Researcher’s equations help guide the study of the positronium molecule

It was predicted in the 1940s but it wasn’t until 2007 that scientists actually saw it in the laboratory. Now, a York researcher has added his own marker to the history timeline of the positronium molecule, an exotic form of matter and antimatter (each molecule is made up of a pair of electrons and a pair of their antiparticles, called positrons). It has similarities to the familiar hydrogen molecule.

Mohsen Emami-RazaviRight: Mohsen Emami-Razavi

When he was a graduate student in York’s Faculty of Science & Engineering under the supervision of Professor Jurij W. Darewych, physicist Mohsen Emami-Razavi (MSc ’00, PhD ’06), now a post-doctoral researcher in York’s Centre for Research in Earth & Space Science, fixed on the idea of developing a generating equation (using Darewych’s formalism) that would predict the results of combining particles of matter and anti-matter. His latest paper describing the behaviour of particles and their antiparticles was published in the April issue of the American Physical Society’s Physical Review A.

His reference paper’s main equation, which one referee said deserves to be in textbooks, explains how the polyelectrons, predicted by John Archibald Wheeler in the 1940s and confirmed recently by physicists David Cassidy and Allen Mills of the University of California, Riverside, combine to form a molecule of positronium and perform a dance of death before being annihilated in the smallest fractions of a second, leaving behind powerful gamma radiations – the force behind future high-energy gamma-ray lasers.

Left: Illustration of the positronium molecule

While other scientists have been working in the lab to create simple forms of these atoms and observe their behaviour, Emami-Razavi turned his mind to formulating the generating equation that could serve as a guide to future experimental work with more complex forms. Cassidy and Mills created the first positronium molecules in 2007 by firing anti-matter called positrons into a thin film of porous silica where they combined with electrons to form two-body positronium atoms (one electron and one positron). Some of these inherently unstable atoms combined to form the four-body positronium molecule (two electrons, two positrons). Emami-Razavi’s equation describes what happens when any number of electrons and positrons combine in the same way – a subject the experimentalists are already working at observing in the lab.

“I am more interested in the equations,” says Emami-Razavi. “I was always thinking about how one can explain a huge phenomenon with a simple equation.”

At the time he wrote his paper, the positronium molecule had not been observed. That perhaps explains why reviewers at five different journals rejected it before it was finally accepted by peers at the Physical Review A. His paper was published in April of this year, months after the positronium molecule was created in the lab for the first time. Emami-Razavi says he doesn’t like to speculate on why the paper took so long to win acceptance but the comments he received on his work suggest it was all too new for the referees to appreciate.

The paper, “Relativistic n-fermion wave equations in Quantum Electrodynamics”, is a tough slog for all but the most experienced mathematician; however a pioneer in the field, Freeman Dyson, after being sent a copy by Emami-Razavi, called the paper “an elegant piece of work.”

For the layperson who finds math more obscure than a foreign language, Emami-Razavi’s work may seem hard to relate to. But, like many researchers who have peered deeply into the mysteries of their field, he regards it as a life narrative rather than an inscrutable puzzle. “What do I see?,” he says when asked to explain the source of his inspiration. “I see an electron and positron dancing together. They have a very short lifespan, nanoseconds or picoseconds. I found that in that time, lots of interactions happen, so many that, for them, it’s a lifetime. Why? Because there is a rule, there is a law. I succeeded in going inside their world because I am dreaming. I have to dream somehow that they have a lifetime in order to understand them.”

Emami-Razavi has plenty of dreaming left to do. He is also working on problems related to the Higgs boson particle, the last unobserved particle of the Standard Model in Particle Physics – and what some call the God particle.

To view a copy of Emami-Razavi’s paper, visit the Physical Review A Web site.

By David Fuller, YFile contributing writer