WARP: WIMP Dark Matter search with Liquid Argon

Princeton University, Princeton NJ

Investigators

Abstract

The recent measurements of the Wilkinson Microwave Anisotropy Probe (WMAP) made the evidence for the existence of dark matter very compelling. Only 4% of the total energy of the Universe is made of baryons, the ordinary matter of which we, and stars, are made. A much larger fraction (22%) is a different kind of matter, which does not collapse into stars and does not shine - hence the term Dark Matter. Dark Matter, though, plays a very important role in the evolution of the Universe. It is subject to the gravitational force, and its presence has indeed allowed the formation of Galaxies. It is what ultimately keeps Galaxies and Clusters of Galaxies together. To date, it is not clear if Dark Matter interacts with the ordinary matter only through the gravitational force or also through other forces. However, particle physics offers an attractive solution in the form of relic neutral particles produced shortly after the Big Bang, pervading the cosmic space, and interacting weakly with ordinary matter. This class of Dark Matter candidates go under the name of Weakly Interacting Massive Particles (WIMPs). The WARP (Wimp ARgon Program) was conceived to search for WIMP Dark Matter. WARP is capable of observing the extremely low energy signals produced by WIMP collisions with ordinary nuclei and, simultaneously, minimizing the signals from cosmic ray and natural radioactivity backgrounds that often limit observation of WIMP dark matter. WARP's two-phase drift chamber uses argon as a target. It has a unique capability that greatly enhances its sensitivity: it uses two fully independent techniques for discriminating minimum ionizing events from nuclear recoils. These rely on the subtle differences produced in the signals observed in WARP by the two different types of events. The viability of the WARP experiment was well established with the successful operation of a small (3.8-kg) yet extremely sensitive prototype detector at the LNGS, the laboratory for astroparticle physics located underneath the peak of the Gran Sasso mountain in central Italy (1 hour by car from Rome) at a depth of 3,800 feet. The WARP collaboration will install in 2006 a much larger (140-kg) detector at LNGS which should improve the sensitivity by about two orders of magnitude for WIMP searches. Education and Outreach are integral to the WARP effort. The PIs of the WARP Program are among the founders of the Gran Sasso-Princeton Summer School. Now in its second year, the School brings 20 promising high-school students from the Gran Sasso region to the Princeton University campus for an intensive 4-week program in physics. The School has become an instant success in Gran Sasso, with more than 300 applicants in 2005, and promises to become a lasting platform for the scientific and cultural enrichment of young Italian men and women. The school helps in establishing a strong link between the US and one of the prime laboratories for underground physics in the world, which hosts a number of US researchers and US funded efforts. The WARP program will also offer in the forthcoming years an opportunity for a number of US students to get directly involved with the project both at Princeton and at LNGS. Research opportunities will include construction and testing of part of the experimental apparatus, and analysis of the data from the experiment. This hands-on experience will give the students a rare window into the nature of the experimental process that produces and tests the very knowledge that they find in their textbooks. The research experience on the project will build the student's confidence, expand their intellectual and cultural horizons, and, hopefully, inspire some to pursue degrees and even careers in science or engineering and to serve as role models for other young students.