Eric Dahl Assistant Professor / Joint with Fermi National Accelerator Laboratory
Prof. Eric Dahl and his group build particle detectors for the direct detection of dark matter. Cosmological and astrophysical observations, ranging in scale from single galaxies to the entire visible universe, all tell use that there is 5x more “dark matter” than ordinary matter in the universe, yet thus far we have not been able to observe individual dark matter particles. We know very little about the particle nature of dark matter, only that it is massive, stable, not made of baryons (protons or neutrons), and at most weakly interacting. This is enough to exclude every type of particle we’ve encountered so far, but hypothetical particles known as WIMPs (Weakly Interacting Massive Particles) are a likely candidate for the dark matter.
Direct detection experiments seek to observe WIMPs from our own galaxy’s dark matter halo as they scatter off of normal matter in detectors here on earth. Professor Dahl's group is part of the PICO collaboration (http://www.picoexperiment.com), one of several collaborations racing to unambiguously identify this signal. PICO uses bubble chambers deployed 6800 feet underground at SNOLAB to look for the ~10-keV nuclear recoils that are the signature of a WIMP interaction. Such an interaction in a PICO bubble chamber will nucleate a single bubble in the superheated fluid target. By tuning the degree of superheat, the chambers can be made completely insensitive to recoiling electrons from beta-decays and gamma-interactions -- backgrounds that plague other WIMP detection experiments. The PICO bubble chambers also employ ultrasonic acoustic detectors to distinguish nuclear recoils from alpha-decays, and high-resolution, high-speed cameras that resolve multi-bubble events from neutrons, giving the PICO experiment world-leading discrimination capability against all “typical” backgrounds to the WIMP signal.
The PICO-2L experiment (3-kg C3F8 target) has produced the most stringent limits to date from a direct detection experiment on the spin-dependent WIMP-proton cross-section. A larger detector, PICO60, has also produced competitive limits with a 30-kg CF3I target, and both detectors will begin their second physics runs at SNOLAB in 2015. The Dahl group at Northwestern leads the test-chamber program in the PICO collaboration, working with groups at Drexel, Queen’s University, and the University of Montreal to understand the exotic backgrounds that currently limit the reach of the PICO detectors.
The Dahl group is also a member of the LZ collaboration. The LZ experiment is a 7-ton xenon time projection chamber to be deployed in the Homestake Mine in South Dakota in 2019, and is the flagship US direct detection experiment for the next generation dark matter searches. The Dahl group also works on novel detector R&D, including a hybrid PICO+LZ detector based on superheated xenon.
- C Amole et al. “Dark Matter Search Results from the PICO-2L C3F8 Bubble Chamber”. ar.Xiv:1503.00008
E. Behnke et al. “First dark matter search results from a 4-kg CF3I bubble chamber operated in a deep underground site.” Phys. Rev. D 86, 052001 (2012). DOI:10.1103/PhysRevD.86.052001
- P. Sorensen and C.E. Dahl. “Nuclear recoil energy scale in liquid xenon with application to the direct detection of dark matter.” Phys. Rev. D 83, 063501 (2011). DOI:10.1103/PhysRevD.83.063501
- >E. Behnke et al. “Improved Limits on Spin-Dependent WIMP-Proton Interactions from a Two Liter CF3I Bubble Chamber.” Phys. Rev. Lett. 06, 021303 (2011). DOI:10.1103/PhysRevLett.106.021303
- P. Sorensen et al. “The scintillation and ionization yield of liquid xenon for nuclear recoils.” Nucl. Inst. Meth. A 601, 339–346 (2009). DOI:10.1016/j.nima.2008.12.197
- J. Angle et al. “First Results from the XENON10 Dark Matter Experiment at the Gran Sasso National Laboratory.” Phys. Rev. Lett. 100, 021303 (2008). DOI:10.1103/PhysRevLett.100.021303
- T. Shutt, C.E. Dahl, J. Kwong, A. Bolozdynya and P. Brusov. “Performance and fundamental processes at low energy in a two-phase liquid xenon dark matter detector.” Nucl. Inst. Meth. A 579, 451–453 (2007). DOI:10.1016/j.nima.2007.04.104
- E. Aprile, C.E. Dahl, L. deViveiros, R.J. Gaitskell, K.L. Giboni, J. Kwong, P. Majewski, K. Ni, T. Shutt and M. Yamashita. “Simultaneous Measurement of Ionization and Scintillation from Nuclear Recoils in Liquid Xenon for a Dark Matter Experiment.” Phys. Rev. Lett. 97, 081302 (2006). DOI:10.1103/PhysRevLett.97.081302