Introduction

The ANAIS Experiment

The direct detection of the Dark Matter of the Universe is a very exciting challenge full of unknowns and uncertainties about its nature. ANAIS is an experiment developed by the Nuclear Physics and Astroparticles group of the University of Zaragoza which pursues this elusive dark matter detection by looking at the annual modulation of the expected interaction rates in a target of sodium iodide, material which produces small scintillations when a particle interacts and deposits some energy. This modulation is a distinctive feature stemming from the Earth revolution around the Sun which changes periodically the relative velocity of the incoming Dark Matter particles to the detector and, because of that, the energy deposited. DAMA-LIBRA experiment at Gran Sasso Underground Laboratory has reported the presence of modulation in its data with a high statistical significance; ANAIS could confirm it and help to understand the different systematics involved.

ANAIS is the large scale conclusion of previous studies carried out with different prototypes by the University of Zaragoza group at the Canfranc Underground Laboratory, Spain. The complete experiment will use 250 kg of NaI(Tl) crystals to study the expected annual modulation in the galactic dark matter signal.

DAMA/LIBRA, experimental effort performed at the Gran Sasso National Laboratory, accumulating more than thirteen annual cycles of data (also with NaI scintillators), obtained (and confirmed with larger statistics) a positive signal for the above mentioned annual modulation. This result is difficult to compare with those obtained with other targets in different experimental approaches since comparison is strongly model dependent. Most current dark matter experiments are using techniques for the discrimination of electron recoiling events, rejecting a large fraction of the registered events. In the case a relevant part of the energy released in the WIMP interaction be converted through this channel, the event would be taken as background. Besides, strong dependence on some analysis parameters (as relative efficiency factors for nuclear recoils) could difficult comparison between results derived using different targets. For all that, ANAIS (that uses the same target and technique that DAMA/LIBRA) appeared in the last roadmap of ApPEC as the experiment that would allow testing such result with an independent experimental set-up and in a model-independent way.

ANAIS aims to set up, at the new facilities of the Canfranc Underground Laboratory, a large scale NaI(Tl) experiment able to observe the DAMA/LIBRA positive result. For this goal, some of the experimental features of ANAIS should be improved and we are focusing efforts in the corresponding directions:

  • Energy threshold below 2 keV should be achieved in order to improve sensitivity for the annually modulated WIMP signal. This goal is addressed by testing new high quantum efficiency photomultiplier tubes, trying to avoid light guides by using very low background phototubes and using new energy estimators at very low energies, as well as a better noise rejection procedure.
  • Background at low energy should be kept as low as possible. After characterization of the available NaI(Tl) crystals, the measured 40K bulk crystal contamination appears as the most dangerous at the very low energies of interest for ANAIS and has forced ANAIS to look for more radiopure crystals, implying the development of new purification techniques and providers.
  • Very stable operation conditions should be guaranteed. Recently, the moving of the ANAIS experiment to the new Canfranc Underground Laboratory facilities was undertaken.

ANAIS is a part of the MULTIDARK project, a nationalwide effort having the goal of approaching the spanish research groups working on the search for the dark matter.

ANAIS is financially supported by Spanish MICINN and MEC (Grants No. FPA2008-03228 and FPA2011-23749 and Consolider-Ingenio 2010 Programme under grants MULTIDARK CSD2009-00064 and CPAN CSD2007-00042) and the Gobierno de Aragón (Group in Nuclear and Astroparticle Physics, ARAID Foundation and C. Cuesta predoctoral grant). We also acknowledge the technical support from LSC and GIFNA staff.