Physicists at PSI Fired Antimuons Into Superfluid Helium and Got a Cold, Parallel Beam of Muonium Atoms Out. It Is the First Step Toward Dropping a Second-Generation Particle and Seeing if Einstein Was Right.

Every test of Galileo's claim that all objects fall at the same rate, from feathers in vacuum chambers to satellites in orbit, has been done with ordinary matter: protons, neutrons and electrons, or their antiparticles. Nobody has ever measured how gravity acts on the heavier, short-lived particles of the second generation. A team at ETH Zurich and the Paul Scherrer Institute in Villigen, Switzerland, has now built the beam that could make that measurement possible, and reported it in Nature Physics.

"We have taken an important step toward carrying out an exciting experiment on this topic," said Anna Soter, the ETH Zurich physics professor who leads the group. "We want to measure the gravitational interaction of the muon."

The muon is the electron's heavier sibling, roughly 200 times its mass and one of the second-generation particles that the Standard Model describes but does not explain. "We physicists do not yet understand why these additional generations exist in the first place," Soter said. "And why are there three in total?" Whether those heavier particles feel gravity exactly as the light ones do is an open question, and the equivalence principle at the heart of Einstein's general relativity, which says gravitational and inertial mass are the same, has never been checked against them.

Measuring the fall of a charged particle is hopeless, because stray electromagnetic fields swamp gravity. So the team uses muonium, a neutral atom formed when a positively charged antimuon captures an electron. "After all, to make something fall, you need something neutral," Soter said. Two obstacles have stood in the way. Muons decay after just 2.2 microseconds, and every previous muonium source sprayed atoms in all directions at a range of speeds, useless for precision work.

The solution, described by lead author Jesse Zhang, is a thin layer of superfluid helium chilled to near absolute zero. "Superfluid helium is what is known as a quantum fluid, in which the individual helium atoms lose their identity and which does not tolerate any impurities within it," Zhang said. Antimuons from PSI's accelerator, which produces the world's most intense continuous muon beams, are fired into the film and slow down. When an antimuon meets a free electron, the muonium atom that forms has a positive chemical potential, meaning the helium wants it gone. The liquid pushes it to the surface, where that stored energy converts into motion and the atom shoots straight up. "So we're using the chemical potential as an atomic cannon," Zhang said.

The result is a beam of muonium atoms that "propagate at similar speeds, almost parallel to one another," Soter said. That is what "cold" means here, and it is what makes a gravity experiment possible. Because muonium can move through the superfluid without collisions, enough atoms reach the surface within their lifetime to form a usable beam.

The next step is an interferometer that exploits the wave nature of the atoms. Earth's gravity should shift the interference pattern by a tiny, calculable amount, and any deviation from that prediction would mean muons fall differently from ordinary matter. "We hope to be able to test the method for the first time with the atomic beam this year," Soter said, "and if all goes well, the actual gravity experiment should follow in two or three years' time." The beam should also allow far more precise laser spectroscopy of muonium, which would pin down the muon's mass and several fundamental constants.

If the muon does fall differently, "that would indeed be surprising, and, in addition to other theories, it could point to the existence of a fifth force," Soter said. Physics currently recognizes four: gravity, electromagnetism, and the strong and weak nuclear forces. A fifth has been proposed many times and never found. Soter is not betting on it. "I am completely open-minded," she said. "I simply want to measure, for the first time, whether the equivalence between gravitational and inertial mass also applies to the second generation of particles. This alone is quite an inspiring piece of work."