A Verdict on the Annual Modulation: Settling a 20-Year Dark Matter Debate

A Verdict on the Annual Modulation: Settling a 20-Year Dark Matter Debate

For over two decades, one of the most tantalizing and controversial results in particle physics has been the signal from the DAMA/LIBRA experiment. Deep under the Gran Sasso mountain in Italy, its sodium iodide detectors have recorded a persistent annual modulation—a subtle, cyclical rise and fall in events that peaks each June and troughs in December. This is exactly the signature one would expect from the Earth moving through a galactic “wind” of WIMP dark matter.

The problem? No other experiment has seen it. This discrepancy, known as the DAMA anomaly, created a deep rift in the field. The results from DAMA/LIBRA contradict other direct detection dark matter experiments under the most conventional scenarios. Was DAMA/LIBRA seeing the first definitive proof of dark matter, or was it an incredibly subtle, long-lived systematic effect?

A new combined analysis from two independent experiments, COSINE-100 and ANAIS-112, has now provided the strongest answer yet: the signal is likely not from dark matter.

A Direct Confrontation with the Anomaly

To test the DAMA/LIBRA claim, you need an identical setup. This was the explicit goal of the COSINE-100 experiment, located in South Korea, and ANAIS-112 in Spain. Crucially, both experiments use the exact same thallium-doped sodium iodide (NaI(Tl)) crystal detectors as DAMA/LIBRA. This allows for a direct, apples-to-apples comparison by removing ambiguities that arise from using different target materials. If the WIMP wind is real, they should see the same modulation.

Independently, neither experiment saw a signal that supported DAMA/LIBRA’s claim. But to increase their statistical power, the collaborations took the next logical step: combining their data.

The Combined Result: A Null Finding

In a new paper, researchers present a combined analysis of the first three years of data from both experiments. By carefully modeling and subtracting the known background events from each detector, they could search the combined “residual” data for any hint of modulation.

The result is unambiguous.

• The combined data is fully consistent with a null hypothesis—that is, no modulation at all.
• The best-fit modulation amplitude they found was $-0.0002 \pm 0.0026$ cpd/kg/keV in the 1-6 keV energy range. This is statistically indistinguishable from zero and fundamentally incompatible with DAMA/LIBRA’s reported amplitude of $0.0105 \pm 0.0011$ cpd/kg/keV in the same range.

The paper goes a step further by performing a simple combination of the newly released, much larger 6-year datasets from both experiments. This larger dataset, totaling nearly 1,000 kg-years of exposure, excludes the DAMA/LIBRA signal with even higher confidence: 4.7σ in the 1-6 keV region and 3.5σ in the 2-6 keV region.

While this result does not identify the source of the DAMA/LIBRA signal, it strongly challenges the interpretation of the DAMA/LIBRA modulation in terms of galactic dark matter. This finding helps to resolve a long-standing puzzle and allows the field to move forward, focusing on other detection strategies as the search for dark matter continues.

References

1. Carlin, N., et al. (2025). “Combined Annual Modulation Dark Matter Search with COSINE-100 and ANAIS-112.” Physical Review Letters 135.12: 121002.
2. Bernabei, R., et al. (2020). “The DAMA project: Achievements, implications and perspectives.” Progress in Particle and Nuclear Physics 114: 103810.