Superfluid Helium Creates Directed Muonium Beam to Test Equivalence Principle for Muons

The Standard Model categorizes elementary particles into three generations. The first builds the visible universe-up and down quarks form protons and neutrons, while electrons create atomic shells. The second and third generations mirror this structure but with significantly higher masses; for instance, a muon is 207 times heavier than an electron. Why nature produces these heavier copies remains an open question. A critical tension exists in whether the equivalence principle applies to these heavy leptons. Einstein's theory relies on the equality of gravitational and inertial mass, yet empirical data for muons is scarce. If particles of different generations fall at different rates, the foundation of general relativity would require revision. To test this, physicists at the Paul Scherrer Institute developed a directed beam of muonium-an exotic atom consisting of a positive muon acting as the nucleus and a single orbiting electron. Because muonium is electrically neutral, researchers can isolate its gravitational interaction from electromagnetic interference.