Can daily modulation in anisotropic crystals identify light dark matter?
In plain words
In a crystal that responds differently along different directions, the signal from dark matter should rise and fall once a day as Earth rotates, while most backgrounds should not. This could distinguish a real signal from the unexplained excess of small events.
Precise statement
For sub-GeV dark matter scattering via phonons or electrons in anisotropic crystals (e.g., $\mathrm{Al_2O_3}$, hexagonal BN, graphene-like or Dirac materials), the rate $R(t)$ modulates with sidereal period because the dark matter wind direction rotates relative to the crystal axes; predicted fractional amplitudes reach tens of percent in some materials. The question is whether a modulation of this size can be measured at sub-eV thresholds with exposure below about $1\,\mathrm{kg}\,\mathrm{yr}$, given present low-energy excess rates, and whether the excess itself shows any sidereal modulation.
What would settle it
A sidereal-period analysis of a low-threshold anisotropic-crystal data set with known orientation, giving an amplitude limit or detection at the predicted level.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
A 2026 preprint (arXiv 2606.04091) estimated halo-model uncertainties of 1-100 percent in single-phonon rates across the mass range, which enter the modulation prediction.
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