Professors Blas Cabrera from Stanford University and Bernard Sadoulet from University of California, Berkeley, have been awarded the 2013 W.K.H. Panofsky Prize in Experimental Particle Physics. The award is in joint recognition of Cabrera and Sadoulet "for their pioneering work and leading roles in the development and use of phonon detection techniques enabling direct searches for weakly interacting massive particles." Cabrera and Sadoulet have collaborated closely and have both served as leaders in the series of Cyrogenic Dark Matter Search (CDMS) experiments.
Blas Cabrera has spent much of his career bringing his deep physical intuition and his knowledge of condensed matter physics to bear on some of the most difficult experimental challenges. He came to Stanford as a graduate student in 1968 to study superconductivity, and earned his Ph.D. with Prof. William Fairbank on experimental gravitational research. About a decade later, Cabrera launched his research into dark matter detection. At the time, there was a suggestion that dark matter may consist of magnetically charged particles, or monopoles. On Valentine’s Day in 1982, his group observed a very interesting event in their SQUID detector, consistent with a magnetically charged particle passing through the detector. Subsequently, his group built two generations of larger and more sensitive detetors, but were never able to reproduce the original finding; they concluded that the event was most likely caused by a spurious effect in the apparatus.
In the meantime, the newly invented theory of inflation transformed our understanding of cosmology and particle physics; the favored candidates for dark matter were weakly interacting massive particles, or WIMPs. In 1985, Blas Cabrera, Lawrence Krauss, and Frank Wilcek wrote a seminal paper on “Bolometric detection of neutrinos,” which appeared in Physical Review Letters. This paper laid out some of the original ideas for low-background cryogenic detectors. Cabrera's work on such detectors has revolutionized a wide range of measurements, and the growing impact of transition edge sensor (TES) technology is a perfect example. The modern era of TES detectors was initiated by a 1995 paper Cabrera wrote with his PhD student Kent Irwin ("A quasiparticle-trap-assisted transition-edge sensor for phonon-mediated particle detection"). These devices are making a critical contribution in WIMP searches (summarized below), but Cabrera and his former students have shown how the devices can impact many other fields. For example, their ability to count, time and energy-resolve individual photons down to infrared energies allows an exciting range of new experiments in photon-starved sciences (see the 1998 Applied Physics Letters paper by Cabrera et al. on "Detection of Single Infrared, Optical, and Ultraviolet Photons Using Superconducting Transition Edge Sensors", with more than 200 citations). Follow-on papers describe the first astronomical observations of pulsars, black hole binaries, and similar objects with TES-based instruments. TES detector arrays have also become the technology of choice in the microwave/far-IR (for cosmic microwave background experiments, run in bolometer mode) and will provide the highest resolution nondispersive (imaging) photon-counting spectrometers for the next generation of large X-ray space telescopes. Further, the photon-counting, energy-resolving performance is enabling important applications in a number of other fields including quantum information, nuclear nonproliferation, biological assay and device micro-forensics.
Cabrera and his group applied TES technology to develop large-scale cryogenic detectors for dark matter searches, using germanium and silicon crystals operating below 0.1 K. At each stage, the CDMS detectors have achieved lower background levels and greater sensitivity. Cabrera was co-spokesperson with Sadoulet for the recently completed CDMS II experiment, which operated 4 kg of germanium and 1 kg of silicon for two years and set the most senstive limits for spin-independent interactions for WIMP masses above 40 GeV/c^2. In 2011, the collaboration published a low-energy-threshold analysis that extended the sensitivity to lower masses, where other experiments had observed potential signals; no signals were seen by CDMS II.
Cabrera serves as spokesperson for the SuperCDMS Soudan experiment, which will operate 10 kg of germanium for two years in the Soudan Mine in Minnesota. The SuperCDMS SNOLAB experiment has been proposed to operate 100 kg of germanium in the deeper SNOLAB facility in Canada. For this phase of CDMS, Cabrera and his group developed double-sided detectors in which ionization electrodes are interleaved with phonon sensors on both sides of the detector, allowing essentially complete rejection of near-surface events and conquering one of the last significant backgrounds in the detectors.
One of Cabrera’s important contributions to the field has been his training of talented experimentalists – over 20 Stanford graduate students and a dozen postdocs – many of whom have gone on to play leading roles in the rapid progress of the field of cryogenic detectors, and the important scientific investigations they enable.
In 2011, Cabrera was named the first holder of the Stanley G. Wojcicki Professorship in experimental physics. On Sunday, October 7, the Physics Department will host a one-day symposium to honor Professor Blas Cabrera and his achievements in teaching, advising and research: https://physics.stanford.edu/cabrerafest-2012
The Panofsky Prize was established in 1985 by friends of Wolfgang K. H. Panofsky, and by the American Physical Society Division of Particles and Fields, Stanford University and SLAC. Wolfgang (Pief) Panofsky served as an Assistant and Associate Professor at University of California, Berkeley, before coming to Stanford as a Professor and then serving as the first Director of the Stanford Linear Accelerator Center from 1961 to 1984.