The dark side of the Sun - Solar reflection of dark matter

When we look up to the night sky, we see a lot of bright objects. Stars, gas clouds, galaxies, or supernovae radiate light that our telescopes can catch. However, we know that the vast majority of matter in space does not send out light at all. We call it 'dark matter' due to its elusive nature. Indeed, dark matter seems to be completely invisible. Timon Emken, author of this summary, tells us more.

We only know of its existence indirectly from the gravitational force it exerts on the visible matter. This is why astroparticle physicists eagerly want to observe dark matter directly inside a laboratory on Earth. A direct discovery would not only be conclusive proof of the existence of dark matter but also reveal more about what it actually is.

The XENON1T detector, one of the largest dark matter observatories
The XENON1T detector, one of the largest dark matter observatories situated deep beneath the Gran Sasso mountain in Italy. Photo: Emken, 2019

So far, our best guess is that dark matter does not consist of the particles that are already known to us but instead of yet unknown, fundamentally different particles. If this is true, dark matter particles would constantly pass through the Earth without leaving a trace.

A dark matter observatory detects rare collisions

Around the world, physicists and engineers have built dark matter observatories searching for these kind of particles. Unlike conventional observatories that observe electromagnetic radiation and are typically found on-top of a mountain or even in space, a dark matter observatory (which physicists rather call 'direct detection experiment') is a machine to detect rare collisions between an incoming dark matter particle and a detector target. Almost all of them are running in deep underground laboratories to shield them of cosmic rays which could mimic a dark matter particle.

One of the most impressive examples for a DM observatory is the XENON1T experiment, a dual-phase time projection chamber using 3.5 tons of liquid xenon as target. It is located 1400m underground beneath the Gran Sasso mountain in Italy. The Dark Matter and Astroparticle Physics group (DMAP) at Fysikum is actively involved in running this experiment and plays a central role in the global XENON collaboration.

Solar reflection of low-mass dark matter

The most-common assumption is that the dark matter particles passing through our detectors are at least as heavy as atomic nuclei. These particles would have enough energy to get detected - a heavy projectile makes a big splash. But what if dark matter particles are lighter than the lightest nucleus, the proton? The impact caused by a low-mass dark matter particle is simply too weak to be observed. For that reason, most dark matter observatories are blind to low-mass dark matter, just like optical telescopes are blind to radiowaves.

A dark matter trajectory simulated with the DaMaSCUS-SUN simulation code. A dark matter particle enters the Sun, scatters multiple times, gets temporarily captured before getting boosted and ejected.

Astroparticle physicist Timon Emken, who is a postdoctoral fellow in the group of Prof. Jan Conrad at Fysikum, has found and analyzed a way to open the eyes of dark matter detectors to see new particles that are much lighter than protons or even electrons.

Dark matter particles can get boosted in a 'solar reflection'

In his recent article in the journal Physical Review D, Timon exploits the idea that dark matter particles can get boosted by the hot solar plasma in a process called 'solar reflection'.

- Already during my PhD studies, we had the idea that our Sun can effectively act as a low-mass dark matter particle accelerator, Timon Emken says. Very fast particles can be observed even when they are very light - a fast, light projectile also makes a big splash just like a slow heavy one. A slow dark matter particle can get a strong kick from a solar electron and get ejected from the Sun with great speed.

Exclusion limits from various direct dark matter search experiments.
Exclusion limits from various direct dark matter search experiments. While the conventional limits (in gray) disappear below  1 MeV, including the Sun as a dark matter accelerator extends the limits of the same experiments to much lower masses.

Timon Emken presented a detailed study of solar reflection for which he developed and analyzed Monte Carlo simulations of dark matter trajectories inside the Sun. These simulations revealed not just how many dark matter particles get reflected by the Sun, but also how fast they get ejected from the Sun. Including these fast particles, even dark matter as light as 1keV/c^2 becomes detectable in the XENON1T experiment. Without taking the accelerating effect of the Sun into account, dark matter would need to be heavier by a factor of 10000 for XENON1T to be able to see them. This illustrates the impact that solar reflection can have for dark matter searches.

Other researchers can use the DaMaSCUS-SUN code

For this work, he developed the "Dark Matter Simulation Code for Underground Scatterings - Sun Edition", short DaMaSCUS-SUN.
- The Monte Carlo simulation tool DaMaSCUS-SUN is a central result of this work and is therefore published along with the paper. This way, other researchers can not only see in full detail what is underlying my results, they can also use it for their own research, says Timon Emken.

The Monte Carlo results would not have been possible without running simulations on high-performance computing clusters. Timon ran his computations on Tetralith, the largest computer of the National Supercomputer Centre (NSC) in Linköping, as well as on vera, a high-performance computing cluster at Chalmers Centre for Computational Science and Engineering (C3SE) in Göteborg. Both are funded by the Swedish National Infrastructure for Computing (SNIC).

Author of this summary

Timon Emken.

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