News from the Frontiers of Cosmology: A companion to the book The Edge of Physics
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Italian experiment continues to claim dark matter detection

Photo: The DAMA/Libra Project

Photo: The DAMA/Libra Project

The controversial DAMA/Libra experiment announced this week that the experiment has further strengthened the case for the existence of dark matter in the galaxy. The problem is that no other direct detection experiment – and there are many of them – has been able to confirm the results.

For more than a decade now, the DAMA collaboration, which runs the experiment in an underground laboratory inside the Gran Sasso Mountain in Italy, has claimed that their experiment has seen evidence of dark matter. Most direct detection experiments look for signs of a dark matter particle hitting one of their sensitive detectors (and none have been found with statistical certainty yet). DAMA on the other hand is not looking for a single particle. Rather, it is looking for a change in the number of particles that hit its detectors on an annual basis.

The idea is that as Earth moves around the Sun, it should encounter a larger flux of dark matter particles on approximately 2 June relative to 2 December. The Sun is moving towards the star Vega in our galaxy. As the Earth revolves around the Sun, in June it encounters dark matter particles with a velocity that is the sum of its speed around the Sun and the speed of the solar system towards Vega. In December, the velocities are subtracted. Assuming that the galaxy is filled with a halo of dark matter particles, this difference in velocities is expected to lead to the change in the flux of dark matter particles encountered by detectors on Earth on 2 June relative to 2 December.

In April 2008, the DAMA/Libra collaboration reported that they had evidence of this phenomenon after 11 annual cycles of observation. Now, they have announced that 2 more additional years of observation have confirmed the same, one year of which was with an upgraded detector.

Much of the dark matter community has steadfastly ignored the DAMA/Libra data, mainly because it has proved impossible to confirm independently, and they think that the annual modulation signal is just some systematic noise (which the DAMA/Libra collaboration denies, saying it has accounted for all systematic noise).

However, there are growing attempts to start reconciling the two sets of results. If the DAMA/Libra experiment is indeed seeing dark matter, then it has to be squared against the negative results of other extremely sensitive experiments like CDMS-II and Xenon10. One candidate is so-called inelastic dark matter. See this paper for details.

The Edge of Physics


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