X-rays Limit Mass of Neutrino Dark Matter December 18, 2001 A team of scientists at the University of California, San Diego (UCSD) has used data from X-ray telescopes to set a limit on the characteristics of a possible dark matter candidate -- sterile neutrinos. They predict that future observations could confirm or disprove the theory that most of the matter in the universe is in the form of sterile neutrinos produced in the first nanosecond of the Big Bang. In a paper published in the December 1 issue of the Astrophysical Journal, the scientists show that sterile neutrinos could betray their presence to sensitive X-ray telescopes because on extremely rare occasions they will disintegrate into a light neutrino and an X-ray. "On average a sterile neutrino will decay only once every septillion years or so," said Kevork Abazajian, lead author of the paper. "The chance of catching one would appear to be impossibly small. However, if sterile neutrinos make up the dark matter that permeates gigantic galaxy clusters, the odds actually become reasonable." A number of astrophysicists have suggested that sterile neutrinos with a mass between 0.2 and 2 per cent of the mass of the electron could explain many observations relating to dark matter and the formation of galaxies that more massive and stable dark matter particles cannot. The decay of sterile neutrinos would produce a sharp peak in the X-ray spectrum from the hot gas clouds present in galaxy clusters. The predicted height of the peak in the spectrum increases with the assumed mass of the sterile neutrino. Data already collected from the Chandra and XMM-Newton X-ray observatories show that the X-ray spectra from the Virgo cluster of galaxies sets a limit on the mass of the hypothetical sterile neutrinos. In particular, assuming that neutrinos and antineutrinos were produced in equal numbers in the Big Bang, the observations rule out the possibility that the dark matter is in the form of sterile neutrinos with masses greater than about 1 percent of the mass of the electron. Much longer observations with these telescopes could lower the mass limit dramatically. The next generation of X-ray telescopes could either directly detect the decay of sterile neutrino dark matter, or effectively rule it out as a dark matter candidate. "In any event," says George Fuller a coauthor of the paper, "it is remarkable and unexpected that we can use modern X-ray observatories to probe very early epochs of the universe, and explore a new regime of particle physics inaccessible in a laboratory." This research was supported by the National Science Foundation and NASA. Science Contacts: Kevork Abazajian NASA/Fermilab Theoretical Astrophysics Group (630) 840-8195 aba@fnal.gov Professor George Fuller CASS/UCSD (858) 534-6329 gfuller@ucsd.edu CASS/UCSD