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We propose an experimental setup to observe coherent elastic neutrino- scattering (CEνAS) using antineutrinos from tritium decay and a liquid target. In this scattering process with the whole atom, that has not been observed so far, the tend to screen the 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 energies of about 9 meV, reducing sizably the number of expected events with respect to the coherent elastic neutrino-nucleus scattering case. This technique allows, in principle, to measure the weak at energy scales never reached before. In addition, such a low-energy experiment could provide a sensitive test of the neutrino , setting, potentially, a stronger limit than the current ones.

The coherent scattering of a neutrino on a target arises when the neutrino wavelength is CEνNS CEνAS comparable to the target size. Kinematic condition: For nuclei (CEvNS) the typical energy of v is MeV. Kinematic condition: ~q Ratom 1 For (CEvAS) the typical energy of v is keV. ~q RN 1 | |· ⌧ | |· ⌧ The scattering cross section against a zero ~q : 3R-Nmomentum1 ~q : 3R-Nmomentum1 | |· ⌧ | |· ⌧ transfer target with mT is transfer : atomic ~q R : nucleus1 radius ~q Ratom 1 | |· N ⌧ | |· ⌧radius d G2 m T = F m C2 1 T R dT ⇡ T V 2E2 R ✓ ⌫ ◆ CEνAS G = Fermi costant F CE⌫NS 1 2 2 2 CE⌫AS CE⌫NS 1 2 2 mT= target mass CV = [(1 4 sin ✓w)ZFZ (q ) NFN (q )] CV = CV + ( 1 + 4 sin ✓w)ZFe(q ) T = recoil energy Coherence condition 2 2 ± R ~q Ratom 1 E = neutrino energy v | |· ⇠ 2 CEvNS is sensitive to the and In CEvAS, the electron component arises, modifying Cv= coupling TR 2 meV/(AR [A])˚ ⇠ atom distribution, since nuclear form factors FZ the coupling by adding a new term dependent on Atomic Effects in Coherent Neutrino Scattering Sehgal, L.M. and and FN play an important role in the the weak mixing angle. The +(-) sign is applied for Wa n n i n g e r M. Phys.Lett. B171 (1986) interaction electron (,) neutrinos .

ATOMIC FORM FACTOR The main difference between the CEvNS and CEvAS cross sections is encased in the interference condition

N 2 This is the parametrization that we used Z (1 4 sin ✓w) for reproducing the atomic form factor of Fe(TR)= 2 1 + 4 sin ✓w 4He To prove the robustness of the result we When this condition is accomplished the compared two form factors obtained with electron part of the coupling compensates the Rothraan Hartree Fock method the nuclear contribution, making the cross (avoiding electron-electron correlations) section vanishing Considering 4He as a target, and the variational method (including 2 sin ϑW = 0.2 3857 [PDG] and the explicitly electron-electron correlations) The cancellation atomic form factor on the left condition we reach the condition of The result does not change, making the COMPLETE SCREENING AT theoretical uncertainty of depends on the P. A. Doyle and P. S. Turner, Acta Crystallographica Section A 24, 390 (1968). 2 the atomic form factor negligible P. J. Brown et al., International Tables for Crystallogra- phyC ch. 6.1, 554 (2006). value of sin θw TR 9 meV A. J. Thakkar and V. H. Smith, Jnr, Acta Crystallographica Section A 48, 70 (1992). '

3 3 To investigate CEvAS scattering To detect a recoil of the order of H He + e + ⌫¯e it’s necessary to use a low-energy meV, the only technology that ! scale neutrino probe, in order to probably will be available in the get the kinematic condition for near future is based on helium reaching the coherence with the evaporation, a method thought whole atom for low-mass dark The Q-value of the process is As soon as the interacting 18.58 keV and the neutrino scatters on the target, the release spectrum is peaked at ~16 keV of energy is propagated through and towards the t/⌧ N⌫ (t)=N3H (1 e ) surface, making evaporate helium · atoms Number of neutrinos depending Detection Using Helium Evaporation and Field Ionization H. J. Maris,G. M. Seidel, D. Stein Phys.Rev.Lett. 119 (2017) 18, 181303 on � = 17.74 yr and on the Direct detection of sub-GeV dark matter using a superfluid 4He target S.A. Hertel, A. Biekert, amount of 3H used as a source J. Lin,V. Velan ,D.N. McKinsey. Phys.Rev. D100 (2019) no.9, 092007

To assess the potentiality of this process, hopefully detectable in the future, we studied the sensitivity under the optimistic case of zero-background, 500 kg of He for the detector, 5 years of data taking and 3 different scenarios of tritium amount: 60 g, 160g and 500 g. Even though building such a setup in the near future seems difficult. the possibility to measure parameters at scales never reached before makes the idea worthy to be explored. The first two scenarios allow for the possibility to distinguish CEvAS from CEvNS at 3σ and 5σ C.L., respectively Weak Mixing Angle CEvAS is highly sensitive to the neutrino Some weak mixing angle magnetic moment because of the 1/T measurements, especially R dependence, which enhances the cross at low scale, are able to section at low energies constrain the presence of CE⌫AS CE⌫AS 2 2 2 new beyond the d d ⇡↵ Z µ⌫ 1 1 2 + 2 (1 Fe(TR)) dTR µ =0 ' dTR me µB TR E⌫ ⌫ 6 ✓ ◆ ✓ ◆ +0.04 CE⌫AS CE⌫AS 2 2 2 2 2 SM d d ⇡↵ Z µ⌫ 1 1 sin ✓W =sin ✓W 0.05 2 + 2 (1 Fe(TR)) +0.025 dTR ' dTR m µB TR E⌫ 2 2 SM µ⌫ =0 e ✓ ◆ ✓ ◆ sin ✓W =sin ✓W 0.029 6 2 2 SM+0.015 In the most optimistic scenario the limit sin ✓W =sin ✓W 0.016 wuold be transfer scale: 13 µ⌫ < 4.1 10 µB (90% C.L.) 5 ⇥ q 2 10 GeV It is two order of magnitude better than the h i' ⇥ current limit

Potentialities of a low-energy detector based on 4He evaporation to observe atomic effects in coherent neutrino scattering and perspectives M. Cadeddu, F. Dordei, C. Giunti, K. A. Kouzakov, E. Picciau, and A. I. Studenikin Phys. Rev. D 100, 073014 K. Kouzakov and A. Studenikin acknowledge the support from the RFBR grant No. 20-52-53022 Contact: [email protected]