observe coherent elastic neutrino-atom scattering and we evaluated the physics potentialities of this apparatus
The observation of a coherent scattering of the whole atom requires the detection of very low atomic recoil energies, of the order of 10 meV
This is achievable thanks to the combination of different critical ingredients: first, the exploitation of the β− decay of tritium that is characterized by a small Q value, ensuring a sufficient flux of low-energy antineutrinos, and second, the usage of a target detector with a new technology based on the evaporation of helium atoms coupled with field ionization detector arrays that allow sensitivity to very small energy deposits
Summary
Coherent elastic neutrino-nucleus scattering (CEνNS) has been recently observed by the COHERENT experiment [1,2], after many decades from its prediction [3,4,5]. The nuclear recoils induced by dark-matter scattering produce elementary excitations (phonons and rotons) in the target that can result in the evaporation of helium atoms, if the recoil energy is greater than the binding energy of helium to the surface Given that the latter can be below 1 meV, this proposed technique represents an ideal experimental setup to observe atomic effects in coherent neutrino scattering. We propose a future experiment that would allow the observation of coherent elastic neutrino-atom scattering (CEνAS) processes and we investigate its sensitivity Since this effect could be visible only for extremely small recoil energies, in order to achieve a sufficient number of low-energy CEνAS events, electron neutrinos with energies of the order of a few keV need to be exploited. We show the potentialities of such a detector to perform the lowest-energy measurement of the weak mixing angle θW, known as the Weinberg angle, a fundamental parameter in the theory of Standard Model electroweak interactions, and to reveal a magnetic moment of the electron neutrino below the current limit
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