On Coherent Neutrino and Antineutrino Scattering off Nuclei

Abstract

Neutrino-nucleus $\nu A \to \nu A$ and antineutrino-nucleus $\bar {\nu }A \to \bar {\nu }A$ interactions, when the nucleus conserves its integrity, are discussed with coherent (elastic) and incoherent (inelastic) scattering regimes taken into account. In the first regime the nucleus remains in the same quantum state after the scattering and the cross-section depends on the quadratic number of nucleons. In the second regime the nucleus changes its quantum state and the cross-section has an essentially linear dependence on the number of nucleons. The coherent and incoherent cross-sections are driven by a nuclear nucleon form-factor squared ${{\left| F \right|}^{2}}$ term and a $(1 - {{\left| F \right|}^{2}})$ term, respectively. One has a smooth transition between the regimes of coherent and incoherent (anti)neutrino-nucleus scattering. Due to the neutral current nature these elastic and inelastic processes are indistinguishable if the nucleus recoil energy is only observed. One way to separate the coherent signal from the incoherent one is to register $\gamma$ quanta from deexcitation of the nucleus excited during the incoherent scattering. Another way is to use a very low-energy threshold detector and collect data at very low recoil energies, where the incoherent scattering is vanishingly small. In particular, for $^{{133}}{\text{Cs}}$ and neutrino energies of 30–50 MeV the incoherent cross-section is about 15–20% of the coherent one. Therefore, the COHERENT experiment (with $^{{133}}{\text{Cs}}$) has measured the coherent elastic neutrino nucleus scattering (CE$\nu$NS) with the inelastic admixture at a level of 15–20%, if the excitation $\gamma$ quantum escapes its detection.