# Research in next decade on dark matter: sky surveys vs particle detectors

The next decade is very promising for cosmology as new surveys such as SPHEREx, Roman, Euclid, DESI which will shed light on inflation, dark matter/energy and more. At the same time there are particle detectors, some of which are still being built, looking to directly detect dark matter, e.g SuperCDMS, XENONnt, LUX-Zeplin, etc. The LHC is also looking to produce dark matter in collisions.

I am wondering, in the next decade, which approach to studying dark matter is projected to be more active or bring us closer to understanding dark matter?

It seems that the large-scale sky surveys are guaranteed to produce a wealth of data and tighten constraints on cosmological parameters, but the particle detector approach is possibly a hit or miss in that the energies required to produce dark matter in an accelerator may be inaccessible and the interactions required to detect it by low-energy recoils may be too weak to be observed.

Perhaps my understanding of the field is incorrect in which case I hope someone might enlighten me.

So there are three main approaches for dark matter detection - Indirect detection, Direct detection, Collider

Now each of these techniques have their merits and drawbacks. For example Indirect detection has a greater potential of detecting dark matter because of the abundance of dark matter in the universe itself and their role in structure formation in early universe. But then it's very hard to find the 'Smoking Gun' signal as there is so little that we understand about the sources of the Galactic and Extra Galactic signals. Till now we have certainly gained many constraints on the abundance, mass ,annihilation cross section (if it annnihilates at all), lifetime (if it decays at all) from various astrophysical signals.

Colliders would be a huge favourite place for dark matter detection if we had confirmed SUSY in the first place. But having said that still we hope to find missing energy signals from dark matter in colliders.

I think the best way to talk about the detection is to first specify what is the mass range that you're talking about. As you may know that different Dark matter candidates fall into different mass ranges. For example WIMPs are thought to be between 10-100 GeV. Light dark matters are what we hope to find in Direct detection or in Colliders. For MACHOs the best way is Indirect Detection. For example recently Primordial Black Holes have gained lot of interest as a potential Dark Matter candidate. There was another buzz of potential dark matter detection with the recent XENON1T results. But there also the Tritium hypothesis seems to have the upper hand. With so many different searches going on, hopefully we will have a confirmed DM candidate soon.

• Thank you for this solid answer. I want to point out though that the figure you show is limited to the WIMP paradigm and not generally true for a random dark matter candidate. Also, MACHO searches or GW PBH studies etc wouldn't typically be considered "indirect detection", which is usually limited to cosmic ray searches (neutrinos, antimatter, gamma rays). This is why there really is a fourth leg, astronomical observations, which includes gravitational lensing studies. Not to mention structure formation simulations which can also give us new insight into the nature of dark matter.
– rfl
Nov 10 '20 at 12:22

At the time of writing, the US particle physics community is trying to address this and similar questions in a large community study. Dark matter being the elephant in the room that it is, it seems clear that we need all possible experiments and detectors and observatories we can get our hands on in order to make progress towards understanding the true nature of dark matter. The various approaches you mention focus on different aspects:

• astronomical observations will measure the distribution of dark matter in unprecedented detail. A lot of this may just confirm the concordance $$\Lambda\mathrm{CDM}$$ model. But keep an eye out in particular at small scales, where some ideas such as self-interacting dark matter predict notable deviations that we can verify or falsify with these observations. Also, precision observatories such as GAIA can e.g. look for microlensing events from dark matter in previously unprobed mass ranges.

• Collider experiments technically won't detect dark matter (since if they find a new particle, one doesn't know its lifetime, and won't know whether it makes up the naturally ocuring dark matter or not) but may give us a path towards new particles. Some experiments do show anomalies already, and promising new experiments are being built. Once we would have such a new particle clearly established, we would have our foot in the door to probing this new world. The issue currently is that we don't know which horse to bet on, but with such a first new ("BSM") particle being discovered, that would really be a door opener.

• Direct detection experiments are in their golden era. Decade-long developments now come to fruition in that these experiments have the sensitivity that is required if the hypotheses they are made to test were true. That is to say, if WIMPs or QCD Axions exist, we now have the tools to discover them in the coming years. At the same time, a lot of new ideas are being worked on to broaden the reach with a recent explosion of dozens of new ideas being tested to probe the broadest possible dark matter parameter space.

You may notice that I didn't answer your questions literally. For the "activity" question, one would need to invent some metric (number of scientists working in the field? number of papers published? research expenditures?) that would be hard to define and even harder to defend. For the "bring us closer" bit, all these experiments will collect data, and even if no signal is found, will be able to rule out currently-allowed parameter space, and since it's hard to predict the future, who can say which experiment will be the one to make the decisive discovery... (in case this question is one about a personal career choices, and in case this is allowed on stackexchange ;) my advice would be to do whatever you find most exciting)