Synopsis

# Dark Matter or Neutrons?

Physics 7, s100
The previously observed signal from a direct-dark-matter-detection search may instead be due to neutrons generated from solar neutrinos and atmospheric muons.

Dark matter, inferred to exist on the basis of the dynamics of galaxies, has never been conclusively detected in a direct way. The DAMA/LIBRA experiment in Italy is one of several ongoing searches for dark-matter particles interacting with ordinary matter. The collaboration has recently reported a modulation in their data with a period of one year, consistent with a dark-matter-related signal. However, new calculations published in Physical Review Letters suggest that such a signal may instead be caused by neutrons created by solar neutrinos and atmospheric muons.

DAMA/LIBRA focuses on measuring annual modulations in dark-matter flux caused by the combined motion of the Earth around the Sun and of the Solar System within the Galaxy. Researchers have observed a temporal variation in their data that exhibits a peak in late May, as expected if signals were due to dark-matter particles originating from the Milky Way’s dark-matter halo. But DAMA/LIBRA’s interpretation of dark matter requires a particle-interaction cross section and mass that are ruled out by other dark-matter experiments. This fact prompted Jonathan Davis at Durham University (UK) to propose an alternative explanation for the DAMA/LIBRA signal. Davis’s calculations show that the combined effect of both solar neutrinos and atmospheric muons could be responsible for the signal; the neutrinos and muons impinge on the lead shielding of DAMA/LIBRA and the surrounding rock and liberate neutrons, which then interact with heavy nuclei to produce recoil events that mimic those arising from dark-matter interactions.

The flux of atmospheric muons has been shown to vary with a period of approximately the Solar activity cycle (i.e., about $11$ years). Additional DAMA/LIBRA data over a longer time baseline are necessary to look for a sign (or lack thereof) of such a periodic mode. – Katherine Kornei

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