From ars technica: “Atomic clocks and solid walls: New tools in the search for dark matter”

Ars Technica
ars technica

Jennifer Ouellette

An atomic clock based on a fountain of atoms. NSF

Countless experiments around the world are hoping to reap scientific glory for the first detection of dark matter particles. Usually, they do this by watching for dark matter to bump into normal matter or by slamming particles into other particles and hoping for some dark stuff to pop out. But what if the dark matter behaves more like a wave?

That’s the intriguing possibility championed by Asimina Arvanitaki, a theoretical physicist at the Perimeter Institute in Waterloo, Ontario, Canada, where she holds the Aristarchus Chair in Theoretical Physics—the first woman to hold a research chair at the institute. Detecting these hypothetical dark matter waves requires a bit of experimental ingenuity. So she and her collaborators are adapting a broad range of radically different techniques to the search: atomic clocks and resonating bars originally designed to hunt for gravitational waves—and even lasers shined at walls in hopes that a bit of dark matter might seep through to the other side.

“Progress in particle physics for the last 50 years has been focused on colliders, and rightfully so, because whenever we went to a new energy scale, we found something new,” says Arvanitaki. That focus is beginning to shift. To reach higher and higher energies, physicists must build ever-larger colliders—an expensive proposition when funding for science is in decline. There is now more interest in smaller, cheaper options. “These are things that usually fit in the lab, and the turnaround time for results is much shorter than that of the collider,” says Arvanitaki, admitting, “I’ve done this for a long time, and it hasn’t always been popular.”

The end of the WIMP?

While most dark matter physicists have focused on hunting for weakly interacting massive particles, or WIMPs, Arvanitaki is one of a growing number who are focusing on less well-known alternatives, such as axions—hypothetical ultralight particles with masses that could be as little as ten thousand trillion trillion times smaller than the mass of the electron. The masses of WIMPs, by contrast, would be larger than the mass of the proton.

Cosmology gave us very good reason to be excited about WIMPs and focus initial searches in their mass range, according to David Kaplan, a theorist at Johns Hopkins University (and producer of the 2013 documentary Particle Fever). But the WIMP’s dominance in the field to date has also been due, in part, to excitement over the idea of supersymmetry. That model requires every known particle in the Standard Model—whether fermion or boson—to have a superpartner that is heavier and in the opposite class. So an electron, which is a fermion, would have a boson superpartner called the selectron, and so on.

Physicists suspect one or more of those unseen superpartners might make up dark matter. Supersymmetry predicts not just the existence of dark matter, but how much of it there should be. That fits neatly within a WIMP scenario. Dark matter could be any number of things, after all, and the supersymmetry mass range seemed like a good place to start the search, given the compelling theory behind it.

But in the ensuing decades, experiment after experiment has come up empty. With each null result, the parameter space where WIMPs might be lurking shrinks. This makes distinguishing a possible signal from background noise in the data increasingly difficult.

“We’re about to bump up against what’s called the ‘neutrino floor,’” says Kaplan. “All the technology we use to discover WIMPs will soon be sensitive to random neutrinos flying through the Universe. Once it gets there, it becomes a much messier signal and harder to see.”

Particles are waves

Despite its momentous discovery of the Higgs boson in 2012, the Large Hadron Collider has yet to find any evidence of supersymmetry. So we shouldn’t wonder that physicists are turning their attention to alternative dark matter candidates outside of the mass ranges of WIMPs. “It’s now a fishing expedition,” says Kaplan. “If you’re going on a fishing expedition, you want to search as broadly as possible, and the WIMP search is narrow and deep.”

Enter Asimina Arvanitaki—“Mina” for short. She grew up in a small Greek Village called Koklas, and, since her parents were teachers, she grew up with no shortage of books around the house. Arvanitaki excelled in math and physics—at a very young age, she calculated the time light takes to travel from the Earth to the Sun. While she briefly considered becoming a car mechanic in high school because she loved cars, she decided, “I was more interested in why things are the way they are, not in how to make them work.” So she majored in physics instead.

Similar reasoning convinced her to switch her graduate-school focus at Stanford from experimental condensed matter physics to theory: she found her quantum field theory course more scintillating than any experimental results she produced in the laboratory.

Central to Arvanitaki’s approach is a theoretical reimagining of dark matter as more than just a simple particle. A peculiar quirk of quantum mechanics is that particles exhibit both particle- and wave-like behavior, so we’re really talking about something more akin to a wavepacket, according to Arvanitaki. The size of those wave packets is inversely proportional to their mass. “So the elementary particles in our theory don’t have to be tiny,” she says. “They can be super light, which means they can be as big as the room or as big as the entire Universe.”

Axions fit the bill as a dark matter candidate, but they interact so weakly with regular matter that they cannot be produced in colliders. Arvanitaki has proposed several smaller experiments that might succeed in detecting them in ways that colliders cannot.

Walls, clocks, and bars

One of her experiments relies on atomic clocks—the most accurate timekeeping devices we have, in which the natural frequency oscillations of atoms serve the same purpose as the pendulum in a grandfather clock. An average wristwatch loses roughly one second every year; atomic clocks are so precise that the best would only lose one second every age of the Universe.

Within her theoretical framework, dark matter particles (including axions) would behave like waves and oscillate at specific frequencies determined by the mass of the particles. Dark matter waves would cause the atoms in an atomic clock to oscillate as well. The effect is very tiny, but it should be possible to see such oscillations in the data. A trial search of existing data from atomic clocks came up empty, but Arvanitaki suspects that a more dedicated analysis would prove more fruitful.

Then there are so-called “Weber bars,” which are solid aluminum cylinders that Arvanitaki says should ring like a tuning fork should a dark matter wavelet hit them at just the right frequency. The bars get their name from physicist Joseph Weber, who used them in the 1960s to search for gravitational waves. He claimed to have detected those waves, but nobody could replicate his findings, and his scientific reputation never quite recovered from the controversy.

Weber died in 2000, but chances are he’d be pleased that his bars have found a new use. Since we don’t know the precise frequency of the dark matter particles we’re hunting, Arvanitaki suggests building a kind of xylophone out of Weber bars. Each bar would be tuned to a different frequency to scan for many different frequencies at once.

Walking through walls

Yet another inventive approach involves sending axions through walls. Photons (light) can’t pass through walls—shine a flashlight onto a wall, and someone on the other side won’t be able to see that light. But axions are so weakly interacting that they can pass through a solid wall. Arvanitaki’s experiment exploits the fact that it should be possible to turn photons into axions and then reverse the process to restore the photons. Place a strong magnetic field in front of that wall and then shine a laser onto it. Some of the photons will become axions and pass through the wall. A second magnetic field on the other side of the wall then converts those axions back into photons, which should be easily detected.

This is a new kind of dark matter detection relying on small, lab-based experiments that are easier to perform (and hence easier to replicate). They’re also much cheaper than setting up detectors deep underground or trying to produce dark matter particles at the LHC—the biggest, most complicated scientific machine ever built, and the most expensive.

“I think this is the future of dark matter detection,” says Kaplan, although both he and Arvanitaki are adamant that this should complement, not replace, the many ongoing efforts to hunt for WIMPs, whether deep underground or at the LHC.

“You have to look everywhere, because there are no guarantees. This is what research is all about,” says Arvanitaki. “What we think is correct, and what Nature does, may be two different things.”

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