The helium atom is like a tightly stretched string: every note it plays is strictly predicted by theory. But in recent years, precise measurements of one such "note"—the energy needed to strip an electron from an excited helium atom — have started to sound slightly higher than they should. The discrepancy is tiny, but it shows up in different helium isotopes, and chance can no longer explain it: 9 sigma is like flipping a coin and getting heads a hundred times in a row.
One possible explanation is a new particle, light and elusive, that subtly "tweaks" the spring of interaction between electrons and the nucleus. Such a particle is not part of the Standard Model, but if it exists, it should affect not only helium but also other systems where electrons are bound to antiparticles. The authors calculated how this hypothetical particle would shift energy levels in positronium — an atom where, instead of a proton, a positron (the electron's antiparticle) orbits the electron.
The calculations show that if the new particle is indeed responsible for the helium anomaly, the transition frequency between two states of positronium should shift by 0.25–0.85 MHz. That's small, but modern spectroscopic techniques can detect such shifts. An experiment to test this prediction would be a decisive test: if the shift is confirmed, physics would have a new force that cannot be explained by known particles.
The spring, slightly stretched in helium, should also twang in positronium. Whether we hear that twang will be shown by experiment.
Positronium was first discovered in 1951 by Martin Deutsch and exists for only about 140 nanoseconds before the electron and positron annihilate, emitting gamma rays.