It’s been a while since there’s been much news on dark matter direct detection. Many of the experiments are working on building bigger and better versions of their detectors. Fortunately, there is still some news. LUX just released a paper with updated WIMP cross section limits after improving things like calibrations and background modeling. The limits have improved and LUX remains the strongest direct detection experiment over a wide range of WIMP masses (assuming there isn’t something strange going on with the individual nucleon couplings).
CMS has posted a new paper onto the arXiv that gives some new limits on theories like dark matter and unparticles. This paper looks for a signal of two charged leptons (with a reconstructed mass consistent with a Z boson) with a lot of missing transverse energy from invisible particles like WIMPs. As usual, they get results consistent with Standard Model prediction and thus set various limits on the theoretical models being studied.
DM-Ice has a recent result looking at annual modulations of muon signals in NaI crystals. DM-Ice is basically intended to be DAMA at the South Pole, where the different location could help provide hints about what is causing DAMA’s annually modulating signal. One common thought is that this is somehow related to the annually modulating signal of muons created by cosmic ray showers high in the atmosphere. This paper looks at both muons and phosphorescence caused by muons, and concludes that the signals from these aren’t enough to explain the DAMA signal. So, this seems to rule out one possible explanation for the DAMA signal.
CDMS has released the latest result from its CDMSlite, in which the detectors are run with a very low energy threshold. To do this, CDMS has to give up its particle ID information by applying a much higher bias voltage to the detectors than usual and then only measuring the phonon signal.
The measurement isn’t background free, and several germanium escape peaks are seen, but they use the optimum interval method to extract a limit. The limit turns out to be the best so far from any direct detection experiment between WIMP masses of 1.6 and 5.5 GeV. Few low background experiments are able to measure events at a threshold close to that of CDMSlite, so the measurement doesn’t need to be background free to give world leading limits.
BBC Earth has a new overview of dark matter written for laypeople. It has a bunch of nice pictures and figures and goes over a lot of the basic concepts like early evidence from Zwicky, the WIMP hypothesis, problems with MOND models, etc.
Xenon has a new paper out looking for an annual modulation in the spectrum of electron recoils in their detector. This is a potentially interesting work because it tests an alternative explanation of the DAMA signal. If instead of interacting with nuclei, WIMP dark matter interacts with electrons, DAMA might still see a signal while most direct detection searches, which look exclusively for nuclear recoils, won’t see anything. Electron analyses are difficult with most low-background dark matter detectors because things like beta decay products will contaminate the signal. Their result clearly seems to exclude the electron recoil hypothesis for the DAMA result, although they also appear to exclude 0-amplitude at over a 99% confidence level. The best phase, however, is quite far from the expected phase of a dark matter signal, so this may not be anything interesting in the end.
The arXiv has a new review of light dark matter direct detection efforts and challenges. Light dark matter, which is maybe a few tens of GeV or less in mass, became a hot topic a few years ago when a number of direct detection experiments started getting results that looked consistent with a light dark matter halo. While the results weren’t necessarily all consistent with one another, people started getting very interested in this idea. Unfortunately, the strongest and most recent limits seem to rule out these possible signals – at least using the standard dark matter halo and interaction models.
The paper talks about recent progress in understanding the possible light dark matter signals. Some of the results are now understood to probably just be backgrounds, but not all of them have been understood yet. One of the most interesting parts is the discussion of the irreducible neutrino background. This will restrict the ability of future experiments to set limits, since once backgrounds appear the limits scale roughly with the square root of the exposure time rather than linearly. Fortunately, the time distributions are different, so the effect is not as bad as it could be.