University of Chicago Contribution to PICO-500 Engineering and Construction

University Of Chicago, Chicago IL

Investigators

Abstract

The problem of the identification of Dark Matter has been with us for more than eight decades. In the interim, astronomical evidence for the existence of this mysterious form of matter has poured in. Unable to absorb or emit light, its presence is made evident by its gravitational pull on luminous objects such as stars, galaxies, and larger structures. If composed of new elementary particles, dedicated detectors housed in underground laboratories -protected there from cosmic radiation- stand a chance of solving this long-standing riddle, now central to particle physics, cosmology and astronomy. PICO-500 is one of these detectors, consisting of one of the largest bubble chambers ever built and housed more than a mile underground in the Canadian SNOLAB laboratory. It is by design insensitive to most known sources of radiation, while highly responsive to the characteristic interactions expected from particle dark matter. This award will allow a US postdoc to play a central role in the construction, commissioning, and extraction of first data from this promising dark matter detector. Superheated liquids like those used in PICO bubble chambers provide the best discrimination of any dark matter detector against electron recoils, the dominant environmental background limiting the sensitivity of these searches. PICO detectors additionally feature an efficient acoustic rejection against signals induced by ubiquitous alpha emitters. This has resulted in a track-record of continuous increase in dark matter sensitivity from the PICO program, as its bubble chambers have grown in size. Recent phenomenological work has emphasized the importance of fluorine-containing PICO targets: these are necessary for a comprehensive exploration of all possible modes of dark matter particle interaction. The completion of the work described in this proposal will extend the sensitivity of PICO down to spin-dependent cross sections of 1E-42 cm^2 (an improvement of 100 beyond present limits). This will provide an exhaustive exploration of supersymmetric dark matter via this channel, and in particular for few GeV/c^2 particles, a mass range of much phenomenological interest. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.