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What in the world is dark matter?

Going underground to reveal the secrets of space


Date: July 31, 2016
Weizmann Direct Vol. 3 Issue 1

What would you do if you discovered that a big chunk of the universe is missing? That’s more or less what happened back in the 1970s, when an American astronomer named Vera Rubin noticed a discrepancy between the predicted motion of galaxies swinging through space and the motion actually observed.

To reconcile Dr. Rubin’s observations with the law of gravity, scientists proposed the existence of “missing” mass–invisible particles with enough gravitational force to pull galaxies together.  Dubbed “dark matter” and believed to make up a whopping 80% of the mass in the known universe, these elusive particles have been playing cat and mouse with scientists ever since.

“Dark matter is difficult to find because it interacts very weakly with light or matter,” says Dr. Ranny Budnik, a member of the Department of Particle Physics and Astrophysics. “Traditional tools – like the accelerators that characterize particles based on their radiation and their interaction patterns–can be effectively blind when it comes to dark matter. That’s why, as experimentalists, we had to come up with something completely different.”

Dr. Budnik is the head of a Weizmann Institute team that helped design an experiment in a laboratory in Italy, where scientists from 20 institutions hope to discover the first direct evidence of dark matter’s existence.  Surprisingly for an endeavor focused on the forces at work in deep space, the facility is located 1.4 kilometers underground.

“To identify dark matter interactions, we needed to isolate our setup from stronger signals that would overwhelm our detector,” Dr. Budnik explains. “Our planet’s voyage through the universe puts it on a collision course with high levels of cosmic radiation that not only bombard the surface, but can also penetrate solid rock. The Earth is constantly moving through a ‘wind’ of dark matter particles – at 200 kilometers per second – but most pass us by without interacting at all. The trick is to detect the rare particles that do interact with our detector, and measure their properties. Only then will it be possible to begin to understand what dark matter really is.”

So what do we know about dark matter? Not nearly enough—yet. Researchers only agree that dark matter is not included in the Standard Model: the “club” of particles – mainly protons, neutrons, and electrons – that physicists rely on to explain what the world is and what holds it together. The magnitude of the mystery leads experimentalists like Dr. Budnik to go to great lengths to catch dark matter in action.

Deep down in the underground lab, the scientists have set up a water tank that is 10 meters in diameter. Inside this tank is the actual detector, made of Xenon, a noble gas kept in liquid phase by extremely cold temperatures. “Interaction with Xenon produces light, and liberates electrons–something we are able to measure because it occurs in an environment where almost all the other radiation has been filtered out,” Dr. Budnik says. “If we get even a handful of ‘hits’ it will be highly significant.”

Dr. Budnik travels to Italy frequently and is looking forward to a new set of experiments planned to begin later this year. “Our latest setup features a larger detector, which means we will be able to gather more dark matter data, faster, than ever before,” he says.  “Unmasking the nature of dark matter–if we can do it–would be a revolution in both particle physics and cosmology.  That’s why I want to be there.”

Pictured above: People in cleanroom gear working on the XENON1T TPC courtesy of the XENON collaboration


Dr. Budnik’s research is supported by the Yeda Sela Center for Basic Research and the Estate of Olga Klein Astrachan.


Dr. Ranny Budnik

Dr. Ranny Budnik