- Light Dark Matter
Light Dark Matter refers to
Dark Matter WIMPcandidates with masses less than 1 GeV. [cite conference |first=M. |last=Cassé |authorlink= |coauthors=Fayet, P. |title=Light Dark Matter |booktitle= |pages= |publisher= |conference=21st IAP Colloquium "Mass Profiles and Shapes of Cosmological Structures" |date=4-9 July 2005 |location=Paris |url= |accessdate= |id=arxiv |id=astro-ph/0510490v1 ] These particles are heavier than Warm dark matterand Hot dark matter, but are lighter than the traditional forms of Cold dark matter. The Lee-Weinberg bound [B. W. Lee and S. Weinberg, "Cosmological Lower Bound on Heavy-Neutrino Masses", Phys. Rev. Lett. 39, 165 (1977)] limits the mass of the favored dark matter candidate, WIMPs, that interact via the weak interaction to about 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 abundandly 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 elecroweak scale have been ruled out by accelerator experiments (ie
CERN, Tevatron), and in decays of B mesons. [cite journal |last=Bird |first=C. |authorlink= |coauthors=Kowalewski, R.; Pospelov, M. |year=2006 |month= |title=Dark matter pair-production in b → s transitions |journal=Mod. Phys. Lett. A |volume=21 |issue=6 |pages=457–478 |doi=10.1142/S0217732306019852 |url= |accessdate= |quote= ]
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. [ C. Boehm and P. Fayet, "Scalar Dark Matter candidates" Nucl. Phys. B 683, 219-263, (2004) [http://arxiv.org/abs/hep-ph/0305261 preprint] ] [ C. Boehm, P. Fayet and J. Silk, "Light and Heavy Dark Matter Particles", Phys. Rev. D 69, 101302 (2004) [http://arxiv.org/abs/hep-ph/0311143 preprint] ] [C. Boehm, "Implications of a new light gauge boson for neutrino physics", Phys. Rev. D 70, 055007 (2004) [http://arxiv.org/abs/hep-ph/0405240 preprint] ]
In recent years light dark matter has become popular due in part to the many benefits of the theory. Sub-GeV dark matter has been used to explain the positron excess in the
galactic centerobserved by INTEGRAL, excess gamma raysfrom the galactic center [cite journal |last=Beacom |first=J. F. |authorlink= |coauthors=Bell, N. F.; Bertone, G. |year=2005 |month= |title= Gamma-Ray Constraint on Galactic Positron Production by MeV Dark Matter |journal=Phys. Rev. Lett. |volume=94 |issue= |pages=171301 |doi=10.1103/PhysRevLett.94.171301 |url= |accessdate= |quote= ] and extragalactic sources. It has also been suggested that light dark matter may explain a small discrepancy in the measured value of the fine structure constant in different experiments [ C. Boehm and Y. Ascasibar, "More evidence in favour of Light Dark Matter particles?", Phys. Rev. D 70, 115013 (2004) [http://arxiv.org/abs/hep-ph/0408213 preprint] ]
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