The mass of the dark matter particle : is it in the keV range ?
1er mai 2010
A new analysis of dark matter particle mass, taking into account theory, observations of dwarf spheroidal satellite galaxies of the Milky Way and numerical simulations tends to suggest that the mass of the dark matter particle could be in the keV scale (by comparison E=mc2 =511 keV is the mass of the electron, or 10-30kg) and the temperature when the dark matter decoupled from ordinary matter and radiation, would be 100 GeV at least. Two scientists of the Observatoire de Paris and the University Pierre et Marie Curie performed this analysis and considered several possibilities for the dark matter particles : at decoupling they could be ultra-relativistic or non-relativistic, at or out of local thermal equilibrium. In all cases, the dark matter particles are "cold" enough to allow galaxy formation. In this model, the dark matter annihilation or self-interaction cross-section is negligible.
Although the problem of missing mass was noticed seventy-five years ago (Zwicky 1933, Oort 1940), the nature of this mass is not yet known. Non-baryonic dark matter represents about 23% of the content of the universe. It is made of unknown particles, which do not emit or absorb light, but are detected indirectly through their gravity. Dark matter in nearby galaxies has a flat radial profile in the center, that is called a core. This is in contradiction with simulations made in the standard model of cold dark matter (CDM), which predict steep radial profile, or cusps. A solution to this problem has been proposed with warm dark matter (WDM) or self-interacting dark matter, that can create cores. However, WDM predicts smaller cores for higher mass systems, in conflict with observations.
Since the beginning, constraints have been put on the possible mass of any dark particle : due to the decrease of the coarse-grained phase space density or PSD for non-interacting particles, their number density is bounded, implying that their mass must be larger than 1 Mev, according to Tremaine & Gunn (1979). This is to account for the maximum phase-space density observed today in dwarf spheroidal galaxies. Since then many authors have revised this constraint, and the minimum mass is a fraction of keV (e.g. Dalcanton & Hogan 2001, Madsen 2001, Boyanovsky et al 2008, Boyarsky et al 2009).
The PSD of the particles decrease from an initial value at the time of decoupling. The temperature of the universe at decoupling Td is still a free parameter. Particles can decouple as ultra-relativistic, if Td mc2, or as non-relativistic if Td <
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In the same framework, lower and upper bounds for the dark matter annihilation cross-section are derived. Given the constraints from the observations (X-rays, optical or lensing observations), the dark matter non-gravitational self-interaction is negligible.
Reference H. J. de Vega, N. G. Sanchez, `Model-independent analysis of dark matter points to a particle mass at the keV scale’, 2010, MNRAS, in press
Other literature Dalcanton J.J., Hogan C.J. : 2001, ApJ 561, 35 Boyanovsky, D., de Vega, H.J., Sanchez, N.G. : 2008, Phys. Rev. D77, 043518 Boyarsky A., Ruchayskiy, O., Iakubovskyi, D. : 2009 JCAP 03, 005 Madsen J. : 2001, PhRvD, 64b7301 Oort, J. : 1940 ApJ, 91, 273 Tremaine S., Gunn J.E. : 1979 PhRvL 42, 407 Zwicky, F. : 1933 Helv. Phys. Acta, 6, 124
Contact
Norma Sanchez (Observatoire de Paris, LERMA, et CNRS)
Dernière modification le 4 mars 2013