COHERENT

Objective

First observation and precision characterization of coherent elastic neutrino-nucleus scattering (CEvNS), 43 years after its prediction.

Method

A suite of small, low-threshold detectors placed in 'Neutrino Alley' about 20 m from the SNS mercury target. The pulsed proton beam produces stopped pions whose decay sequence gives a well-characterized spectrum of νμ (prompt) and νe + ν̄μ (delayed). Different detector technologies — CsI[Na], liquid argon, germanium — target the keV-scale nuclear recoils predicted by CEvNS with timing cuts to separate prompt and delayed neutrino populations from beam-correlated backgrounds.

Key results

• August 2017: first observation of CEvNS at 6.7σ using a 14.6-kg CsI[Na] detector — 43 years after Freedman's 1974 prediction.
• 2020: CEvNS confirmation in a liquid argon target, demonstrating the expected N² dependence on neutron number.
• Ongoing measurements with multiple targets (CsI, Ar, Ge, NaI) to map the N² scaling and extract nuclear form factors.
• Constraints on non-standard neutrino interactions and on neutrino magnetic moment from CEvNS spectral shapes.

Significance

CEvNS observation completed the experimental verification of a Standard Model process that had been predicted but not detected for four decades. The process — a neutrino scattering coherently off an entire nucleus via Z-exchange — is the largest neutrino cross-section at sub-50-MeV energies and is the foundation for reactor CEvNS programs, for neutrino floor limits in dark-matter detection, and for applied neutrino research including the Master Equation framework for neutrinovoltaic conversion.

The Spallation Neutron Source

The SNS is a 1.4 MW proton accelerator at Oak Ridge National Laboratory. Its 1-GeV proton pulses strike a mercury target, producing neutrons for materials-science applications worldwide. As a by-product, stopped pions produced in the target decay at rest, emitting neutrinos through the two-step chain ${\pi }^{+}\to {\mu }^{+}+{\nu }_{\mu },{\mu }^{+}\to e^{+}+{\nu }_e+{\bar{\nu }}_{\mu }$ The prompt muon-neutrino pulse coincides with the proton beam (sub-microsecond timing); the delayed ${\nu }_e$ and ${\bar{\nu }}_{\mu }$ follow with the 2.2 μs muon lifetime. This clean two-component pulse shape, combined with the 60 Hz beam pulsing, gives a powerful timing handle for background rejection.

”Neutrino Alley”

Access is via a basement utility corridor named “Neutrino Alley” about 20 m from the target, shielded by roughly 12 m-water-equivalent of basement concrete. Here the COHERENT collaboration has deployed a series of small, low-threshold detectors:

• CsI[Na], 14.6 kg — sensitive to recoils from both Cs and I nuclei; used for the 2017 first observation
• Liquid argon, 24 kg (CENNS-10) — different N, cross-check of N² scaling
• Germanium, multiple kg — lower threshold than CsI, favoring low recoil energies
• NaI[Tl], planned at multi-tonne scale — sensitive to both Na and I

Each target measures CEvNS at a different neutron number $N$, allowing direct test of the $\sigma \propto N^2$ prediction.

The 2017 result

The COHERENT collaboration announced in Science in August 2017 a $6.7\sigma$ observation of CEvNS in the CsI[Na] detector. The event rate of 134 observed above background over 308 live days matched Standard-Model predictions at the $\sim 10\%$ level.

The delayed neutrinos (${\nu }_e+{\bar{\nu }}_{\mu }$ from ${\mu }^{+}$ decay) dominated the signal because their energies (10–50 MeV) sit well above the experimental nuclear-recoil threshold. The timing analysis extracted signal from background at high significance; cross-checks included alternative analysis streams and varied cuts.

Physics reach

CEvNS at a pulsed-neutrino source gives access to several physics targets beyond the cross-section itself.

Form factors. The coherent amplitude includes a nuclear form factor $F(q^2)$ that falls off as the momentum transfer approaches the inverse nuclear radius. Measuring the recoil spectrum over a range of targets maps the neutron distribution — a largely independent probe of nuclear structure, complementary to parity-violating electron scattering.

Non-standard interactions. Any beyond-Standard-Model modification of neutrino-quark couplings would distort the CEvNS rate and spectrum. COHERENT’s measurements already constrain large regions of the NSI parameter space at the tree-level weak scale.

Neutrino magnetic moment. A finite neutrino magnetic moment would contribute an electromagnetic CEvNS component with a different recoil-energy dependence. COHERENT’s low-energy sensitivity provides competitive limits.

Weak mixing angle at low Q². The ratio of CEvNS to purely Standard-Model expectations extracts ${\sin }^2{\theta }_W$ at a kinematic regime distinct from atomic parity violation or Z-pole measurements.

Reactor-CEvNS context

A parallel set of experiments — CONUS (Brokdorf/Leibstadt), CONNIE (Angra-2, Brazil), Dresden-II, RED-100, nuGEN — pursue CEvNS at reactor sites. Reactor ${\bar{\nu }}_e$ energies are lower (0–10 MeV), placing their recoil spectra at the edge of current detector thresholds. The reactor measurement is complementary: a cleaner flux, a simpler flavor content, and lower energies favor sensitivity to some beyond-SM scenarios but make the experimental effort significantly harder.

Together, the two approaches — pulsed source at SNS and steady source at reactors — define the CEvNS experimental landscape through the 2030s.