Dark matter
cold are particles travelling at velocities slower than the speed of light; hot at ultra-relativistic velocities (close to the speed of light); warm somewhere in-between.
The most widely accepted view is that dark matter is cold. This works with our best model of the universe – the ΛCDM model. 3 Within cold dark matter, there exist three likely classes of dark matter: WIMPs, 4 axions and MACHOs. 5 WIMPs are currently the most researched source for dark matter. They are hypothetical particles, which only interact through gravity, the weak force and possibly other forces weaker than the weak force – a very low interaction cross-section. 6 WIMPs are an attractive solution for the identity of dark matter, as they fit neatly into the proposed model of the early universe. Based on assumptions of the early universemade by supersymmetry, little dimensions and little Higgs scenarios, there were massive particles in thermal equilibrium with ordinary matter. As the universe expanded and cooled, these particles would have decayed into lighter, more stable particles. If the lightest of these were stable with a neutral charge, then it would persist to this day. 7
One proposed WIMP particle appeared to be a very promising solution for the dark matter problem called the ‘WIMP miracle’. 8 Astrophysicists postulated a particle had a mass in the 100 GeV range and could only interact through the weak and other weaker forces. By coincidence, theoretical physicists working on supersymmetric extensions to the standard model (a theory about the relationship between bosons and fermions) predicted a similar particle. Unfortunately, the failure to produce evidence of supersymmetric particles in experiments at the LHC, as well as constraints on the properties of WIMPs due to experiments at LUX, has cast doubt on this solution.
Figure 7: Constraints placed by detectors around the world on the Wimp-nucleon cross section versus mass of WIMPs
Experiments such as the CDMS detector in the US and noble gas scintillators throughout the world have provided very specific bounds on the properties of WIMP as a source for dark matter with most sensitive experiments at XENON providing extremely small constraints on spin dependent and independent cross sections. These constraints have vastly reduced the possible solutions for WIMP particles – ruling out most, if not all, supersymmetry or extra dimensions dark matter (see figure 3 for constraints).
3 Λ denotes a cosmological constant while CDM refers to cold dark matter . 4 Weakly Interacting Massive Particles. 5 Massive astrophysical compact halo objects. 6 The probability an interaction will take place in a collision between two particles. 7 Sadoulet (2007). 8 Seigel (2019).
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