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WIMPs are bigger than we thought

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Physicists from Brown University are claiming to have set a lower limit for the mass of dark matter.

Their analysis of data from the NASA Fermi Gamma-ray space telescope, to be published in Physical Review Letters on December 1 (pre-press copy here), suggests a lower mass limit of 40 GeV (giga-electron volts) for dark matter.

This limit, the researchers say, cast doubt on experiments that have claimed to spot dark matter in underground laboratories. For example, in September the CRESST experiment in Italy said that it had spotted 67 particle interactions that may be “WIMPs” (weakly interacting massive particles), one particle type hypothesized as a possible dark matter candidate.

The Brown University researchers, assistant professor Savvas Koushiappas and graduate student Alex Grainger-Smith, analyzed the annihilation cross-section of WIMPs in seven Milky Way dwarf galaxies. These are good place to look for WIMPs, it seems, because their anomalous motion suggests that there’s more stuff in them than their visible ordinary-matter mass.

In addition, the number of photons in the galaxies is measurable (El Reg: Think about that for a second. Whatever that number is, it’s mind-bendingly big – and it’s measurable. Wow.), which then permits a calculation of the dwarf galaxies’ quark production.

Those quarks, it seems are the key. From the University’s announcement: “quark production … allowed them to establish constraints on the mass of dark matter particles and the rate at which they annihilate.”

That annihilation, the Brown University researchers say, gives rise to their calculation of the lower mass limit of 40 GeV (although if I’m reading the Arxiv paper correctly, the error band still allows a mass of 19GeV) – far higher than the 7 GeV to 12 GeV suggested in experiments such as the CRESST research (others include DAMA/LIBRA and CoGeNT). ®

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