Truth of the matter: The Majorana particle mystery

Can a single entity be matter and antimatter at the same time? It looks like it

OH, MATTER is matter and antimatter is antimatter, and never the twain shall meet. That line has a poetic ring of truth about it - perhaps more so than Rudyard Kipling's original about east and west. After all, if matter and antimatter do meet, their mutual destruction is assured as they "annihilate" in a flash of light.

Or do they? Almost as soon as antimatter burst onto the scene 80 years ago, another possibility was aired: that certain particles, dubbed Majorana particles after their proposer, might be matter and antimatter at the same time. Proving this would be a big deal. It could help us to pin down the identity of the dark matter thought to dominate the cosmos, and discriminate between candidates for a better, all-encompassing theory of how stuff works. It might even explain the greatest material mystery: why matter exists at all.

Yet so far searches for these intriguing, ambiguous entities have turned up nothing. Some think that they pass through us in their millions each second, but lack the clinching proof. Others say a positive identification will come from the Large Hadron Collider, at the CERN particle physics laboratory near Geneva, Switzerland. As yet, nothing doing.

Solid sighting

Now, though, these matter-antimatter hybrids seem to have been sighted - not in cosmic rays or the detritus of particle collisions, but trapped in the innards of a solid superconductor. Has the mystery of the Majorana particles finally been solved?

Ettore Majorana had a talent for enigma. The mercurial Italian physicist's disappearance, somewhere en route from Palermo to Naples in the spring of 1938, still excites lively discussion. Suicide? Kidnap? A recluse's ruse to escape the public eye?

The particles that bear his name are no less enigmatic. Their origin lies in a seemingly innocuous modification Majorana made to an equation derived by the British theoretical physicist Paul Dirac in 1928. The Dirac equation marries quantum mechanics and Einstein's relativity to describe how electrons behave - and with them all other "fermion" particles, the building blocks of matter.

The Dirac equation was a revelation. First, it showed that electrons in a magnetic field act in one of two ways, distinguished by different values of a quantum-mechanical property called spin. But these spin states were only two of four possible guises for the electron that the equation made possible. The other two looked just the same, but had some sort of "negative" energy.

It wasn't immediately clear what this could mean. That changed in 1932, when American physicist Carl Anderson discovered an electron curving entirely the wrong way as it passed through the magnetic field of his cosmic-ray detector. He had found positrons: particles just like electrons, but with the opposite, positive, electric charge. Antimatter had made its debut.

Antimatter has since become a staple of science fact and fiction, beguiling for its habit of destroying itself and matter whenever the two should meet. It harbours great mysteries: exactly equal amounts of matter and antimatter should have been made in the big bang, so by rights everything should have annihilated. Why some matter survived to make stars, planets and people remains one of cosmology's great existential questions.

In Dirac's original formulation, only electrically charged particles had antiparticles. Majorana's tweak produced antiparticles for chargeless particles, too. Indistinguishable even by their charge, such a particle and its antiparticle would be absolutely identical. In fact, they would be one particle embodying all the qualities of both simultaneously.

The idea sounds faintly absurd - but it can be tested. "If a particle is its own antiparticle, then if two of them are brought together they can annihilate," says theorist Frank Wilczek of the Massachusetts Institute of Technology. Majorana particles would eat themselves.

That's not technically unprecedented. Today's standard model of the workings of matter predicts that absolutely every particle has an antiparticle: the chargeless, massless photon, for example, is its own antiparticle, and two photons annihilate themselves on the rare occasions they interact. But the photon is a force-carrying "boson"; seeing a matter-making fermion eating itself would be another thing entirely.

So far we have been denied the spectacle. The hottest tip is that neutrinos might be Majorana particles in disguise. These aloof, chargeless particles pass through Earth in their billions each second without interacting with anything. We know of three types and each seems to have an antineutrino equivalent that participates in particle reactions very differently. But many favoured routes to a unified theory of all of nature's forces suggest that this is an illusion. "Neutrinos and antineutrinos might be the same thing, just seen in different states of motion," says Wilczek.

The trouble is that the very elusiveness of neutrinos makes it nigh-on impossible to say that conclusively (see "Nothing doing"). Now, though, a result from an unexpected quarter could at last have given us something solid to go on.

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