Observation of coherent elastic neutrino-nucleus scattering (2017)
Observation of coherent elastic neutrino-nucleus scattering (2017)
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The coherent elastic scattering of neutrinos off nuclei has eluded detection for four decades, even though its predicted cross-section is the largest by far of all low-energy neutrino couplings. This mode of interaction provides new opportunities to study neutrino properties, and leads to a miniaturization of detector size, with potential technological applications. We observe this process at a 6.7-sigma confidence level, using a low-background, 14.6-kg CsI[Na] scintillator exposed to the neutrino emissions from the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.
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The discovery of a weak neutral current in neutrino interactions (1973) implied that neutrinos were capable of coupling to quarks through the exchange of neutral Z bosons. Soon thereafter it was suggested that this mechanism should also lead to coherent interactions between neutrinos and all nucleons present in an atomic nucleus. This possibility would exist only as long as the momentum exchanged remained significantly smaller than the inverse of the nuclear size (Fig. 1A), effectively restricting the process to neutrino energies below a few tens of MeV. The enhancement to the probability of interaction (scattering cross-section) would however be very large when compared to interactions with isolated nucleons, approximately scaling with the square of the number of neutrons in the nucleus (2, 3). For heavy nuclei and sufficiently intense neutrino sources, this can lead to a dramatic reduction in detector mass, down to a few kilograms.
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The present CEνNS measurement involves neutrino energies in the range ~16-53 MeV, the lower bound defined by the lowest nuclear recoil energy measured (fig. S9), the upper bound by SNS neutrino emissions (fig. S2).
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Coherent elastic neutrino-nucleus scattering (CEνNS) has evaded experimental demonstration for forty-three years following its first theoretical description. This is somewhat surprising, in view of the magnitude of its expected cross-section relative to other tried-and-tested neutrino couplings (Fig. 1B), and of the availability of suitable neutrino sources: solar, atmospheric and terrestrial, supernova bursts, nuclear reactors, spallation facilities, and certain radioisotopes (3). This delay stems from the difficulty in detecting the low-energy (few keV) nuclear recoil produced as the single outcome of the interaction. Compared to a minimum ionizing particle of the same energy, a recoiling nucleus has a diminished ability to generate measurable scintillation or ionization in common radiation detector materials. This is exacerbated by a trade-off between the enhancement to the CEνNS cross-section brought about by a large nuclear mass, and the smaller maximum recoil energy of a heavy target nucleus.

The interest in CEνNS detection goes beyond completing the picture of neutrino couplings predicted by the Standard Model of particle interactions. In the time since its description, CEνNS has been suggested as a tool to expand our knowledge of neutrino properties. These studies include searches for sterile neutrinos (4–6), a neutrino magnetic moment (7, 8), non-standard interactions mediated by new particles (9–11), probes of nuclear structure (12), and improved constraints on the value of the weak nuclear charge (13). In addition to these, the reduction in neutrino detector mass may lead to a number of technological applications (14), such as non-intrusive nuclear reactor monitoring (15). CEνNS is also expected to dominate neutrino transport in neutron stars, and during stellar collapse (16–18). Direct searches for Weakly Interacting Massive Particles (WIMPs), presently favored dark matter candidates, rely on the same untested coherent enhancement to the WIMP-nucleus scattering cross-section, and will soon be limited by an irreducible CEνNS background from solar and atmospheric neutrinos (19). The importance of this process has generated a broad array of proposals for potential CEνNS detectors: superconducting devices (3), cryogenic detectors (20–22), modified semiconductors (23–25), noble liquids (26–30), and inorganic scintillators (31), among others.

The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory generates the most intense pulsed neutron beams in the world, produced by the interactions of accelerator-driven high-energy (~1 GeV) protons striking a mercury target. These beams serve an array of neutron-scattering instruments, and a cross-disciplinary community of users. Spallation sources are known to simultaneously create a significant yield of neutrinos, generated when pions, themselves a byproduct of proton interactions in the target, decay at rest. The resulting low neutrino energies are favorable for CEνNS detection (3, 32, 33). Three neutrino flavors are produced (prompt muon neutrinos νμ, delayed electron neutrinos νe, and delayed muon anti-neutrinos Embedded Image), each with characteristic energy and time distributions (fig. S2), and all having a similar CEνNS cross-section for a given energy. During beam operation, approximately 5 × 1020 protons-on-target (POT) are delivered per day, each proton returning ~0.08 isotropically-emitted neutrinos per flavor. An attractive feature is the pulsed nature of the emission: 60 Hz of ~1 μs-wide POT spills. This allows us to isolate the steady-state environmental backgrounds affecting a CEνNS detector from the neutrino-induced signals, which should occur within ~10 μs windows following POT triggers. Similar time windows preceding the triggers can be inspected to obtain information about the nature and rate of steady-state backgrounds, which can then be subtracted (31, 34). A facility-wide 60 Hz trigger signal is provided by the SNS, at all times.
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http://science.sciencemag.org/content/early/2017/08/02/science.aao0990.full
http://science.sciencemag.org/content/early/2017/08/02/science.aao0990.full

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