Dark Matter Beyond Kinetic Equilibrium: When Standard Relic-Density Calculations Fail

by Dr Shiuli Chatterjee (Narodowe Centrum Badań Jądrowych)

Tuesday 26 May 2026, 13:00 → 14:00 Europe/Brussels

E/3rd floor-E.349 - Seminar room (E.349) (Marc de Hemptinne (chemin du Cyclotron, 2, Louvain-la-Neuve))

The relic abundance of dark matter (DM) is measured to percent-level precision and provides a powerful probe of physics beyond the Standard Model. Predicting this abundance is traditionally based on the integrated Boltzmann equations that track number densities, under the assumption that dark matter remains in kinetic equilibrium with the Standard Model bath throughout its chemical decoupling. While this is typically valid for canonical WIMP (weakly interacting massive particle) scenarios, it need not hold in general. Departures from kinetic equilibrium can lead to relic-density predictions that differ by up to an order of magnitude, even in simple and well-motivated models.
 
More generally, the dark sector may contain multiple particles, one or more of which contribute to the observed DM abundance. In such scenarios, the existence of multiple number changing processes (for example, within the dark sector) further challenges the assumption of kinetic equilibrium of DM during its chemical decoupling, and thereby poses challenges to the standard relic abundance calculation.
 
In this talk, I will present two recent studies of these effects. First, I will discuss a phenomenologically motivated two-component dark matter scenario, showing how conversion processes within the dark sector modify not only particle abundances but also their momentum distributions. In this case, departures from kinetic equilibrium can change the predicted total DM abundance by more than 100%, while in most of the interesting parameter space being in the range from around -20% to 50%. Second, I will discuss sequential freeze-in in a minimal two-scalar dark sector, focusing on how the out-of-equilibrium phase-space evolution of an intermediate mediator affects dark matter production and the final relic abundance. Together, these studies illustrate the importance of phase-space level methods for reliably tracking dark matter evolution whenever kinetic equilibrium cannot be guaranteed a priori.

Zoom: https://cern.zoom.us/j/62591938215?pwd=qSqjURfb693pfZR7aFvYR2vJw3YnuD.1