On the Possibility of Separating Coherent and Incoherent (Anti)neutrino Scattering on Nuclei

Abstract

The discovery of coherent neutrino–nucleus scattering in the COHERENT experiment opened a source of new information for fundamental investigations in the realms of neutrino and nuclear physics, as well as in the realms of searches for new physics beyond the Standard Model. Owing to substantial momentum transfers, a feature peculiar to the kinematical region of this experiment is that the effect of coherence is mixed with a sizable incoherent contribution rather than being seen in a pure form. On one hand, this leads to additional systematic uncertainties in studying the neutrino component of the coherence effect as such. On the other hand, this makes it possible to study a dynamical transition between the coherent and incoherent scattering modes and, in principle, to separate them experimentally. In our opinion, a consistent measurement of the coherent and incoherent cross sections for (anti)neutrino scattering on a nucleus in the same experiment seems a unique possibility, and its implementation would of course provide new data for neutrino physics, as well as for nuclear and new physics. In the present study, it is shown that this possibility is implementable not only in experiments that explore coherent neutrino and antineutrino scattering on various nuclei at accelerators, where the neutrino energy reaches several hundred MeV units but also in reactor experiments, where antineutrino energies do not exceed 10 MeV. The respective estimation is based on the approach that controls qualitatively a ‘‘smooth transition’’ of the cross section for (anti)neutrino–nucleus scattering from a coherent (or elastic) to an incoherent (inelastic) mode. In the former case, the target nucleus remains in the initial quantum state, while, in the latter case, its quantum state changes. Observation of a specific number of photons that have rather high energies and which remove the excitation of the nucleus after its inelastic interaction with (anti)neutrinos is proposed to be used as a signal from such an inelastic process. An upper limit on the number of such photons is obtained in this study.

Concept of Coherence in Neutrino and Antineutrino Scattering off Nuclei

“…Methods for observing the incoherent signal from the neutrino-nucleus scattering were discussed in [1,3,5]. For a massive nonrelativistic χ lepton, there is nothing fundamentally different.…”

“…On the other hand, the incoherent contribution can be directly measured (estimated) by purposeful detection of photons emitted by target nuclei excited in inelastic processes [5] with the number of these photons being proportional to the inelastic-to-elastic νA scattering ratio. These photons should be time-correlated with the incident neutrino beam and have a range of energies typical of the nucleus A, which are usually much higher than the nuclear recoil energy, thus noticeably simplifying their detection.…”

“…Obviously, the approach to the scattering of neutrinos and antineutrinos off nuclei used in [1][2][3]5] can be generalized to the case where a massive neutral particle is considered instead of a massless (anti)neutrino and where the rather weak interaction of this particle with nucleons of the nucleus is described by the phenomenological Lagrangian taking into account possible Lorentz structures (scalar, pseudoscalar, vector and axial vector). The relevant importance of considering this case follows, for example, from the necessity of correctly understanding the balance of elastic and inelastic interaction in the problem of direct detection of massive weakly interacting neutral particles of dark matter via their (nonrelativistic) scattering from target nuclei of the corresponding detector.…”

“…Note that CEνNS-events considered in [1][2][3]5] are an irremovable background for experiments aimed at direct detection of dark matter particles under terrestrial conditions (see, for example, [6][7][8][9][10], since the main and so far, the only signature of both processes is the target nucleus recoil energy. For this reason, it seems expedient to consider these two processes, which proved to be interrelated, within a single approach.…”

“…Thus, the first objective of this work is to generalize the above massless neutrino approach [1][2][3]5] to the case of interaction between massive neutral weakly interacting χ-particles and nuclei χA → χA ( * ) in the nonrelativistic approximation. The second objective is to investigate those regions of parameters (including kinematic ones) where the inelastic (incoherent) process χA → χA * can noticeably compete with the elastic (coherent) process χA → χA, which is traditionally believed to be generally dominant.…”