LUX: The LUX dark matter detector, which operated at the Sanford Underground Research Facility in South Dakota, wrapped up its search in 2016. The quest for a direct detection of a dark matter particle goes on. Matt Kapust / Sanford Lab

World’s leading dark matter physicists convene at Brown

At the 12th annual Identification of Dark Matter Conference being held this week at Brown, physicists are working to understand the missing mass of the universe.

PROVIDENCE, R.I. [Brown University] — More than 150 research physicists from across the world are gathering at Brown this week to discuss the ongoing search for dark matter, the mysterious stuff thought to account for most of the matter in the universe.

The 2018 Identification of Dark Matter Conference (IDM2018) is an opportunity for scientists to review the latest data from dark matter searches, discuss the direction of future searches and share new theoretical ideas about the potential properties of dark matter particles. This is the 12th in a long-standing conference series, which originally started in 1996 in the U.K. and has since been held in Stockholm, Amsterdam, Chicago and elsewhere. This is the first time it’s been held at Brown.

Brown physics professor Rick Gaitskell is chair of the organizing committee for this year’s event. He is co-leader of the LUX Experiment, which concluded its search for dark matter in 2016, and is a principal investigator on the LUX-ZEPLIN (LZ) experiment, which replaces LUX and begins its search in 2020. A major piece of the LZ central detector is currently being built in cleanrooms at Brown.

As the conference kicked off, Gaitskell shared this thoughts on the state of dark matter science and more.

Q: Could you give us a quick recap on what dark matter is and how we know it’s out there?

Rick Gaitskell
Rick Gaitskell

Dark matter dominates the universe, accounting for over five-sixths of its mass. The protons, neutrons and electrons you and I are made of are merely the flotsam and jetsam on the ocean of dark matter. The dark matter is responsible for forming the structure of all galaxies, including our own, in the early universe. Theories based on the dynamic gravitational behavior of dark matter do a very good job of predicting the observational data we have of our universe.    

However, the challenge that we have faced for my entire research career is trying to identify the fundamental nature of the dark matter. What processes created it? How does it interact beyond simple gravitation? Does it interact, decay or annihilate with itself? This conference is dedicated to trying to answer those questions. We are looking for new results that will provide a key to explaining dark matter.

A leading theoretical candidate for what a dark matter particle might be is the WIMP (weakly interacting massive particle). These are particles that don’t emit or reflect light and only interact with other forms of matter on extremely rare occasions. They pervade the universe. There are literally millions of WIMPs passing through your body right now. The trick is detecting one of the rare occasions on which a WIMP does interact with ordinary matter.

Q: A number of presentations at IDM2018 center on dark matter searches that are active now. What’s the state of those searches?

Highly competitive! For instance, the search for dark matter using direct detection has been ongoing for over 30 years. During that time, we have been building more and more sensitive experiments. The latest generation of detectors is over a million times more sensitive than the first-generation detectors. That’s an amazing rate of improvement for any experimental program. Several of the current projects are now looking at how to improve even further, by another factor of 10 immediately, and then another factor of 10 within six years. This will be necessary to test the range of dark matter models being conjectured.

Since the early detectors were built, several new ideas about what dark matter could be have been proposed. At IDM2018, there have been many talks from new experimental projects about these new avenues of investigation. We’ve heard from projects looking at high-mass WIMPs, low-mass WIMPs as well as non-WIMP candidates like axions, sterile neutrinos and a maelstrom of other possible dark matter particles.

Amazingly, we still have no conclusive, unambiguous evidence of direct detection of dark matter, from any of them.

Q: Most dark matter searches to date have focused on WIMPs — yet as you mention, there’s been no definitive detection. Is the dominance of the WIMP model starting to waver?

The idea of the WIMP and WIMP-like particles grows out of the theory of supersymmetry (SUSY), which has been favored for more than 30 years. Tim Tait, a professor at the University of California Irvine and one of the opening speakers at the conference, showed a sketch of a stained glass window that celebrates SUSY. He was at pains to point out that it will be hung in the cathedral of dark matter, not the mausoleum. In spite of SUSY’s age, it remains a compelling model. We continue to test the multitude of models that it inspires.

That said, there are other theories for dark matter candidates that allow for different signals to look for. Perhaps the most notable is the axion, but even more exotic candidates exist. The great thing about the upcoming LZ experiment and others like it is that they are sensitive to many potential candidates. So while WIMPs are the focus, we can and will perform an array of other searches as well.

New WIMP models continue to be tested. Based on projection from many talks at the conference, they will continue to be tested over the next 10 years plus.

Q: There were a few presentations of data from LUX, which wrapped up its search two years ago. Are you still learning things from the LUX data?

There have been a number of presentations from researchers who are on LUX or work that originated with studies on LUX. LUX’s primary analysis focus is now to look for dark matter candidates that go beyond the standard WIMP dark matter. LUX is reporting results at Brown that include new results on very light dark matter particle searches, lepto-philic particles searches, and potentially particles that couple through a much broader range of effective field theories (than the standard WIMP assumptions). We are also reporting results for searches for axion and axion-like dark matter particles. At present all the search results represent exclusion limits where we have ruled possible dark matter models out. However, we will continue to search for positive signals using the LUX data and when it comes on stream with the new LZ experiment.

Q: What’s going to be different about LZ compared to previous searches?

LZ will not only be a much larger version (by 20 times) of its predecessors, LUX and ZEPLIN, but also a much smarter one, applying lessons learned by those earlier experiments. The detector is like a multi-layered onion where the outer skins not only absorb more background radiation, but can also tag events that are therefore definitely not dark matter. LZ will also be able for the first time to observe the coherent scattering of neutrinos from the sun. Neutrinos are very weakly interacting and yet in LZ they become a “background” for our dark matter signal search.

Q: What’s been the highlight of the conference so far? Any particularly hot topics?

From a physics point of view, the talks have all been great. Precision observations from the Gaia satellite are now being exploited to go beyond simple gravitational models of dark matter to learn something about fundamental physics of dark matter. We are seeing the same finesse and sensitivity being applied in the analysis of cosmic microwave background (CMB) and 21-cm radio wave measurements associated with hydrogen line emission in the early universe.

We have seen the analyses of data from direct detection experiments exploited in a number of new ways to look for more exotic models of dark matter. We have to search under every possible stone to find the first new signature of its fundamental nature.

There have also been a number of very elegant talks about improved ways of how to differentiate dark matter signal in detector from other background process. This is vital if we are to convince the community that when we do get the first positive signals for the detection of dark matter (beyond gravitation) that we really are measuring it, and not some unexpected manifestation of a more conventional background.

From a non-physics perspective, these conferences are always great for discussing new ideas and forging new collaborations. The tour of the RISD gave the global astroparticle physics community the possibility to discuss the intersection between dark matter and the abstract expressionism of Rothko. It was stirring stuff.