May 21, 2013
Lori Ann White
Theorists from the Kavli Institute for Particle Astrophysics and Cosmology are helping dark matter sleuths decide where to start their search.
“Now that it looks like the hunt for the Higgs boson is over, particles of dark matter are at the top of the physics ‘Most Wanted’ list. Dozens of experiments have been searching for them, but often come up with contradictory results.
Theorists from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint SLAC-Stanford institute, believe they’ve come up with an algorithm – a mathematical description of how the individual particles behave – that could help narrow the search for these elusive particles, which are thought to make up more than 25 percent of the matter and energy in the universe.
It starts with assumptions, said Yao-Yuan Mao, lead author of a paper published in The Astrophysical Journal that outlines their new search tool. Assumptions are a good starting point when you don’t know where to look. A popular assumption about dark matter is that it’s made up of WIMPs, Weakly Interacting Massive Particles. The “M” in WIMP accounts for gravity’s ability to herd these particles around; the “P” and “I” hint at why they’re so hard to detect otherwise.
KIPAC theorists (l to r) Louis Strigari, Risa Wechsler and Yao-Yuan Mao discussing dark matter velocity distributions. (Credit: Luis Fernandez.)
Most dark matter detectors are based on the assumption that, every once in a while, a WIMP must smack into the nucleus of an atom of visible matter, making the nucleus vibrate and releasing a signal. Such disruptions can be detected. But what that disruption looks like and how often it happens depends on yet more assumptions. How heavy is the dark matter particle? How fast is it moving?
Left panel: Air molecules whiz around at a variety of speeds, and some are very fast. When they collide with both heavy and light elements – for example, xenon (purple) and silicon (orange) – these fast moving particles have enough momentum to affect both nuclei. Right panel: Dark matter particles are moving more slowly and are less able to affect the heavy xenon nucleus. As a result, detectors made from lighter materials like silicon may prove to be more effective at picking up signals of dark matter. (Credit: Greg Stewart/SLAC National Accelerator Laboratory)
Another common assumption that touches on these issues, said Mao, is that collections of WIMPs behave as an ideal gas, a collection of particles that hang out together and occasionally bounce off each other. Sometimes a lucky bounce gives a particle more energy, sending it zooming off at a greater speed. How often particles pick up more energy and more speed depends on how much you turn up the heat or put on the pressure.
But, as far as scientists can tell, turning up the heat and putting on the pressure doesn’t affect WIMPs. Only gravity does.
“The Ideal Gas Law doesn’t describe a system of particles, like dark matter particles, that don’t seem to transfer energy to each other,” said Mao. This incorrect description can distort the carefully built picture upon which a search for WIMPs is based. In particular, it means predictions of their velocities can be off by a significant amount, but velocities affect what a detector will see.
Mao and his colleagues have used simulations to provide new insight into how fast WIMPs are expected to move.”
See the full article here.
SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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