Warm dark matter (WDM) is a hypothesized form of
dark matter that has properties intermediate between those of
hot dark matter and
cold dark matter, causing structure formation to occur bottom-up from above their free-streaming scale, and top-down below their free streaming scale. The most common WDM candidates are
sterile neutrinos and
gravitinos. The WIMPs (
weakly interacting massive particles), when produced non-thermally, could be candidates for warm dark matter. In general, however, the thermally produced WIMPs are
cold dark matter candidates.
keVins and GeVins
One possible WDM candidate particle with a mass of a few
keV comes from introducing two new, zero charge, zero
lepton numberfermions to the
Standard Model of Particle Physics: "keV-mass inert fermions" (keVins) and "GeV-mass inert fermions" (GeVins). keVins are overproduced if they reach thermal equilibrium in the early universe, but in some scenarios the
entropy production from the decays of unstable heavier particles may suppress their abundance to the correct value. These particles are considered "inert" because they only have suppressed interactions with the
Z boson.
Sterile neutrinos with masses of a few keV are possible candidates for keVins.
At temperatures below the
electroweak scale their only interactions with standard model particles are
weak interactions due to their
mixing with ordinary
neutrinos. Due to the smallness of the mixing angle they are not overproduced because they freeze out before reaching thermal equilibrium. Their properties are consistent with astrophysical bounds coming from structure formation and the
Pauli principle if their mass is larger than 1-8 keV.
In February 2014, different analyses[1][2] have extracted from the spectrum of X-ray emissions observed by
XMM-Newton, a monochromatic
signal around 3.5 keV. This signal is coming from different
galaxy clusters (like
Perseus and
Centaurus) and several scenarios of warm dark matter can justify such a line. We can cite, for example, a 3.5 keV candidate annihilating into 2 photons,[3] or a 7 keV dark matter particle decaying into a photon and a neutrino.[4]
In November 2019, analysis of the interaction of various galactic halo matter on densities and distribution of stellar streams, coming off the satellites of the Milky Way, they were able to constrain minimums of mass for density perturbations by warm dark matter keVins in the GD-1 and Pal 5 streams. This lower limit on the mass of warm dark matter thermal relics mWDM > 4.6 keV; or adding dwarf satellite counts mWDM > 6.3 keV [5]
See also
Dark matter – Hypothetical form of matter that interacts with gravity, but not with light or electromagnetic field
Hot dark matter (
HDM) – Theoretical form of dark matter particles which travel near the speed of light
Cold dark matter (
CDM) – Hypothetical type of dark matter in physics
^Banik, Nilianjan; Bovy, Jo; Bertone, Gianfranco; Erkal, Denis; de Boer, T.J.L (2021). "Novel constraints on the particle nature of dark matter from stellar streams". Journal of Cosmology and Astroparticle Physics. 2021 (10): 043.
arXiv:1911.02663.
doi:
10.1088/1475-7516/2021/10/043.
S2CID207847306.