The Case of the Missing Matter


Andrea Albert, talking to Jodi Cooley-Sekula and Alex Drlica-Wagner.

As a physicist trying to find clues that may lead to an understanding of what dark matter is, I feel like Sherlock Holmes, or one of those CSI detectives. We know that given our standard theory of gravity, there must be more mass out there holding things like our Milky Way Galaxy together than we can see–a lot more mass. Our suspicion is that this missing mass is composed of a new kind of “dark matter”. This is an exciting time to be a dark matter physicist because we have some intriguing leads and are developing searches to solve this mystery – the trail is very hot right now!



So what can this mysterious dark matter be? Dark matter could be a new elementary particle that behaves by a different set of physical laws than the "normal matter" that makes up you and me. Physicists have built sensitive machines to look for rare interactions involving these elusive dark matter particles, and a few tantalizing hints have been reported.

A popular theory is that dark matter is made of particles called Weakly Interacting Massive Particles (WIMPs). WIMPs are particles that are 10 to 1,000+ times more massive than a proton and interact via the weak force–the force behind nuclear decay. Since WIMPs interact with the weak force and not the electromagnetic force, they don’t emit light, so we can’t see them with telescopes like we can see other objects.

Looking for WIMPs means we need the talents of both astro- and particle- physicists. Current dark matter searches combine techniques used in both astrophysics and particle physics experiments. Three of them are direct detection, collider production, and indirect detection.

Direct Detection

Direct detection experiments take a big sample of normal matter (like a tank of pure liquid xenon) and wait for a dark matter particle to whack into it. Depending on the size of the detector, we may only expect a dark matter collision a few times a year or less. Recently, a variety of experiments have seen hints that may be these rare dark matter signals. These tentative signals all seem to come from rather lightweight dark matter particles--only about 10 times the mass of the proton.

The problem is that other experiments should’ve also seen these signals and haven’t. That’s why we’re building and running new, more sensitive experiments to follow up on these recent hot leads.

Collider Production

Second, we may be able to produce dark matter particles by smashing two particles together to create a dark matter particle from the collision energy. Dark matter is one of many new classes of particles the Large Hadron Collider (LHC) experiments ATLAS and CMS are looking for. Searches at the LHC attempt to identify dark matter particles by looking for “missing” energy.

The challenge is avoiding red herrings like neutrinos–another type of particle that looks just like a dark matter particle. Fortunately, neutrinos are well-studied, so we can predict how many neutrino events we expect to see. Therefore, if we see elusive particles beyond what we expect from neutrinos, those extra particles are likely dark matter.

So far no dark matter particles have been seen by ATLAS or CMS, but the LHC has only been running at half power. In a year or two it will start colliding protons at even higher energies, and dark matter may leave us another clue.

Indirect Detection

Last but not least, instead of waiting for dark matter particles to appear in our experiments on Earth, we can use our own Milky Way Galaxy as a laboratory! While dark matter interactions are rare, with a large enough amount of dark matter, you expect to see a signal from one dark matter particle interacting with another. In other words, although you’re unlikely to find someone who has won the lottery if you only ask 10 people, your chances improve a lot if you ask 1,000 people, and get even better if you ask 1,000,000 people.

We suspect that dark matter interactions may create normal matter like gamma rays, electrons, protons and neutrinos that we can observe with instruments like the Fermi Gamma-ray Space Telescope, the Alpha Magnetic Spectrometer, or IceCube. Similar to the other search methods, known processes can also create these particles so it’s challenging to determine which signals came from dark matter and which signals came from other interactions.

Nonetheless, we have seen some tantalizing hints from recent indirect searches. For example, there seem to be some extra gamma rays coming from the Galactic center that may be from dark matter interactions. Also, something is creating more high-energy (greater than about 10 billion electronvolts) positrons (electrons’ antimatter partner) than we expected–and dark matter could be a culprit.

As we learn more about our Universe, it is becoming quite clear that there is more matter out there than we can see. This dark matter may be a new kind of matter, something different than the matter that makes up you and me. Recent searches to detect the rare interactions of dark matter particles have uncovered some exciting hints, inspiring follow-up studies with new clever analysis techniques and more sensitive detectors. With a solid hunch and the tools to follow the trail of clues, scientists all around the world are on the case, working hard to find and explain dark matter.


You can watch all the talks in this session on the KIPAC youtube channel.

You can also read more about KIPAC@10 on the conference blog home page.