Coherent elastic neutrino-nucleus scattering

I describe physics potential and experimental prospects for coherent elastic neutrino-nucleus scattering (CEvNS), a process which has not yet been observed. Germaniumbased detectors represent a promising technology for CEvNS experiments. I focus primarily on stopped-pion neutrino sources. 1. Coherent elastic neutrino-nucleus scattering Coherent elastic neutrino-nucleus scattering (“CEvNS”1) is a process involving the neutralcurrent scattering of a neutrino with an entire nucleus [1, 2]. It has a well-defined rate in the standard model, given by (for a spin-zero nucleus and neglecting radiative corrections): dσ dT (E, T ) = GF 2π M [ 2 − 2T E + ( T E )2 − MT E2 ] QW 4 F (Q) . (1) where E is the incident neutrino energy, T is the nuclear recoil energy, M is the nuclear mass, F is the ground-state elastic form factor, Qw is the weak nuclear charge, and GF is the Fermi constant. For nuclei in the medium-A range, the process remains largely coherent (i.e., all nucleon wavefunctions in phase) for neutrino energies up to a few tens of MeV. The cross section is relatively high, boosted by an ∼ N2 enhancement, compared to other chargedand neutral-current processes in the same energy range. It potentially even has practical applications [3, 4], thanks to the large cross section. Although observation of this process has been sought for decades, the experimental signature— very low energy nuclear recoils in the tens-of-keV or less regime— is a challenging one. Only with recent detector developments has good sensitivity to very-low-energy nuclear recoils been feasible. In particular, the experimental search for WIMP dark matter, which has a similar nuclear recoil signature (and in fact for which neutrinos represent a background floor [5, 6]), has driven detector development to enable CEvNS observation. 1 Note there exist a number of abbreviations for this process in the literature, e.g., CNS, CNNS, CENNS. I favor a version with “E” for “elastic” to distinguish the process from inelastic coherent pion production, which is commonly confused with CEvNS by members of the high energy physics community. I prefer to replace the first “N” with “v”, for “neutrino”, for two reasons: first, “NN” means “nucleon-nucleon” to many in the nuclear physics community. Second, this disambiguates from CENNS, which is the name of an experimental collaboration. Finally, the Roman letter “v” is less cumbersome than the Greek letter “ν”. 2nd Workshop on Germanium Detectors and Technologies IOP Publishing Journal of Physics: Conference Series 606 (2015) 012010 doi:10.1088/1742-6596/606/1/012010 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 Neutrino energy (MeV) 0 10 20 30 40 50 60 M ax im u m r ec o il en er g y (k eV ) -2 10 -1 10 1 10 2 10 Recoil energy (keV) -2 10 -1 10 1 10 N u m b er o f ev en ts ( A .U .) 1 10 2 10 3 10 4 10 5 10 Figure 1. Left: maximum recoil energy for a 30-MeV neutrino coherently scattering from 76Ge. Right: nuclear recoil spectra for 30-MeV (top, red) and 3-MeV (bottom, green) incident neutrinos scattering on 76Ge. 2. Sources and detectors The CEvNS cross section increases with energy, so in general to detect CEvNS, one wants a relatively high-energy neutrino source. Moreover, the maximum nuclear recoil energy is given by Emax = 2E2 ν M (see Fig. 1), and the higher the recoil energy of the nucleus, the easier it is in general to detect. Figure 1 shows an example recoil spectrum for 3 and 30 MeV neutrinos, given the same flux, demonstrating both the higher rate and higher recoil energies for higher neutrino energy. However, the neutrino incident energy must not be too high, or else inelastic interactions will begin to dominate (i.e., neutrinos will scatter off individual nucleons inside the nucleus, rather than with the nucleus as a whole— the neutrino wavelength shrinks with increasing energy, and the coherence condition is Q ∼< 1/R). For germanium, neutrino energies must be less than about 50 MeV to ensure coherence. Desirables for a neutrino source for CEvNS detection include: • High flux • Well-understood spectrum • Multiple flavors (helpful for testing new physics) • A pulsed source if possible, for background rejection • Ability to get close to the source; flux increases as inverse square of distance • Practical aspects, such as access and control. Natural sources of neutrinos suitable for CEvNS detection exist: a nearby core-collapse supernova will create an intense burst, feasibly visible over background, in dark matter detectors (e.g., [7]). Diffuse supernova background neutrinos, solar neutrinos, and the lowenergy end of the atmospheric neutrino flux are also in principle observable (geoneutrinos are in principle observable also, but their energies are so low as to produce extremely low-energy recoils). However such steady-state fluxes of natural neutrinos are relatively small and would require enormous detectors, and subject to large backgrounds (perhaps including dark matter!) 2nd Workshop on Germanium Detectors and Technologies IOP Publishing Journal of Physics: Conference Series 606 (2015) 012010 doi:10.1088/1742-6596/606/1/012010