Tag Archives: direct detection

SuperCDMS Collaboration Possible Detection of Dark Matter

Hard on the heels of the Alpha Magnetic Spectrometer (AMS) positron excess and possible dark matter report, we now have a hint of direct dark matter detection from the SuperCDMS Collaboration this month. A recent blog here on discusses the detection of excess positron flux seen in the AMS-02 experiment on board the Space Shuttle. The two main hypotheses for the source of excess positrons are either a nearby pulsar or dark matter in our Milky Way galaxy and halo.


Photo: CDMS-II silicon detector, Credit: SuperCMDS Collaboration

CDMS-II (CDMS stands for Cryogenic Dark Matter Search) is a direct dark matter detection experiment based in the Soudan mine in Minnesota. The deep underground location shields the experiment from most of the cosmic rays impinging on the Earth’s surface. Previously they had reported a null result based on their germanium detector and ruled out a detection. Now they have more completely analyzed data from their silicon detector, which has higher sensitivity for lower possible dark matter masses, and they have detected 3 events which might be due to dark matter and report as follows.

“Monte Carlo simulations have shown that the probability that a statistical fluctuation of our known backgrounds could produce three or more events anywhere in our signal region is 5.4%. However, they would rarely produce a similar energy distribution. A likelihood analysis that includes the measured recoil energies of the three events gives a 0.19% probability for a model including only known background when tested against a model that also includes a WIMP contribution.”

So essentially they are reporting a possible detection with something ranging from 95% to 99.8% likelihood. This is a hint, but cannot be considered a firm detection as it rises to the level of perhaps 3 standard deviations (3 sigma) of statistical significance. Normally one looks to see a 5 sigma significance for a detection to be well confirmed. If the 3 events are real they suggest a relatively low dark matter particle mass of around 8 or 9 GeV/c² (the proton mass is a little under 1 GeV/c², and the Higgs boson around 126 GeV/c²).


Figure: Error ellipses for CDMS-II and CoGeNT, assuming a dark matter WIMP explanation. Blue ellipses are the 68% (dark blue) and 90% (cyan) confidence levels for the CDMS-II experiment. The purple ellipse is the 90% confidence level for CoGeNT.

The figure shows the plane of dark matter (WIMP, or weakly interacting massive particle) cross-section on the y-axis vs. the WIMP mass on the x-axis. Note this is a log-log plot, so the uncertainties are large. The dark blue region is the 1 sigma error ellipse for the CDMS experiment and the light blue region is the 90% confidence error ellipse. The best fit is marked by an asterisk located at mass of 8.6 GeV/c² and with a cross-section a bit under 2 x 10^-41 cm². But the mass could range from less than 6 to as much as 20 or more GeV/c². And the cross-section uncertainty is over two orders of magnitude.

However, this is quite interesting as the error ellipse for the mass and interaction cross-section from this CDMS-II putative result overlaps well with the (smaller) error ellipse of the CoGeNT results. The CoGeNT experiment is a germanium detector run by a different consortium, but based in the same Soudan Underground Laboratory as the CDMS-II experiment! COGENT sees a possible signal with around 2.8 sigma significance as an annual modulated WIMP wind, with the modulation in the signal due to the Earth’s motion around the Sun and thus relative to the galactic center. The purple colored region in the figure is the CoGeNT 90% confidence error ellipse, and it includes the CDMS-II best fit point and suggests also a mass of roughly 10 GeV/c².

The DAMA/LIBRA experiment in Italy has for years been claiming a highly significant 9 sigma detection of a WIMP (dark matter) wind, but with very large uncertainties in the particle mass and cross-section. However both the COGENT results and this CDMS-II possible result are quite consistent with the centroid of the DAMA/LIBRA error regions.

And both the CoGeNT and DAMA experiments are consistent with an annual modulation peak occurring sometime between late April and the end of May, as is expected based on the Earth’s orbit combined with the Sun’s movement relative to the galactic center.

What we can say at this point is the hottest region to hunt in is around 6 to 10 GeV/c² and with a cross section roughly 10^-41 cm². Physicists may be closing in on the target area for a confirmed weakly interacting dark matter particle detection. We await further results, but the pace of progress seems to be increasing.

References: – Recent first results from AMS for positron excess – SuperCDMS Collaboration web site – “Dark Matter Search Results Using the Silicon Detectors of CDMS II” – Kevin McCarthy’s presentation at the American Physical Society, April 15, 2013 – CoGeNT website – Discussion of CoGeNT 2011 results  – DAMA/LIBRA results summary, 2013


Do we have a CoGeNT direct detection of Dark Matter?

CoGeNT detector during installation

CoGeNT detector during installation (Credit: Pacific Northwest National Laboratories)

(cogent = clear, logical, convincing)

The race to demonstrate direct detection of WIMP (weakly interacting massive particle) dark matter is heating up with this month’s release of results from the CoGeNT experiment, located in a mine in northern Minnesota*. They have just published results collected during the first 15 months of data taking. CoGeNT, as the Ge in the name indicates, uses a detector made of germanium.

There are quite a few such experiments that seek to measure the impact of WIMP dark matter as it directly strikes nucleons, that is, protons and neutrons, in some target material. The cross sections expected for such direct impact are extremely low, thus the experiments require relatively large detectors, high sensitivity, and long runs to gather sufficient statistical evidence of impacts and separate good events from background events due to other causes. The most favored candidate is a WIMP with mass somewhat under 10 GeV to perhaps as high as 200 GeV (the proton rest mass is .938 GeV, a GeV is a billion electron volts, and the mass is stated in energy equivalent units).

While XENON, CDMS (located in the same Soudan laboratory in Minnesota) and other direct experiments have not detected dark matter, for a number of years the DAMA/LIBRA project in Italy has been claiming the detection of an annual modulation of a dark matter signal. The modulation is said to be due to movement of the Earth toward and then away from the galactic center as it orbits the Sun each year, with the signal peaking in the second quarter of the year.

The CoGeNT experiment is also now claiming a detection of an annual modulation with about 2.7 or 2.8 sigma (standard deviations) of statistical significance, which is at the margin of a good detection. DAMA/LIBRA, which uses a thallium-doped sodium iodide crystal (salt) detector, claims a very high statistical significance of 8.9 sigma. Generally, 3 sigma of significance is considered sufficient for a good detection and 5 sigma would be considered a solid detection. The DAMA/LIBRA events have until now been unconfirmed, and have appeared to be in conflict with limits from other experiments including XENON and CDMS.

The CoGeNT results are consistent with DAMA/LIBRA in two respects. First, they together imply a relatively low mass of 5 to 12 GeV for the dark matter WIMP. Second, both the CoGeNT and DAMA experiments are consistent with an annual modulation peak occurring sometime between late April and the end of May, as is expected based on the Earth’s orbit combined with the Sun’s movement relative to the galactic center.

While the CDMS results appear to set limits which contradict both the CoGeNT and DAMA results, there are a number of uncertainties in the actual sensitivity of the respective experiments that may allow resolution of the apparent discrepancy.

We eagerly await further results from CoGeNT and from other experiments including CRESST and COUPP that are well suited to measurement of a relatively low mass WIMP particle such as CoGeNT is claiming to have detected.

*The mine is located in a state park, and tours down into the mine run during the summer months. It is also near to the beautiful Boundary Waters Canoe area that crosses into Canada, where I took a ten day canoe excursion as a Boy Scout, decades ago.


C. Aalseth et al. 2011, “Search for an Annual Modulation in a P-type Point Contact Germanium Dark Matter Detector”

D. Hooper and C. Kelso 2011 “Implications of CoGeNT’s New Results for Dark Matter”