In astronomy and cosmology, light dark matter refers to dark matter Weakly interacting massive particle (WIMP) candidates with masses less than 1 GeV. These particles are heavier than warm dark matter and hot dark matter, but are lighter than the traditional forms of cold dark matter. The Lee-Weinberg bound limits the mass of the favored dark matter candidate, WIMPs, that interact via the weak interaction to GeV. This bound arises as follows. The lower the mass of WIMPs is, the lower the annihilation cross section, which is of the order, where m is the WIMP mass and M the mass of the Z-boson. This means that low mass WIMPs, which would be abundantly produced in the early universe, freeze out (i.e. stop interacting) much earlier and thus at a higher temperature, than higher mass WIMPs. This leads to a higher relic WIMP density. If the mass is lower than GeV the WIMP relic density would overclose the universe.
Some of the few loopholes allowing one to avoid the Lee-Weinberg bound without introducing new forces below the electroweak scale have been ruled out by accelerator experiments (i.e. CERN, Tevatron), and in decays of B mesons.
A viable way of building light dark matter models is thus by postulating new light bosons. This increases the annihilation cross section and reduces the coupling of dark matter particles to the Standard Model making them consistent with accelerator experiments.
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