Positronium is positively puzzling.
A new measurement of the exotic “atom” — consisting of an electron & its antiparticle, a positron — disagrees with theoretical calculations, scientists report on August 14 Physical Review Letters. And physicists are at a loss to define it.
A flaw in either the calculations or the experiment seems unlikely, researchers say. And new phenomena, like undiscovered particles, also don’t provide a simple answer, adds theoretical physicist Jesús Pérez Ríos of the Haber Institute of the Max Planck Society in Berlin. “Right now, the simplest I can tell you is, we don’t know,” says Pérez Ríos, who wasn’t involved the new research.
Positronium consists of an electron, with a -ve (negative) charge , circling in orbit with a positron, with a +ve (positive) charge — making what’s effectively an atom without a nucleus. With just 2 particles and free from the complexities of a nucleus, positronium is appealingly simple. Its simplicity means it are often wont to precisely test the idea of QED (quantum electrodynamics), which explains how electrically charged particles interact.
A team of physicists from University College London measured the separation between 2 specific energy levels of positronium, what’s referred to as its good spectrum . The researchers formed positronium by colliding a beam of positrons with a target, where they met up with electrons. After manipulating the positronium atoms with a laser to place them within the appropriate energy state , the team hit them with microwave radiation to induce a number of them to leap to a different energy state .
The researchers pinpointed the frequency of radiation needed to form the atoms take the leap, which is like finding the size of the gap between the energy levels. While the frequency predicted from calculations was about 18,498 MHz, the researchers measured about 18,501 MHz, a difference of about 0.02%. as long as the estimated experimental error was only about 0.003%, that’s a good gap.
The team looked for experimental issues that would explain the result, but came up empty. Additional experiments are now-needed to help investigate the mismatch, says physicist Akira Ishida of the University of Tokyo, who wasn’t involved the study. “If there’s still significant discrepancy after further precise measurements, things becomes far more exciting.”
The theoretical prediction also seems solid. In quantum electro-dynamics, making predictions involves calculating to a particular level of precision, leaving out terms that are smaller and harder to calculate. Those additional terms are expected to be too small to account for the discrepancy. But, “it’s conceivable that you simply might be surprised,” says theoretical physicist Greg Adkins of Franklin & Marshall College in Lancaster, Pa., also not involved the research.
If the experiments and therefore the theoretical calculations inspect , the discrepancy could be thanks to a new particle, but that explanation also seems unlikely. A new particle’s effects probably would have shown up in earlier experiments. For Ex., says Pérez Ríos, positronium energy levels might be affect from a hypothetical axion-like particle. That’s a light-weight particle that has the potential to define dark matter, an invisible sort of matter thought to permeate the universe. But if that sort of particle was causing this mismatch, researchers would even have seen its effects in measurements of the magnetic properties of the electron and its heavier cousin, the muon.
That leaves scientists still checking out a solution , says physicist David Cassidy, a coauthor of the study. “It’s getting to be something surprising. I just don’t know what.¬”
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