A study by the Center for Astroparticles and High Energy Physics (CAPA-Unizar) establishes the most restrictive limit, refuting the signal from the DAMA/LIBRA experiment, the only one to date that claims to have detected a signal from galactic dark matter.
The new, highly significant result, published in Physical Review Letters, represents the culmination of two decades of work by scientists at the University of Zaragoza.
https://journals.aps.org/prl/abstract/10.1103/ntnl-zrn9
If the initial results from the University of Zaragoza’s ANAIS experiment at the Canfranc Underground Laboratory, published in 2019, cast doubt on the evidence of the existence of dark matter obtained by the Italian DAMA/LIBRA experiment, the new results from six years of data refute them. This is the conclusion of a new study conducted by researchers at the University of Zaragoza’s Center for Astroparticle and High Energy Physics (IUI CAPA), coordinated by María Luisa Sarsa Sarsa and María Martínez Pérez, co-directors of the ANAIS experiment.
This new result, published on July 28 by the prestigious scientific journal Physical Review Letters, is of great significance and has a significant impact on the field of astroparticle physics and represents the culmination of two decades of work by scientists at the University of Zaragoza. In parallel, the results were presented by María Martínez this August at two major international conferences: SUSY 2025 in California, United States, and TAUP 2025 in Sichuan, China.
For more than two decades, the DAMA/LIBRA experiment, located at the Gran Sasso Laboratory in Italy, has claimed to detect galactic dark matter by observing an annual modulation in the detection rates of its 250 kg sodium iodide detectors. This modulation could be attributed to the dark matter wind caused by the change in relative velocity between the Earth (and the detectors located on it) and the dark matter particles distributed in the galactic halo, due to the Earth’s motion around the Sun with a period of one year.
The nature of dark matter is one of the greatest enigmas facing cosmology, particle physics, and astrophysics today. It cannot be explained with any of the particles that constitute the standard model of particle physics, requiring the existence of new hypothetical particles within the framework of extensions of the standard model, such as WIMPs: weakly interacting massive particles.
Despite extensive international experimental efforts, this dark matter has not been directly detected. Only the DAMA/LIBRA experiment maintains a signal that has not been confirmed by any other experiment, but neither has it been refuted because the comparison depends on the particle model considered and the distribution of particles in our Galaxy’s halo.
For the first time, an independent experiment, ANAIS-112, at the Canfranc Underground Laboratory in Spain, using similar sodium iodide detectors and after analyzing six years of data, provides highly significant results (above the four-sigma level) that are inconsistent with those of DAMA/LIBRA, independently of the particle and halo models, using the same material, sodium iodide, as the detector.
“The signal observed by DAMA/LIBRA is fully compatible with what is expected to be produced by dark matter particles distributed in our galaxy’s halo. This result could have opened the door to new theories, outside of established frameworks. The fact that another experiment using the same material as a detector observed nothing rules out the interpretation in terms of dark matter. However, understanding the DAMA/LIBRA observation is still far from complete. To date, there is no convincing explanation for the result,” note María Luisa Sarsa Sarsa and María Martínez Pérez, professors in the area of Atomic, Molecular, and Nuclear Physics in the Department of Theoretical Physics, and researchers in the Nuclear Physics and Astroparticle Group, affiliated with the IUI CAPA-Unizar.
“We are collaborating with Italian, American, Korean, and Australian researchers with the goal of understanding the DAMA/LIBRA observation,” the co-directors of the ANAIS experiment point out. “The DAMA/LIBRA experiment will stop collecting data at the end of 2024, and we are trying to ensure that some of its detectors can be studied at the Canfranc Underground Laboratory.”
ANAIS continues collecting data at the Canfranc Underground Laboratory with the goal of achieving a sensitivity of five sigma with the full data set, which will correspond to approximately eight years of effective measurement time with its 112.5 kg detection mass.
In parallel, in a joint effort with the international COSINE-100 collaboration, a combined analysis of partial data from the two experiments (ANAIS-112 and COSINE-100) corresponding to three years has been developed. This analysis, also incompatible with the DAMA/LIBRA modulation signal, has been accepted for publication in the same journal, Physical Review Letters. This combined analysis was led by Sophia Hollick, a PhD student at Yale University, who has been on a research visit at the University of Zaragoza since September 2023.
ANAIS-112 is an experiment proposed and operated at the Canfranc Underground Laboratory by researchers from the University Research Institute Center for Astroparticles and High Energy Physics (IUI CAPA) of the University of Zaragoza. The R&D development of the detectors with the necessary sensitivity and the fine-tuning of the experiment have required a twenty-year effort and have been possible thanks to the funding of MICIU/AEI/10.13039/501100011033 (projects Nos. PID2022-138357NB-C21 and PID2019-104374GB-I00), the European Union NextGenerationEU/PRTR (AstroHEP) and the Government of Aragon.