RUI: A Search For Dark Matter With Superheated Liquid Targets

Indiana University, Bloomington IN

Investigators

Abstract

One of the most enduring mysteries in science is the explanation for the evidence that, on all scales larger than a single star, far more gravitating material is seen to exist than can be accounted for by the atoms in these systems. This material, called Dark Matter (DM), is intrinsically invisible and has only conclusively been seen by its gravitational effects in aggregate. The objective of this project is to test the hypothesis that DM is an exotic particle, constituting about 85% of the mass of the Universe. If DM is observed in this detector, the project would be able to characterize basic properties such as its mass and the strength with which it interacts with ordinary matter. Many undergraduates have participated in the work at IUSB as have high school teachers and students. They have participated in all aspects of "big science" at an undergraduate institution. The group has aided the broader scientific community by fabricating specially designed transducers for a wide array of operating environments. An astrophysics experiment is now using acoustic particle ID in a bubble chamber to study exotic reactions relevant to large stars. The Muon Ionization Cooling Experiment uses them to understand electrical breakdown in RF cavities. A competing dark matter experiment uses them to eliminate boiling background. A neutron parity violation spin rotation experiment will use them to understand bubble migration in liquid helium. This award provides funds to enable the IUSB group to continue to contribute to the detector R&D, installation, commissioning and running of the PICO bubble chambers and analysis of the resulting data. PICO uses superheated liquids to search for the tiny energy depositions left by nuclei recoiling due to collisions with dark matter. These depositions cause the liquid to explosively transform into gas. The bubble chamber technique represents a unique approach in the field, since it is inherently immune to sparsely ionizing particles (such as electrons), which are serious backgrounds to other DM detectors. This will be especially important if robust observation claims are made by experiments in the future. It has also been demonstrated by the group that the target fluids are easily changed, which will be important in determining the properties of any observed DM particle. The IUSB group has designed and fabricated all of the acoustic transducers for the detectors. The group recently discovered that one can even distinguish what kind of nucleus has decayed. IUSB also participates in the commissioning and running of the various bubble chambers at the deep site and energy threshold experiments at the Fermilab test-beam facility. A new design for transducers mounted in the dome over the bellows system will be made for a ton-scale detector. The group will also continue to contribute to the analysis of data already collected with the COUPP-60 and PICO-2L detectors.