The scientist who first detected the neutrino called the strange new particle “the most tiny quantity of reality ever imagined by a human being.” They are so absurdly small and interact with other matter so weakly that about 100 trillion of them pass unnoticed through your body every second, most of them streaming down on us from the sun.
And yet, new experiments to hunt for dark matter are becoming so sensitive that these ephemeral particles will soon show up as background. It’s a phenomenon some physicists are calling the “neutrino floor,” and we may reach it in as little as five years.
The neutrino floor applies only to direct detection experiments, which search for the scattering of a dark matter particle off of a nucleus. Many of these experiments look for WIMPs, or weakly interacting massive particles. If dark matter is indeed made of WIMPs, it will interact in the detector in nearly the same way as solar neutrinos.
We don’t know what dark matter is made of. Experiments around the world are working toward detecting a wide range of particles.
“What’s amazing is now the experimenters are trying to measure dark matter interactions that are at the same strength or even smaller than the strength of neutrino interactions,” says Thomas Rizzo, a theoretical physicist at SLAC National Accelerator Laboratory. “Neutrinos hardly interact at all, and yet we’re trying to measure something even weaker than that in the hunt for dark matter.”
This isn’t the first time the hunt for dark matter has been linked to the detection of solar neutrinos. In the 1980s, physicists stumped by what appeared to be missing solar neutrinos envisioned massive detectors that could fix the discrepancy. They eventually solved the solar neutrino problem using different methods (discovering that the neutrinos weren’t missing; they were just changing as they traveled to the Earth), and instead put the technology to work hunting dark matter.
In recent years, as the dark matter program has grown in size and scope, scientists realized the neutrino floor was no longer an abstract problem for future researchers to handle. In 2009, Louis Strigari, an astrophysicist at Texas A&M University, published the first specific predictions of when detectors would reach the floor. His work was widely discussed at a 2013 planning meeting for the US particle physics community, turning the neutrino floor into an active dilemma for dark matter physicists.
“At some point these things are going to appear,” Strigari says, “and the question is, how big do these detectors have to be in order for the solar neutrinos to show up?”
Strigari predicts that the first experiment to hit the floor will be the SuperCDMS experiment, which will hunt for WIMPs from SNOLAB in the Vale Inco Mine in Canada.
While hitting the floor complicates some aspects of the dark matter hunt, Rupak Mahapatra, a principal investigator for SuperCDMS at Texas A&M, says he hopes they reach it sooner rather than later—a know-thy-enemy kind of thing.
“It is extremely important to know the neutrino floor very precisely,” Mahapatra says. “Once you hit it first, that’s a benchmark. You understand what exactly that number should be, and it helps you build a next-generation experiment.”
Much of the work of untangling a dark matter signal from neutrino background will come during data analysis. One strategy involves taking advantage of the natural ebbs and flows in the amount of dark matter and neutrinos hitting Earth. Dark matter’s natural flux, which arises from the motion of the sun through the Milky Way, peaks in June and reaches its lowest point in December. Solar neutrinos, on the other hand, peak in January, when the Earth is closest to the sun.
“That could help you disentangle how much is signal and how much is background,” Rizzo says.
There’s also the possibility that dark matter is not, in fact, a WIMP. Another potentially viable candidate is the axion, a hypothetical particle that solves a lingering mystery of the strong nuclear force. While WIMP and neutrino interactions look very similar, axion interactions would appear differently in a detector, making the neutrino floor a non-issue.
But that doesn’t mean physicists can abandon the WIMP search in favor of axions, says JoAnne Hewett, a theoretical physicist at SLAC. “WIMPs are still favored for many reasons. The neutrino floor just makes it more difficult to detect. It doesn’t make it less likely to exist.”
Physicists are confident that they’ll eventually be able to separate a dark matter signal from neutrino noise. Next-generation experiments might even be able to distinguish the direction a particle is coming from when it hits the detector, something the detectors being built today just can’t do. If an interaction seemed to come from the direction of the sun, that would be a clear indication that it was likely a solar neutrino.
“There’s certainly avenues to go here,” Strigari says. “It’s not game over, we don’t think, for dark matter direct detection.”
