**Title:** New Insight on Neutrino Dark Matter Interactions from Small-Scale CMB Observations

**Authors:** ** **Philippe Brax, Carsten van de Bruck, Eleonora Di Valentino, William Giarè, and Sebastian Trojanowski

**First Author’s Institution: ** Institut de Physique Théorique, Université Paris-Saclay, Gif-sur-Yvette Cedex, France

**Status**: Submitted to ArXiv [28 Feb 2022]

Despite our best efforts, most powerful telescopes, and biggest atom-smashers, dark matter (DM) slipperily continues to avoid direct detection. While we can measure its effects on the surrounding universe, and theorize that it must exist in order for modern space to look the way it does, we have yet to figure out exactly what it’s made of. The main reason it is so hard to determine what dark matter is made of is that it seems to only interact with other particles through gravity. If it interacted through any other means (i.e. the electromagnetic, weak, or strong forces) we believe we should have found it in the shattered remains of particles annihilated in colliders. At the very least, our accelerator-based experiments show us that if DM does interact through anything other than gravity, its cross-section is incredibly small. But what if we could use the oldest light in the universe we can see, the Cosmic Microwave Background (CMB), to infer how dark matter could interact with other particles? Today’s paper does just that, looking for an interaction between neutrinos and DM (νDM) in the CMB.

**Interaction Signatures in the CMB**

How does one show that neutrinos can interact with DM from just the CMB observations? From Figure 1 you can see that the CMB looks a lot like TV static (fun fact, that’s because it quite literally is), so the first step is to manipulate that mess into something more palatable. We do this by converting the 2D sky map into a 1D graph, called the angular power spectrum. The spectrum encapsulates how different the temperatures at different points on the CMB are from each other on average, as a function of angular separation. In other words, how smooth or rough the CMB is at different angular scales. For example, in Figure 2, we see a major peak at about 1 degree (see the upper x-axis); this is the main degree of “patchiness” that our eyes pick out when we look at the CMB map. (For an in-depth understanding, see this article.) Typically, the x-axis of the power spectrum is given in terms of the multipole moment , but all you need to know is that high corresponds to small angular scales and vice versa.

Conveniently, our models of cosmology can predict what they expect the power spectrum to look like. The authors therefore ask, what would the power spectrum look like if we include a νDM interaction in our model of the universe? With some key assumptions, one can include this νDM correction as a singular term, called , in the model proportional to the cross-section of the interaction and mass of the hypothetical DM particle. = 0 means DM and neutrinos do not interact, larger values mean they interact more frequently. The authors then study the shape of the angular power spectrum as a function of different values of , see the larger plot of Figure 3.

Evidently, the difference between a universe without νDM interactions and one with is indistinguishable at most angular scales on the CMB. However, it seems that the difference becomes apparent at high multipoles, which is the same thing as saying very small angles. Great! So we just look at these small angular scales and see what the CMB tells us, right? Well, that’s the idea, but it turns out these tiny angular scales quickly become difficult to measure, hence the Planck satellite not covering ranges higher than > 2500 (see Figure 2). Thankfully, the ground-based Atacama Cosmology Telescope’s most recent data release is capable of probing these high multipole regions. What happens, then, if we fit our νDM model to this power spectrum? Surprisingly, the data seems to prefer a non-zero cross-section between neutrinos and dark matter, meaning the two species could interact! Specifically, they estimate that the size of this term is log () = . Due to the limits set by the data, the authors are only able to rule out a cross-section of zero at a 1-sigma level, when generally 5-sigma accuracy is required to be certain. Hence, an interacting DM is not confirmed, only tantalizingly hinted at. Nonetheless, the data’s preference for a non-zero interaction is exciting and encourages further study. For instance, what would a new standard model where a neutrino-DM interaction existed look like?

**Warning, Particle Physics ahead, abandon all hope ye who enter here**

Assuming a dark matter particle with a mass of ~1 GeV (1.79 kg), the size of the parameter governing the interaction corresponds to a cross-section size of about a nanobarn, hence my hilarious title. However, due to the close relationship between leptons (electrons, muons, and tauons) and neutrinos, a neutrino interaction with a cross-section of this size should have produced dark matter particles in our electron collider experiments already. We thus have to be careful how we add dark matter into the Standard Model Lagrangian to avoid these constraints: enter the sterile neutrino. The sterile neutrino is a* *hypothetical 4th neutrino particle that interacts only via gravity. If this sterile neutrino was coupled to DM particles and was itself quite heavy, it could explain why we see cosmological signals of νDM interactions but not in a collider, as this sterile neutrino could be too heavy for our current colliders to produce, hence preventing any decays into DM particles. They would also be too heavy for other neutrinos to oscillate into, further explaining our lack of detection. While other models beyond this use of the sterile neutrino are possible, the authors note that the interaction strength calculated when using a model of this kind agrees well with other papers that use different cosmological detection methods.

### What’s Next?

While it is far too soon to declare that dark matter has made friends with neutrinos, the data’s preference for a potential interaction is exciting. With upcoming CMB experiments – such as the Simons Observatory – we can make better measurements of the high multipole region of the CMB, allowing for an even more precise measurement of . If a non-zero interaction is detected, our improved understanding of DM’s connection to the rest of the standard model could point us in the right direction for finding DM particles in an accelerator. These exotic particle interactions could lead us to new exciting extensions to the Standard Model and rewrite our understanding of the evolution of the universe!

*Astrobite edited by Jack Lubin*

*Featured image credit: Kat Nurminsky *