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Geant4 Joins the Hunt for Dark Matter

(Image - pattern)
Much as sunlight shining through a glass of water makes light patterns on a tabletop, phonons travel through a germanium crystal in preferred directions, creating patterns like the one seen above. (Simulated image courtesy Daniel Brandt.)

Deep underground, the Cryogenic Dark Matter Search experiment hunts for the brief—and rare—flash of a dark-matter particle zipping through detectors. Yet what would such a flash look like? And how can the CDMS collaboration carefully distinguish it from background particles like those originating from the cosmic rays that zing through the detector with far greater frequency than dark-matter particles?

Using the Geant4 software toolkit, originally designed to simulate high-energy particle physics experiments, a small team of SLAC researchers is working with the CDMS collaboration to answer just these questions for the future experiments.

CDMS began its search for dark matter at Stanford in the mid-1990s, and in the intervening years has evolved and improved in sensitivity. The next experiment, SuperCDMS at Soudan, is now under construction in Minnesota's Soudan Mine. There, protected from most cosmic rays and other pesky background noise by miles of dirt and rock, 15 kilograms of 1-inch thick germanium crystals will be cooled to nearly absolute zero as they wait for dark matter particles to pass through.

Detecting elusive dark matter particles with a germanium detector is possible because the interior of every crystal, be it the Hope Diamond or a grain of table salt, is constantly vibrating as the atoms transfer vibrations from one to the next. These waves, or "phonons," travel through the crystal, each carrying a discrete unit of vibrational energy.

The CDMS collaboration's germanium crystal detector is no different, and occasionally a dark-matter particle traveling through the germanium lattice will crash into the nucleus of a germanium molecule, sending out a shower of thousands of phonons and electrons. By simulating such an interaction—as well as similar interactions caused by other non-dark-matter particles—researchers can separate evidence of dark-matter particles from more mundane phenomena.

In the past, the CDMS collaboration has based its simulations of how phonons travel though the detector in Matlab, a programming language that helps scientists undertake numerical computations and simulations. Geant4 has been used for modeling the dark-matter and background radiation up until the point that it strikes the detector.

Daniel Brandt, a postdoctoral researcher at the Kavli Institute for Particle Astrophysics and Cosmology, is adding the ability to simulate phonons to Geant4. This work builds on SLAC's strong roles in both the CDMS collaboration—for which SLAC will manage the detector systems for a proposed future experiment at SNOLAB—and Geant4—for which SLAC plays a leading role in development, maintenance and user support on a global scale. Using Laboratory Directed Research and Development funds, Brandt seeks to create a more complete picture of how the CDMS detector will react to dark-matter particles.

Adding the capability to model the entire process in Geant4 would allow a more seamless picture of the expected dark-matter detection process, said MIT postdoctoral associate Steven Leman, who heads the CDMS detector Monte Carlo simulation group. "One of the greatest benefits is that this work would start to provide a solid-state physics framework to Geant4 for the physics and engineering communities at large."

Geant4 is already a robust simulation toolkit, Brandt said. Originally designed to help understand high-energy physics detectors, Geant4's set of customizable modules helps experimenters simulate particles passing through and interacting with matter. Those interactions can happen in just about any situation, be it a high-energy particle collision, a particle striking a space-based particle detector or a medical radiation procedure. SLAC pioneered the use of Geant4 in the simulation of high-energy physics experiments with the BaBar experiment. In the intervening years, the toolkit has also been used by the Fermi Gamma-ray Space Telescope, Enriched Xenon Observatory and International Linear Collider collaborations.

Once Brandt adds the phonon capability, Geant4 will be able to simulate the entire process of a dark-matter particle flying through the air, colliding with the germanium detector, and triggering a wave of phonons.

"Knowing what phonons do when we release them into the crystal is very difficult," said KIPAC physicist and CDMS SLAC Department Manager Eduardo do Couto e Silva. "But this may become possible when we combine the CDMS collaboration's very sophisticated understanding of the physics with this really powerful tool."

Over the past year, Brandt has demonstrated that it's technically feasible to introduce phonons into Geant4. Now he's writing the computer code that will integrate phonon processes into the toolkit. A year from now, Brandt plans to have that test code completed, and in two years he plans to have a final product that can be added to any Geant4 system in the world.

"This really wouldn't have been possible anywhere else," said Brandt, who is working closely with Makoto Asai, leader of the SLAC Geant4 team and spokesperson of the international Geant4 Collaboration. "The turn-around time is quite short here, because we have the expertise," Brandt said. "Last week I mentioned that Geant4 was missing something I needed. It's there now."

The project, which was first suggested in discussions between do Couto e Silva and KIPAC physicist and Stanford Professor Blas Cabrera, "brings people together who normally would work quite separately," do Couto e Silva said. "There's something about the environment at SLAC that does this—a combination of existing expertise and the environment of the lab makes this possible."

Brandt, Asai and do Couto e Silva hope that something special will not only help the CDMS collaboration confirm the first sightings of dark matter particles, but will also aid researchers in many different areas of science.

"There's an enormous potential for physics, biophysics and even materials science to use Geant4," said Asai. "Everybody has been reinventing the wheel, and there's really no need. There's great potential to use it in biomedical, space applications, and even at LCLS."

—Kelen Tuttle
SLAC Today, October 14, 2010