February 21, 2014
Confirmation Of LUX Dark Matter Results
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
Dark matter research, like all experiments involving particle and astrophysical detections, relies on sorting out the desired events (the source events) from the noise (the background events). Since the interactions occur at a quantum level, the statistical process of sorting through the data is laborious, but also, more importantly, relies on your ability to calibrate and understand the instrument.
Back in October the Large Underground Xenon (LUX) detector, a system designed to detect candidate dark matter particles called WIMPs (Weakly Interacting Massive Particles), published their results from the first 90 days of experimental operation. Despite being the most sensitive dark matter detector to date, the collaboration of scientists concluded that there was no statistically significant evidence of WIMPs identified in the data. This was a bit of a surprise as other, less sensitive efforts, had turned up hints of a signal.
In order to prove the data further, an effort was undertaken to increase the calibration accuracy, thereby lowering the effective background rate. This would allow for greater sensitivity in the statistical calculation, and if any WIMP events had been measured in the detector, this recalibration would increase the chance of finding them. “The new calibration improved our calibration accuracy by about a factor of 10,” said Rick Gaitskell, professor of physics at Brown and co-spokesperson for LUX. “It demonstrates that our first dark matter search result, which showed no sign of low-mass particles, is absolutely robust.”
These new results were achieved by further isolating the signal types that would be recorded when a WIMP particle would interact with a xenon molecule in the detector. Since dark matter particles do not interact electromagnetically – the traditional type of interaction used to trace particle interactions – these types of detectors look for direct contact collisions, where a dark matter particle hits the nucleus of an atom (in this cases xenon) inside the instrument.
This interaction will excite the nucleus causing it to recoil and radiate energy in the form of light and an ion charge that can be captured by an array of photo sensors. The challenge, however, is that dark matter is not the only particle type that will cause these recoil emissions. So the team re-calibrated the instrument to be specifically sensitive to this specific interaction. “One of the important things we need to do is to calibrate the detector for what a WIMP-like recoil would look like,” said James Verbus, a graduate student at Brown who led the new calibration work. “You want to be able to measure your detector response for WIMP-like events.”
To achieve this new calibration, the research team used neutron beams as an approximation for WIMP particles passing through the detector. Since physicists believe that neutron interactions will be similar to those of the WIMPs, the researchers were able to measure and characterize the neutron interaction events as a function of energy. This procedure is unique in that the team used the actual LUX detector itself for the study, whereas previous experiments had done the calibration in a separate chamber and not within the measurement region itself. “Because our detector is so big and detects recoil positions so well,” Verbus said, “we can just fire neutrons directly into LUX and get an absolute measurement of energy.”
While the LUX result does not completely rule out dark matter, or more specifically WIMP, models of the Universe, it does eliminate some of the contending theories. “There are literally thousands of models of particle physics lying bloodied in the gutter,” noted Gaitskell.
The results of the new analysis were presented Wednesday, Feb. 19, 2014, at the Lake Louise Winter Institute in Alberta, Canada.