Just over the border from Georgia, in the Caucasus Mountains of southern Russia, lies a small town called Neytrino. For the last half-century, its main business has been the study of the tiniest bit of matter in the universe, an ephemeral subatomic particle called the neutrino.

This is the home of the Baksan Neutrino Observatory, a warren of tunnels and laboratories burrowed three kilometers into a mountain, sheltered from the outside universeby 3,700 meters of rock. There, vats of liquid wait to record the flight of neutrinos from the center of the sun, from exploding stars, atomic reactors and the Big Bang itself, carrying messages through time.

Neutrinos are mostly impervious to the forces, like electromagnetism, with which other denizens of nature interact. Neutrinos cruise unmolested through rocks, the earth and our bodies. The most delicate measurements so far indicate that an individual neutrino weighs less than a millionth of what an electron weighs.

Baksan’s men and women share an underground union with scientists scattered around the world: the Sanford Underground Research Facility in Lead, South Dakota; the Gran Sasso National Laboratory, beneath the mountain of that name in Italy; the Sudbury Neutrino Observatory in Ontario, Canada; the Super-Kamiokande, deep within Mount Ikeno, Japan; and IceCube, an array of detectors buried in ice at the South Pole. All of them are trying to listen to quantum whispers about the nature of reality.

One of Baksan’s biggest accomplishments to date was to catch neutrinos emitted by thermonuclear reactions in the center of the sun in nearly 60 tons of liquid gallium. The experiment, called SAGE, for Soviet-American Gallium Experiment, proved that scientists actually do know what powers our sun. Since the fall of the Soviet Union, the scientists in Baksan have had to fend off both thieves and the Russian government to keep their gallium, an element that goes for some $500 a kilogram.

Physicists know that neutrinos come in at least three types, known as electron, muon and tau neutrinos, depending on their subatomic origin. To add to the confusion, neutrinos can molt from one type to another. Physicists are arguing over whether there is evidence for a fourth type, called sterile neutrinos. That is the object of a new experiment underway in the Baksan tunnels.

Although neutrinos are the lightest and flimsiest particles of the universe, they are also among the most numerous, outnumbering the protons and electrons that make up us and ordinary matter by a billion to one. And so neutrinos contribute about as much mass to the universe as the visible stars.

The discovery of a high-energy neutrino from a distant galaxy passing through the IceCube detector at the South Pole elicited headlines around the world in July.

An extra population of neutrinos discovered by scientists in a cave in the Caucasus would affect basic calculations of the expansion of the universe.

Tracking one of the universe’s most elusive particles.

© 2018 The New York Times