Authors: The VERITAS Collaboration
Status: Accepted to PRD
Making, shaking, and breaking dark matter
With plenty of evidence that dark matter makes up ~85% of the matter in our Universe, it’s no longer much of a debate within the physics community that dark matter exists. However, we still don’t know what dark matter is or what the best way of detecting it might be. Our current method for understanding dark matter is starts with theorists coming up with a hypothetical model, usually new particles, but sometimes weirder things too! Experimentalists then do their best to look for these dark matter candidates. If they don’t find it, they can rule out the candidates with masses and cross-sections (how likely it is that the particle interacts with normal matter) that they expect their experiments to be sensitive to – until the whole model becomes implausible.
There are typically three ways of experimentally looking for dark matter, colloquially dubbed “make it, shake it, or break it.” The “make it” method typically uses particle colliders and tries to make dark matter by smashing particles together. The “shake it” method looks for rare dark matter interactions in a large tank of material. Both of these are called direct detection methods since the dark matter particle is either being produced or detected directly in the detector. The third “break it” method is the subject of today’s paper, and involves an assumption that two dark matter particles can collide and annihilate to make a standard model particle. This particle can go on to interact with normal matter (what astronomers call baryonic matter) to create other particles, like photons (i.e., light)!
Where in the Universe is a good dark matter target?
One thing we know about dark matter is that it rarely interacts with anything. For indirect detection, we need high densities of dark matter to collide with each other, so it’s best to look up into space for the sources of dark matter. From simulations, we think that many galaxies and galaxy clusters tend to form around dark matter halos – the invisible glue that holds much of our Universe together (see Figure 1). The tricky thing with indirect detection is that we need to separate any light we see (which comes from the photons produced in the processes associated with dark matter annihilation) from the ordinary light we see from stars and other normal stuff in these galaxies.
Today’s paper looks for dark matter in dwarf spheroidal (dSph) galaxies, which are old galaxies that appear very dim due to a lack of active star formation. The stars that are part of these galaxies have a curiously high velocity dispersion, which usually indicates that they’re orbiting around a very large mass distribution. However, similar to many observations of galaxies, the problem is that the few stars that belong to the galaxy can’t account for this mass, indicating they are composed of a large amount of dark matter. dSphs are great to study since there’s a lot of them orbiting the Milky Way (close by, astronomically speaking), they’re expected to have a lot of dark matter, and they don’t have a lot of starlight. Therefore, there is a better chance that any light we see is likely from dark matter annihilation rather than another astrophysical process (depending on the wavelength).
Deeper limits than ever before!
The authors are looking for gamma-ray photons that are expected to come from dark matter annihilation. Dark matter annihilation signatures appear in the source’s spectrum (measured brightness as a function of energy) as spectral lines from bursts of gamma rays being produced or as a steady spectrum with a cutoff corresponding to the dark matter mass for more continuously produced gamma rays. The dark matter mass roughly corresponds to the gamma-ray energy in natural units (i.e., a 1 TeV mass dark matter particle will approximately make a 1 TeV energy gamma-ray).
They use the VERITAS ground-based gamma-ray telescope to observe the seventeen dwarf spheroidals with the highest expected dark matter content (quantified by the source’s J-factor). VERITAS is sensitive to gamma-rays between 100 GeV and >30 TeV, which corresponds to a dark matter mass range of 200 GeV to 30 PeV – although this really depends on the annihilation channels (which particles the dark matter annihilates into) and the model that predicts the expected gamma-ray flux (brightness) for that annihilation channel.
The authors searched for these dark matter signatures in 638 hours of data collected on seventeen dSphs – these data were taken over eleven years of VERITAS observations (gamma-ray fluxes are really low, so require a LOT of observing time!). None of the sources showed any excess of gamma-rays at the source location, compared to the expected background, meaning that VERITAS does not detect emission from any of the dSphs individually. Additionally, they combine all seventeen sources to provide a deeper limit than any of the sources individually. This stacking works since if any of the dSphs are producing gamma-rays through a particular dark matter annihilation channel, we should expect the same process to be happening in all the other dSphs. Once factors like different exposure times or the J-factor of each source are taken into account, they can essentially be treated as the same dataset.
In this study, the authors were able to employ improved analysis techniques, applied to more extensive data with improved calculations of the J-factor for each dSph. Consequently, they produce deeper constraints on the dark matter mass and cross-section than ever before. Figure 2 shows the improvement for the dSph Segue 1 compared to the previous VERITAS result from 2017.
The big picture
The limits derived from the combined non-detection of dark matter in all seventeen dSphs in today’s paper are comparable to those from telescopes that operate at similar energies (see Figure 3). However, today’s paper extends these limits to include ultra-heavy dark matter (mass > 194 TeV), which is an energy range that hasn’t been explored by other instruments.
Although there’s still a lot we don’t know about dark matter, limits like those in today’s paper help us rule out models for dark matter that seem implausible. Slowly but surely, instruments are approaching the thermal relic cross-section, which is the expected cross-section of dark matter that was produced abundantly in the hot early universe, which then “froze in” as the Universe cooled. For dark matter produced this way, the number of total dark matter particles has essentially remained the same ever since. Finding this dark matter would be a signature of the weakly interacting massive particle (WIMP), which is historically a popular dark matter model. As the constraints on the characteristics of dark matter particles continue to become tighter (such as in today’s paper), WIMPS become closer to being ruled out by studies.
For direct detectors, moving these limits down requires detectors to go bigger, literally, in terms of detector size (and cost) in order to obtain the required sensitivities. However, indirect detection searches improve with a better understanding of the astrophysical sources in our Universe and understanding of J-factors — the dark matter content of these sources; while this is perhaps challenging intellectually, it is nowhere near as costly as building larger detectors or particle accelerators.
Disclaimer: Today’s author was an author on the paper as a member of the VERITAS collaboration but was not directly involved in this project.
Edited by Abbé Whitford
Featured image credit credit: NASA/Canva image library
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