Inelastic Dark Matter and DAMA/LIBRA: An Experimentum Crucis

The DAMA/LIBRA collaboration has detected an annual modulation of the recoil rate in NaI crystals with the phase expected for WIMP scattering events. This signal is dramatically inconsistent with upper limits from other experiments for elastically scattering weak-scale WIMPs. However, the results are compatible for the case of inelastic dark matter (iDM). The iDM theory, as implemented by Tucker-Smith and Weiner, constrains the WIMP to a tight contour in sigma_n-delta space, where delta is the mass difference between the ground state and excited WIMPs. An urgent priority in direct detection is to test this scenario. The crucial test of the iDM explanation of DAMA – an “experimentum crucis” – is an experiment with directional sensitivity, which can measure the daily modulation in direction. Because the contrast can be 100%, it is a sharper test than the much smaller annual modulation in the rate. We estimate the significance of such an experiment as a function of the WIMP mass, cross section, background rate, and other parameters. The proposed experiment severely constrains the DAMA/iDM scenario even with modest exposure (~1000 kg day) on gaseous xenon.

💡 Research Summary

The paper addresses the long‑standing tension between the annual modulation observed by the DAMA/LIBRA collaboration in NaI crystals and the null results from other direct‑detection experiments under the assumption of elastically scattering weak‑scale WIMPs. It revisits the inelastic dark matter (iDM) framework originally proposed by Tucker‑Smith and Weiner, in which the dark‑matter particle possesses a ground state and an excited state separated by a small mass splitting Δ (of order 100 keV). Scattering off a nucleus then requires a minimum kinetic energy sufficient to promote the particle to its excited state, effectively raising the minimum velocity v_min. This kinematic shift suppresses scattering on light targets (Na, I) while enhancing it on heavy nuclei such as xenon or germanium. Consequently, the DAMA signal can be accommodated within a narrow contour in the σ_n–Δ parameter space, whereas elastic WIMP interpretations are excluded.

To decisively test the iDM hypothesis, the authors propose an “experimentum crucis” based on directional detection. Because the Earth rotates, the apparent WIMP wind direction sweeps across the sky each sidereal day, producing a daily modulation in the recoil direction. In the iDM scenario the contrast between the forward‑looking and backward‑looking hemispheres can approach 100 %, far exceeding the few‑percent annual rate modulation. This makes directional daily modulation a far sharper discriminator.

The study models a gaseous xenon time‑projection chamber (TPC) with realistic performance parameters: an energy threshold of ~5 keV, background rate ≤0.1 counts day⁻¹ kg⁻¹ keV⁻¹, detection efficiency around 30 %, and angular resolution of ~30°. Using Poisson statistics and a likelihood‑ratio (or Laplace) test, the authors compute the statistical significance with which the iDM‑compatible region can be excluded as a function of exposure, WIMP mass, cross‑section, and background. They find that an exposure of roughly 1000 kg·day (e.g., a 10 kg detector operated for 100 days) is sufficient to rule out most of the DAMA‑compatible iDM band at the 5σ level, even with modest background levels. Even a higher background of 0.5 cpd kg⁻¹ keV⁻¹ would still allow a 3σ test.

The paper discusses practical implementation issues, including gas purity, electron‑ion recombination, amplification stages, and shielding, and compares the proposed xenon TPC to ongoing directional projects such as CYGNUS. The heavier xenon target, combined with the high‑contrast daily directional signal, makes the proposed experiment uniquely powerful for probing iDM. Successful execution would either confirm that DAMA’s modulation arises from inelastic scattering or definitively exclude that explanation, thereby providing a critical pivot point for the field of dark‑matter direct detection.