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By Hamish Johnston
For more than a decade physicists working on the DAMA/LIBRA experiment (and its predecessor DAMA/NaI) in Italy have collected and analysed data that show an annual oscillation in the signal from their dark matter detector.
The team argues that this is the first direct detection of dark matter particles – and that the yearly variation in the number of detections (see image above) is caused by Earth’s motion through the halo of dark matter that surrounds the Milky Way.
While no-one argues with the existence of the annual signal – after 13 years of data collection it has a whopping “nine sigma” statistical significance – there have been many arguments over whether the signal is related to dark matter.
For one thing, DAMA/LIBRA appears to be at odds with several other experiments that are also trying to detect dark matter – and this has led some physicists to dismiss the oscillation as the result of a yet-to-be discovered systematic error in the experiment.
However, these dissenting experiments are different from DAMA/LIBRA, and so far have only yielded null results. It’s possible, therefore that DAMA/LIBRA has managed to succeed where others have failed.
The latest addition to the debate comes from DAMA/LIBRA scientists who have posted a vigorous defence of their measurements and analysis on the arXiv preprint server. In the paper the team argue that there are “no systematics or side reactions” that are able to mimic the annual signal that they see.
There are several other experiments that may have caught tantalizing glimpses of dark matter. The latest is the CoGeNT collaboration in the US, which earlier this year reported seeing a number of events in their germanium detectors that could be dark matter.
While these results are very preliminary, and have been questioned by those working on other experiments, some scientists believe that CoGeNT could be seeing the same dark matter particles as DAMA/LIBRA. Your can read a preprint of their analysis, where they argue that the DAMA/LIBRA and CoGeNT results can both be explained by a dark matter particle with a mass of approximately 7 GeV.
Meanwhile at DAMA/LIBRA, the team is set to install new photomultiplier tubes that will boost the sensitivity of the experiment. The group has also secured funds to do research and development on a next-generation detector.
The dark matter of the Galaxy is very likely just made up of baryons, locked up in Macho objects of earth mass. This is predicted by gravitational hydrodynamics and explains a wealth of observations.
A dedicated Macho search is planned. In this light it is difficult to see a relation between the DAMA/LIBRA results and dark matter.
Dark matter of galaxy clusters is well described by neutrinos of ca 1.5 eV, explaining why all the GeV-TeV searches do not get to a clear point. This case will be tested in KATRIN 2015 by measuring the mass of the electron antineutrino in tritium decay.
A yearly variation in nuclear deacay half lives has been observed by the Brookhaven Laboratory, and associated independently with the change in the Earth-Sun distance as outlined in this article http://arxiv.org/PS_cache/arxiv/pdf/0808/0808.3283v1.pdf. I wonder if the DAMA/LIBRA researchers have considered a link between their observed yearly variation the yearly variation in radioactive half life. In other words, do these two phenomena have the same origin?
Jonny