The ANDES Deep Underground Laboratory

Xavier Bertou

Underground science is living in exciting times. This year we celebrate 100 years from the first flight of Victor Hess leading to the discovery of cosmic rays. While cosmic rays have helped a lot in the understanding of particles in the early years, they are now a source of noise for many high-energy physics experiments. To escape from this unwanted background, tens of different experiments have moved to laboratories thousands of feet below ground level, to look for elusive neutrinos or dark matter particles.

Many of these studies are aiming at a better understanding of the neutrino. How does it oscillate? What kind of particle is it? What mass does it have? And after the surprising result of the Opera experiment at the Gran Sasso underground laboratory last year, do they really travel faster than light?

Not less interesting is dark matter, expected to be 85% of the total matter of the Universe. Dark matter searches are performed in deep underground laboratories all over the world, and while no clear direct signal has been seen yet, 3 different experiments are seeing a counting rate modulation which could well actually come from the movement of the earth through the dark matter halo of our galaxy. However, such a modulation could also be faked by more mundane effects related to atmospheric changes. To get a definitive answer, this modulation should be measured in the southern hemisphere. A similar modulation would be the sign of a genuine dark matter signal, while an opposite modulation is explained by an atmospheric effect. Unfortunately, all the current underground laboratories are located in the northern hemisphere.

In the last decade, strong efforts have been made in Latin America towards a better integration, and the booming economies of the region have been looking forward to the Asian market. Linking commercially the productive areas of Brazil and Argentina to Asia implies the construction of complementary accesses to the Pacific through the Andes. One of the main projects being pushed forward is the Agua Negra tunnel, to be dug under the Andes between the Chilean 4^th region of Coquimbo and the Argentine province of San Juan. It will be a double 14 km long road tunnel, and its deepest point will have more than 1750 m of rock overburden. This tunnel is, in other words, an ideal location for a world class deep underground laboratory, the first in the southern hemisphere.

The construction of the ANDES deep underground laboratory has been pushed forward since 2010. While not initially thought of as an acronym, ANDES can stand for Agua Negra Deep Experiment Site. The foreseen underground layout will consist of various halls for a total volume of more than 60,000 m³ over about 4,000 m² to harbour all the necessary equipment for several major experiments. The dark matter program will of course be a highlight of the facility, and repeating any modulation seeing experiments in the southern hemisphere will be the first target. In the neutrino sector, a large detector able to detect low energy neutrinos is being designed, mainly focused on neutrinos from solar and supernovae origin, as well as geoneutrinos. Geoneutrinos are neutrinos produced by the decay of radioactive products in the Earth (mainly potassium, uranium and thorium), and are thought to be of extreme relevance for the heat budget of the Earth. ANDES is at a unique location to make this measurement, as not only the geoneutrino flux is expected to be high due to the thick Earth crust at the level of the Andes, but also because one of the main backgrounds in this measurement, neutrinos from nuclear power plants, is absent (there are few nuclear power plants in the region, compared to other laboratories where such measurements are conducted).

The location of ANDES is also unique as it is located close to an active seismological region. In addition to the measurement of geoneutrinos, this is an ideal site for a full geophysics underground laboratory, linked to both the Chilean and the Argentine network of seismographs. The laboratory will also host an ultra low radiation measurement area, important to select high purity material for neutrino and Dark Matter experiments, but also for environmental studies.

Taking into account that the Agua Negra tunnel is an important project of regional integration, ANDES was seen as a similar opportunity, at a time where science is well promoted in most Latin American countries. In order to build and operate the ANDES laboratory, a larger body, the CLES (Latin American Consortium for Underground Science, in Spanish and/or Portuguese), is being considered. The CLES is thought of as the natural body to operate, finance, and plan the scientific roadmap for ANDES, and as a way to promote the academic integration among Latin American countries participating in the project. The four countries currently being actively part of the CLES are Argentina, Brazil, Chile and Mexico, but it is foreseen more will join in the future. It is a unique opportunity not only to host international experiments and to build an international laboratory, but also to create an international body in charge of coordinating these activities together with academic development and integration, human resources formation, and a rich technical, social and outreach program.

Underground science is really living in exciting times.

Longitudinal section of the planned Agua Negra road tunnel. ANDES would be located at the deepest point, with more than 1750m of rock overburden.


3D conceptual view of a possible layout of ANDES and its connection to the two Agua Negra road tunnels.

Xavier Bertou is head of the department in the high energy research and technology unit of CNEA, associated researcher of CONICET, and coordinates the ANDES initiative.  He works at Centro Atómico Bariloche in Argentina.