Tag Archives: Dark Matter

Recent MALBEK Result on the ArXiv

About a week ago, the Majorana collaboration released a new MALBEK conference proceeding presenting a preliminary WIMP dark matter limit curve. MALBEK uses p-type point contact germanium detectors to search for new physics. In particular, the MALBEK detectoris basically identical to the germanium detectors used by the CoGeNT collaboration. A few years ago, CoGeNT famously released a result that appeared to display hints of the annually modulating event rate expected from a galactic WIMP dark matter halo. With the same type of detector in a different location, MALBEK could potentially hope to confirm or reject the existence of the proposed CoGeNT signal.

There’s nothing groundbreaking here, but MALBEK has obtained results that reject the CoGeNT result with certain analysis choices but not with others. They use a wavelet-based pulse shape discrimination variable to remove surface events but don’t have a very good way yet to quantify the efficiency for bulk nuclear recoils or the contamination from background.

Juan Collar (from CoGeNT) has already responded. He criticizes the rather opaque nature of the MALBEK pulse shape analysis. MALBEK resorted to these methods because it was found that a simple variable such as the pulse rise time does not separate surface and bulk events at energies below 2 keV. CoGeNT is able to separate these events due to a much lower amount of electronics noise.

At any rate,  the best results in the low mass region from CDMS and LUX already seem to rule out the CoGeNT dark matter hypothesis by a fairly wide margin. CDMS even uses germanium so one can’t argue that maybe the Ge cross section is enhanced compared to the standard assumptions for the Xe cross section.

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DoE and NSF Announce Support for Next-Gen Dark Matter Searches

Yesterday, the Department of Energy and National Science Foundation announced the major dark matter searches that they will be supporting for the coming years. The experiments to be supported are LZ (LUX-ZEPLIN), SuperCDMS (Cryogenic Dark Matter Search), and ADMX (Axion Dark Matter Search). Additional funding will be available for smaller R&D efforts.

My take on this is that the funding agencies have gone with the most conservative approach, supporting proven technologies and experiments based in North America.

LZ is a planned multiton dual-phase xenon TPC. It’s basically a scaled-up version of detectors like LUX and XENON-100, which have been getting some of the best WIMP search results in recent years. Currently, LUX is the most sensitive direct detection experiment for spin-independent interactions across a wide range of WIMP masses. These experiments use liquid xenon as their target material and pull electrons left from ionization into a gaseous region, where a high electic field causes electroluminescence that can be measured by photodetectors. The scintillation left from a nuclear recoil can also be measured by those detectors, giving them two energy channels to use for position and energy reconstruction and particle ID. LZ will be a continuation of the noble gas TPC program, and is expected to be constructed at the Homestake Mine in South Dakota.

SuperCDMS is a scaled-up version of CDMS, an experiment using cryogenic germanium detectors that are sensitive to both ionization and thermal excitation.Germanium detectors have less mass than the TPCs but potentially have a much lower energy threshold allowing for better sensitivity to low mass WIMPs. SuperCDMS is planned to be constructed at SNOLAB in Ontario, which has a long history of operating large experiments.

I’m not entirely sure what these choices mean for other experiments such as XENON-1ton, COUPP, DarkSide, DEAP/CLEAN. These experiments will not be getting US funding, but many of them have significant support from other countries. They’ll have to get enough non-US support to keep going. If US collaborators are unable to secure funding, it’s quite likely that some of these experiments will end up being forced to merge with others or will just shut down completely due to a lack of personnel.

DoE and NSF will continue to support ADMX, which searches for axion dark matter and is basically the only player in the axion DM field right now. This is not surprising, as it’s an easy way for the US to host a world-leading experiment. Finally, while there will continue to be funding for R&D efforts the specific groups and projects have not been announced, so it will be interesting to see what technologies are developed over the next few years.