Two new ground-breaking measurements reported results in the last 24 hours! Here are very quick summaries.
A group of atomic physicists, called the ACME collaboration, has performed the best search so far for the electric dipole moment (EDM) of the electron. Unfortunately they didn’t find the EDM, but the limit
- |de| < 8.7 10-29 e cm
is 12 times stronger than the previous one. While this is still a billion times larger than what is expected in the Standard Model of particle physics (the equations used for the known elementary particles and forces), there are various types of as-yet unknown particles and forces that could easily produce a much larger electron EDM, through new violations of T symmetry (or, almost equivalently, CP symmetry). These effects could have been large enough to have been discovered by this experiment, so those types of possible phenomena are now more constrained than before. Fortunately, there’s more to look forward to; the method these folks are using can eventually be improved by another factor of 10 or so, meaning that a discovery using this technique is still possible.
This morning the LUX dark matter experiment reported new results, and knocked everyone’s socks off. They have understood their backgrounds from radioactivity much better and more quickly than most of us expected, using new calibration methods and a much better characterization of their backgrounds than has previously been possible. Although they have a detector only a bit larger than XENON100 and have only run the detector underground for three months, compared to the year or so that XENON100 ran previously, their limits on the rate for a dark matter particle to hit a Xenon nucleus beats XENON100’s results by a factor of 2 for a dark matter particle of mass 1000 GeV/c², increasing to about a factor of 3 for a dark matter particle in the 100 GeV/c² mass range, and soaring to a factor of 20 for a dark matter particle in the 10 GeV/c² mass range. Consequently, LUX pretty definitively rules out the possibility, hinted at by several dark matter experiments (as discussed in the second half of the article I wrote about this in April), of a dark matter particle in the 5 – 20 GeV/c² mass range. (See the figure below.) While XENON100 seemed to contradict this possibility already, it didn’t do so by a huge factor, so there were questions raised as to whether their result was convincing. But the sort of ~10 GeV/c² dark matter that people were talking about is ruled out by LUX by such a large factor that finding ways around their result seems nigh impossible. And again, there’s more to look forward to; by 2015 their results should improve by another factor of 5 or so… so they get another shot at a discovery, as will XENON1T, the successor to XENON100.
Congratulations to both groups for their spectacular achievements!