Neutron-rich nuclei have more neutrons than protons. The excess neutrons extend slightly beyond the proton distribution, forming a “neutron skin” — a thin layer where the nuclear matter is predominantly neutrons. The thickness of this skin constrains the nuclear equation of state, which in turn constrains neutron star structure.
Measuring the skin requires a probe that sees neutrons differently from protons. Neutrinos interact through the weak force, which couples to neutrons more strongly than protons. Coherent elastic neutrino-nucleus scattering (CEvNS) — the entire nucleus recoiling as a unit — is sensitive to the neutron distribution.
Chuliá and colleagues (arXiv:2602.20436) show that the key variable is qR — the product of momentum transfer and nuclear radius. Pion-decay-at-rest (piDAR) neutrinos keep qR below 1, where the nucleus scatters coherently and the recoil spectrum is smooth. Shape information is washed out. But kaon-decay-at-rest (KDAR) neutrinos at 236 MeV push qR above 1, into the diffractive regime, where the recoil spectrum develops structure — minima and maxima that depend on the nuclear form factor.
In the diffractive regime, the scattering spectrum is genuinely shape-sensitive. The positions of the diffractive features encode the neutron distribution radius. A 10 ton-year exposure could measure neutron skin thicknesses to 0.02-0.09 fm precision — competitive with parity-violating electron scattering but with entirely different systematic uncertainties.
The general principle: below a coherence threshold, a probe sees the target as a point. Above the threshold, it resolves structure. The transition from coherent to diffractive scattering is not a degradation of the signal — it is the onset of spatial resolution. Structure appears when the probe's wavelength drops below the object's size.