Abstract
We study the question of whether coherent neutrino scattering can occur on macroscopic scales, leading to a significant increase of the detection cross section. We concentrate on radiative neutrino scattering on atomic electrons (or on free electrons in a conductor). Such processes can be coherent provided that the net electron recoil momentum, i.e. the momentum transfer from the neutrino minus the momentum of the emitted photon, is sufficiently small. The radiative processes is an attractive possibility as the energy of the emitted photons can be as large as the momentum transfer to the electron system and therefore the problem of detecting extremely low energy recoils can be avoided. The requirement of macroscopic coherence severely constrains the phase space available for the scattered particle and the emitted photon. We show that in the case of the scattering mediated by the usual weak neutral current and charged current interactions this leads to a strong suppression of the elementary cross sections and therefore the requirement of macroscopic coherence results in a reduction rather than an increase of the total detection cross section. However, for the νe scattering mediated by neutrino magnetic or electric dipole moments coherence effects can actually increase the detection rates. Effects of macroscopic coherence can also allow detection of neutrinos in 100 eV — a few keV energy range, which is currently not accessible to the experiment. A similar coherent enhancement mechanism can work for relativistic particles in the dark sector, but not for the conventionally considered non-relativistic dark matter.
Highlights
Observe neutrinos with a hand-held detector rather than with ton- or kiloton-scale ones — a spectacular achievement
We study the question of whether coherent neutrino scattering can occur on macroscopic scales, leading to a significant increase of the detection cross section
We have considered the possibility of achieving macroscopic coherence in neutrino detection experiments
Summary
How about scattering with coherence on macroscopic scales? Clearly, this would require measuring even much smaller recoil energies and so does not look practical. The total cross section obtained by summing over all the scatterers in the target will scale as N 4/3, i.e. the cross section increase due to the coherence effects is ∼ N 1/3 While this is much smaller than an extra factor of ∼ N one could naively expect, it still would mean a very strong enhancement of the detection cross section. Macroscopic coherence holds and the cross section becomes very large only for neutrino scattering in a very narrow forward cone, which corresponds to unmeasurably small recoil energies of the target particles. As is seen from the above discussion, one reason why it is difficult to achieve macroscopic coherence in neutrino scattering processes is that one usually measures the recoil energy of the target particles, which for small recoils is suppressed compared to the recoil. One such possibility was suggested in the 1980s by Joseph Weber [9,10,11]
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