PI: Will Handley

One of the most fundamental open questions in modern physics is the nature of dark matter (DM), the invisible substance making up about 85% of the matter in the Universe. While we know DM exists due to its gravitational effects on visible matter, we have yet to identify the particle(s) responsible. A popular class of DM candidates are weakly interacting massive particles (WIMPs), but decades of searches have yielded no definitive detection. This motivates the exploration of alternative DM candidates, such as those with sub-GeV masses.

This project investigates the status of sub-GeV DM particles using the DiRAC Data-Intensive service at Cambridge. We focus on models where DM interacts with ordinary matter via a new force-carrying particle, the “dark photon”. This dark photon mixes with the electromagnetic force, allowing DM to interact, albeit very weakly, with electrons and atomic nuclei.

A key challenge for sub-GeV DM models is that if DM particles and antiparticles exist in equal numbers, their annihilations in the early Universe and today can produce observable signals that are in conflict with data. Our research has systematically investigated ways to avoid these constraints. We found two particularly promising scenarios:

1. Asymmetric Dark Matter: If there is an asymmetry between the number of DM particles and antiparticles, similar to the asymmetry between matter and antimatter in the visible Universe, annihilation signals can be suppressed. Our analysis revealed that such an asymmetry significantly expands the viable parameter space for sub-GeV DM.
2. Scalar Dark Matter: If the DM particle is a fundamental scalar, its annihilation rate is naturally suppressed at low velocities. This allows symmetric scalar DM to evade the strongest constraints even without an asymmetry.

Using Bayesian model comparison, we showed that both asymmetric fermionic DM and symmetric scalar DM models are preferred over symmetric fermionic DM. This preference arises because these models require less fine-tuning of parameters to fit the data.

Allowed parameter regions for asymmetric fermionic DM

Our results have crucial implications for future DM searches. Many experiments are currently searching for DM by looking for its interactions with ordinary matter. Our work has shown that existing constraints already disfavour the most commonly used benchmark scenarios for sub-GeV DM. We have therefore proposed a new benchmark point that is consistent with current data and provides an attractive target for future experiments. Our findings will guide the design and interpretation of future searches, bringing us closer to uncovering the nature of dark matter.