Physicists at ETH Zurich and the Paul Scherrer Institute have created a narrow, cold-like beam of muonium, an exotic atom containing a second-generation muon, overcoming a major technical barrier to measuring how it responds to gravity.The researchers produced muonium inside superfluid helium cooled to about 0.2 kelvin. The quantum liquid expelled the atoms vertically into vacuum at similar speeds, producing the controlled beam needed for an atom-interference experiment.The team has not yet measured muonium\u2019s gravitational acceleration. Its planned experiment could become the first direct test of whether Einstein\u2019s weak equivalence principle applies to matter whose mass is dominated by a second-generation particle.<\/p>\n
For centuries, experiments have supported a remarkable rule of nature: objects fall the same way in a gravitational field regardless of what they are made of. Physicists have tested that principle with ordinary matter to extraordinary precision, and more recently with antimatter.<\/p>\n
One important piece of the particle world has remained out of reach. Scientists have never directly measured how gravity acts on matter dominated by a second-generation particle, such as the muon.<\/p>\n
Researchers at ETH Zurich<\/a> and the Paul Scherrer Institute<\/a>, or PSI, have now cleared a major obstacle to doing exactly that. They created a highly controlled beam of exotic muonium atoms that could make a direct gravity experiment possible.<\/p>\n The advance, reported in Nature Physics<\/a>, does not yet show whether muonium falls normally. Instead, it provides the unusual atomic beam required to put one of Einstein\u2019s foundational ideas to a completely new test.<\/p>\n The three doctoral students Robert Waddy, Jesse Zhang and Paul Wegmann (from left to right) are aligning the experiment at PSI, to the beamline. (CREDIT: Anna Soter \/ ETH Zurich) <\/p>\n \u201cWe have taken an important step toward carrying out an exciting experiment on this topic,\u201d said ETH Zurich physics professor Anna Soter. \u201cWe want to measure the gravitational interaction of the muon.\u201d<\/p>\n Muonium puts a heavier relative of the electron into the test<\/p>\n Ordinary matter consists largely of particles from the first generation of the Standard Model, including electrons and the quarks that make up protons and neutrons. Nature also contains two additional generations whose particles are heavier.<\/p>\n The muon belongs to the second generation and behaves in many respects like a heavy relative of the electron. A positively charged muon, or antimuon, can bind with an electron to create a neutral atom called muonium.<\/p>\n That combination makes muonium unusually valuable for gravity experiments<\/a>. Its mass is overwhelmingly dominated by the muon, while its electrical neutrality prevents tiny stray electric fields from simply overwhelming the far weaker gravitational force researchers want to measure.<\/p>\n The question goes directly to the weak equivalence principle. This principle states that gravitational mass and inertial mass are equivalent, producing the familiar result that different objects experience the same gravitational acceleration under equivalent conditions.<\/p>\n Experiments using ordinary matter have supported this principle with exceptional precision. Antihydrogen experiments have also found gravitational behavior consistent with ordinary matter. Muonium would extend the test into a fundamentally different part of the Standard Model.<\/p>\n Jesse Zhang (left) and Paul Wegmann complete detector wiring in the dilution refrigerator. (CREDIT: Anna Soter \/ ETH Zurich) A lifetime of 2.2 microseconds creates an enormous challenge<\/p>\n Muonium comes with one severe disadvantage: the muon survives for only about 2.2 microseconds before decaying.<\/p>\n That gives researchers very little time to create an atom, move it into vacuum and observe gravity changing its motion. Conventional muonium sources made the problem even harder because their atoms emerged over a broad range of speeds and directions.<\/p>\n An interferometer capable of detecting gravity<\/a> needs something much more orderly. Researchers require an intense beam whose atoms travel at similar speeds in nearly the same direction.<\/p>\n Previous materials could produce muonium in vacuum, but narrowing those beams sufficiently would discard so many atoms that a gravity experiment became impractical.<\/p>\n Soter’s team turned to an unusual material to solve the problem: superfluid helium chilled almost to absolute zero.<\/p>\n \u201cWe have managed to produce the muonium atoms in a \u2018cold\u2019 state, which is what makes the gravity experiment possible in the first place,\u201d Soter said. Here, cold describes the narrow range of velocities and directions in the resulting beam rather than simply the atoms’ temperature.<\/p>\n Professor Anna Soter and doctoral student Paul Wegmann are assembling cryogenic silicon detectors for the experiment. (CREDIT: Robert Waddy \/ ETH Zurich) Superfluid helium acts like an atomic cannon<\/p>\n The researchers cooled a thin layer of helium to approximately 0.2 kelvin, only a fraction of a degree above absolute zero. At such temperatures, helium becomes a quantum fluid with highly unusual properties.<\/p>\n Antimuons generated by PSI’s accelerator were directed into this liquid and slowed. Once an antimuon captured an electron from its ionization trail, it formed muonium beneath the helium surface.<\/p>\n Muonium is energetically unfavorable inside the superfluid<\/a>. The atom therefore experiences a positive chemical potential that effectively drives it toward and out of the liquid.<\/p>\n \u201cWhen the muon reaches the surface, the chemical potential is converted into kinetic energy,\u201d explained lead author Jesse Zhang. \u201cSo we’re using the chemical potential as an atomic cannon.\u201d<\/p>\n Measurements showed muonium moving upward through the detector after escaping the helium, while an empty-target control showed no equivalent signal. A preliminary analysis placed the atoms\u2019 propagation speed in vacuum at about 2.1 kilometers per second, with diffusion through the helium occurring on the microsecond timescale.<\/p>\n Most importantly, the resulting beam was far narrower in velocity and direction than a conventional thermal source.<\/p>\n Experimental setup for observing Mu emission from He II. (CREDIT: Anna Soter et al, Nature Physics) Interference could reveal how the muon falls<\/p>\n The next stage involves sending the muonium beam through an interferometer. Instead of treating atoms simply as particles, the instrument takes advantage of their wave-like quantum behavior.<\/p>\n A series of extremely fine gratings would produce an interference pattern. Earth’s gravity<\/a> should shift that pattern vertically by a tiny amount while the muonium travels between the gratings.<\/p>\n Measuring that displacement would allow the researchers to infer muonium’s gravitational acceleration.<\/p>\n Calculations based on the new beam suggest the proposed setup could eventually reach roughly 1% relative precision after about 100 days of measurements under projected PSI beam conditions. The researchers first need to demonstrate the required interferometry and control experimental systematics before that gravity measurement can happen.<\/p>\n