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SIMP v WIMP: Novel thermal relic mechanism for dark matter creation in the early universe

Posted December 3, 2014

Our current understanding of the universe is that the majority of its mass consists of dark matter (DM) – but there’s a wrinkle: Despite having an idea about some of its properties – dark matter is cold, massive, has neither color nor electrical charge, and does not self-interact very strongly, so that it is detected through its gravitational interactions with ordinary matter and radiation – scientists don’t know what dark matter actually is. That said, and as might well be expected, dark matter theories abound, one being that dark matter is a thermal relic from the early universe, in which all particles are in thermal equilibrium until expansion and cooling occurs. At that point, particle interaction rates slow, causing them to freeze-out – and while unstable particles vanish, stable particles reach what’s known as their thermal relic density that remains. In this scenario, the most promising dark matter candidates are weakly interacting massive particles (WIMPs) – but even though extensions of the standard model often include such particles, no particle known today matches WIMP properties.

SIMP paradigm

A schematic description of the SIMP paradigm. Credit: Hochberg, Y et al. (2014) Mechanism for Thermal Relic Dark Matter of Strongly Interacting Massive Particles. Phys. Rev. Lett. 113, 171301.

Recently, however, researchers at University of California, Berkeley, Tel Aviv University, Israel and Stanford University presented a new thermal relic dark matter mechanism which arises when a nearly secluded dark sector – a grouping of new particles that don’t couple directly to the Standard Model – is thermalized (that is, attains thermal equilibrium) with the Standard Model after reheating.

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