Dark matter has never been directly observed in the laboratory. The international XENON collaboration, that includes researchers from the Universities of Bern and Zurich, has not found evidence for it either. However, its latest results solve a long-standing scientific controversy in the field.
Although dark matter constitutes a significant part of the matter in our Universe, all evidence is indirect, via its gravitational pull on luminous matter. Since 14 years, however, scientists of the DAMA/LIBRA collaboration claim the discovery of the enigmatic dark matter particles. New results from the XENON project are now in strong conflict with these observations. «The XENON100 detector, that we used for our analysis, is one of the world’s most sensitive devices. Still, we did not find the dark matter so far», says Marc Schumann from the Albert Einstein Center for Fundamental Physics in Bern. Schumann is co-author of two new studies that were published in Science and in Physical Review Letters. A central assumption of most experiments is that dark matter particles would reveal themselves by occasionally colliding with atomic nuclei in the detector. In addition, the movement of the Earth around the Sun should induce an annual modulation in the signal rate, as explained by co-author Laura Baudis from the Physik-Institut of the University of Zurich: «In summer we expect more, in winter less events.» The DAMA/LIBRA experiment has measured precisely such a seasonal modulation with their Sodium-Iodine detector. However, the interpretation of the modulation as being due to dark matter particles is in conflict with other experiments, according to Baudis. To alleviate this conflict, theorist have postulated that dark matter particles would not scatter off atomic nuclei, but would interact with the electrons of the atomic shell.
Shielded by 1400 m of rock
Since DAMA/LIBRA can not distinguish between interactions with atomic nuclei and electrons, the XENON collaboration has now analysed its data for signatures of dark matter interactions with electrons. The XENON100 detector uses 62 kg of liquid xenon as detection medium, measuring the tiny charge and light signals expected to be induced by the rare collisions of dark matter particles with xenon atoms. «In contrast to DAMA/LIBRA, the XENON100 detector can distinguish well between interactions with nuclei and electrons», explains Marc Schumann. The experiment is operated in the Italian Gran Sasso Underground Laboratory, where the cosmic ray flux is reduced by 1400 m of rock. The XENON team has analysed its data for scatters off electrons and for temporal variations. Schumann: «The search for such variations did not show any significant modulation for periods up to 500 days - in strong contradiction to DAMA/LIBRA.» The XENON scientists have also calculated the expected signal in their detector, if dark matter were the explanation of DAMA/LIBRA. The results are unambiguous, as explained by Laura Baudis: «We have not observed any signal, only the expected background noise.» Thus none of the tested models withstands the XENON100 challenge, as the two Swiss scientists further elaborate. This clearly excludes an explanation of DAMA/LIBRA where dark matter would interact with electrons.
«The net is not yet sufficiently fine-meshed»
Do these new results imply that dark matter may not exist? Marc Schumann denies categorically: «You have to imagine the dark matter particles as small fish in the ocean. We know that they are there, but our nets are simply not yet sufficiently fine-meshed to catch them.» Since XENON100 has reached its sensitivity limit, the collaboration is now installing a 100 times more sensitive detector. This instrument, XENON1T, will start a new era for dark matter searches by the end of this year, as Laura Baudis states. The researchers from the Universities of Bern and Zurich are also active in this new project.
Exclusion of Leptophilic Dark Matter Models using XENON100 Electronic Recoil Data, XENON Collaboration, arXiv:1507.07747.
Search for Event Rate Modulation in XENON100 Electronic Recoil Data, XENON Collaboration, arXiv:1507.07748, Physical Review Letters