New Scientist | Physicsgg

Truth of the matter: The Majorana particle mystery

14 May 2012 by Michael Brooks and Richard Webb
Magazine issue 2864
For similar stories, visit the Quantum World Topic Guide

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

(Image: Andy Gilmore)

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.

Take half an electron…

A superconductor might seem an implausibly material stage upon which to spy antimatter. A positron would certainly be hard-pressed to survive among the myriad electrons that swarm through any sort of conductor. But from the early days of quantum physics it has been clear that certain materials harbour their own version of anti-electrons: holes.

“The hole is an absence of an electron where an electron would usually be,” says Marcel Franz, a physicist at the University of British Columbia in Vancouver, Canada. These holes move freely through certain conductors, and carry a positive charge equal and opposite to that of the electron. Understanding how a silicon transistor works is impossible without accepting that holes exist. When an electron and a hole meet, they “annihilate”: the electron jumps into the hole and neither electron nor hole is there to conduct any more.

The spectral existence of holes suggests a way to make a Majorana particle. Start off with half an electron and half a hole, combine the two bits and you have a fermion that is chargeless and has zero energy overall. “This solid-state Majorana would be a ‘nothing’ particle,” says Leo Kouwenhoven of Delft University of Technology in the Netherlands. “It is one big zero.”

But hang on: an electron is an elementary particle that you can’t just split apart. That is true, but doesn’t take into account the weird things that go on in superconductors. At very low temperatures, electrons and holes lose their individual identities and start behaving essentially as a larger quantum particle that flows collectively through the material without resistance. “It’s a little bit like a Mexican wave in a stadium,” says Kouwenhoven. “You can describe it as lots of individuals jumping up and sitting down singly. Or you can describe it as one wave.”

The crucial Majorana-making trick was worked out in 2010. It involves inducing superconductivity in a material in which electrons have very little room for manoeuvre in the first place, such as a one-dimensional wire (Physical Review Letters, vol 105, p 077001 and p 177002). Then it is as if the Mexican wave is disrupted at its ends. Things start to peel off – and in these broken-off bits you can find something that’s a little bit electron, a little bit hole, and every bit a Majorana. “Theoretically, there is no doubt that Majoranas should appear in these set-ups under the right conditions,” says Franz.

Kouwenhoven and his team might now have snared them. When they allowed superconductivity to “leak” from a superconductor into a neighbouring, restricted nanoscale semiconducting wire, entities popped up at the ends of the nanowire with zero energy and zero charge. Applying an electrical or magnetic field did not budge them, which is exactly the behaviour expected of a “nothing” matter-antimatter hybrid particle (Science, DOI: 10.1126/science.1222360). In February this year, David Goldhaber-Gordon and his team at Stanford University, California, also claimed evidence for Majorana particles in a subtly different material setting (arxiv.org/abs/1202.2323).

Further tests are needed to confirm the nature of the finds, but hopes are high. “Observation of Majoranas in solid-state devices will provide proof that these particles can exist in nature,” says Franz, who is not involved with the experimental teams. “My guess is we’ll have it in the next year or two.”

These manufactured Majoranas are sought after in their own right as the working bits of potentially super-powerful quantum computers (see “Twisted logic”). But there is a sense they are still not quite the real deal: a magnificent big cat caged in a zoo rather than freely roaming the savannah.

Besides those still stalking the neutrino in the hope of observing Majorana-like behaviour, the LHC’s particle hunters would seem to have the best chance to bag that beast. The LHC is searching for the Higgs boson, which would complete the standard model, and also for signs of a greater theory. The leading candidate is the construct known as supersymmetry, which proposes that every standard-model particle has a heavier, undiscovered “superpartner”. For every fermion, there’s a super-boson, and for every boson there’s a super-fermion.

Fact of the matter

Take the Higgs boson. It is the chargeless particle thought to give all others mass. But bring two “Higgsinos”, their super-fermion partners, together and you should get a spectacular mass-destroying show: they will annihilate into a slew of other particles. Other supersymmetric particles should also act as Majoranas. Such particles are prime candidates for the “weakly interacting massive particles”, or WIMPs, that might make up dark matter, the puzzling three-quarters of the universe’s mass that we cannot see. “If so, Majorana particles would be the most common in the universe,” says Kouwenhoven. The constant annihilation of these dark Majoranas could account for anexcess of high-energy cosmic positrons seen by detectors.

So far, the LHC has seen no evidence for anything supersymmetrical, let alone Majorana-like. But Wilczek thinks that could change over the next few years as the machine ramps up its power to maximum. “We’re not there yet, but we’re close,” he says.

Determining whether neutrinos are Majorana particles, meanwhile, might finally tell us why there is something rather than nothing. If neutrinos and antineutrinos are distinct particles, equal numbers of each would have been produced in the big bang. In the highly energetic conditions of the early universe, each would have decayed into equal numbers of all manner of other particles and antiparticles. But if neutrinos and antineutrinos are the same particle, it could decay into particles or antiparticles at will. There is no guarantee that those decays would have happened at the same rates: decays to particles might have slightly outnumbered decays to antiparticles. “It needs only a tiny difference, but this could be what makes the universe as we see it,” says Silvia Pascoli, a particle physicist at Durham University, UK.

Proving this scenario directly would require a particle accelerator that could recreate the incredibly hot and dense first fraction of a second of the cosmos – a machine 10 million times more powerful than the LHC. Given that, perhaps we should be grateful for any glimpse afforded us in a supercooled wire in a down-to-earth lab. The Majorana mystery continues, but we are a step closer to its resolution.

Nothing doing

Might neutrinos be matter-antimatter hybrids? Their “Majorana” status (see main story) has long been suspected. Definitive proof, however, would only come from seeing neutrinos annihilating each other. But it is difficult enough to get neutrinos to interact with detectors, and so measure their properties. Getting two neutrinos to interact with each other under earthly conditions is that problem squared.

One solution might be to observe a radioactive process known as neutrinoless double-beta decay. Conventional beta-minus decay involves the emission of an antineutrino, but a few nuclei can undergo two successive decays, producing two antineutrinos. If the neutrino is its own antiparticle – and so the antineutrino is a neutrino under a different name – then these antineutrinos might meet and annihilate on emission, resulting in no neutrino product. “It’s a very clumsy way to investigate the Majorana question, but it’s the best thing people have come up with,” says Frank Wilczek of the Massachusetts Institute of Technology.

It is also fiddly, patience-sapping work: a neutrinoless decay is expected to happen to any one susceptible atom only about once in 10²⁵ years. In 2001, a group of German and Russian physicists suggested that they had seen a few instances of the process, afterobserving decays of germanium-76 over 10 years (Modern Physics Letters A, vol 16, p 2409), but this result remains disputed.

The Majorana Collaboration now aims to break the deadlock. Involving more than 100 physicists from four countries, its goal is to keep a watchful eye on a tonne or so of germanium; a 40-kilogram prototype, the Majorana Demonstrator, is being built at the Los Alamos National Laboratory in New Mexico (arxiv.org/abs/1109.4790).

Twisted logic

It’s not only testing basic principles of matter and antimatter that motivates researchers to hunt down Majorana particles. “Majorana states are hotly pursued because they would allow for something called topological quantum computing,” says Laurens Molenkamp of the University of Würzburg in Germany.

Physicists have long dreamed of encoding information in the quantum states of particles, for example in the direction of their quantum-mechanical “spin”. Quantum theory’s fuzzy logic means that particles can exist in multiple states at once, meaning more computing bang for your bit. There is a huge stumbling-bock, however: quantum states are extraordinarily delicate, breaking down at the slightest disturbance from their environment.

Not so Majorana bits. A consequence of the way they arise in superconductors (see main story) is that they always come in pairs that, although spatially separated, encode the same information. That offers built-in robustness through redundancy: if one of the pair gets its information wiped, the other still has it.

Not much use if, as soon as the two bits meet, they blow each other up. But the beauty of Majorana particles is that they don’t have to interact for you to use them for computing. You can change the quantum states of a Majorana pair in concert simply by moving them around each other. By guiding these twisty movements in a pre-determined way, you can effectively run a whole series of computing steps: an algorithm.

If the recent sightings are confirmed, it will be full speed ahead with attempts to harness this sort of “topological” number-crunching. “Five years ago it was pure fantasy,” says theorist Frank Wilczek at the Massachusetts Institute of Technology. “The theoretical ideas have been way ahead, but now the experiments are catching up – that’s very exciting.”

Truth of the matter: The Majorana particle mystery – physics-math – 14 May 2012 – New Scientist.

Ανακαλύφθηκαν φερμιόνια Ματζοράνα;

Το νανοκαλωδιο (η οριζόντια μπάρα στο κέντρο) στο οποίο κάτω από ορισμένες συνθήκες όταν διοχετευθεί ηλεκτρικό ρεύμα τα ηλεκτρόνια συμπεριφέρονται ως φερμιόνια Majorana. DELFT UNIVERSITY OF TECHNOLOGY

Ένα ειδικά σχεδιασμένο τρανζίστορ προσφέρει τις πρώτες σαφείς ενδείξεις για την ύπαρξη των «φερμιονίων Ματζοράνα», μιας κατηγορία παράξενων σωματιδίων που αποτελούν τα αντισωματίδια των εαυτών τους.

Φερμιόνια είναι χονδρικά τα σωματίδια από τα οποία αποτελείται η μάζα, σε αντιδιαστολή με τα λεγόμενα μποζόνια, τα οποία χονδρικά είναι φορείς των φυσικών δυνάμεων.

Όλα ή τουλάχιστον τα περισσότερα φερμιόνια είναι «φερμιόνια Ντίρακ» (ονομάστηκαν έτσι από τον μεγάλο Άγγλο φυσικό Πολ Ντίρακ), τα οποία εξ ορισμού είναι διαφορετικά από τα αντισωματίδιά τους, τα οποία υπάρχουν στον κόσμο της αντιύλης. Όταν ένα σωματίδιο συναντήσει ένα αντισωματίδιο, και τα δύο εξαϋλώνεται σε μια έκρηξη ενέργειας.

Το 1937, όμως, ο Ιταλός θεωρητικός φυσικός Εττόρε Ματζοράνα (ο οποίος εξαφανίστηκε μυστηριωδώς το 1938 στη διάρκεια ενός ταξιδιού με καράβι) προέβλεψε θεωρητικά ότι υπάρχουν φερμιόνια που δεν ακολουθούν τον κανόνα του Ντίρακ.

Εβδομήντα πέντε χρόνια μετά, οι φυσικοί ακόμα προσπαθούν να επιβεβαιώσουν ή να διαψεύσουν την πρόβλεψη του εξαφανισμένου φυσικού.

Σαν σωματίδια

Προηγούμενες θεωρητικές μελέτες προέβλεψαν ότι, ακόμα και αν δεν υπάρχουν συγκεκριμένα θεμελιώδη σωματίδια που συμπεριφέρονται ως φερμιόνια Ντίρακ, η κίνηση των ηλεκτρονίων σε κάποια ηλεκτρονικά κυκλώματα μιμείται τη συμπεριφορά των φερμιονίων Ματζοράνα.

Αν οι φυσικοί κατάφερναν να δείξουν ότι μεγάλοι αριθμοί ηλεκτρονίων συμπεριφέρονται όπως τα αντίθετα των εαυτών τους, θα έδειχναν ουσιαστικά ότι τα φερμιόνια Ματζοράνα όντως υπάρχουν.

Αυτό ακριβώς επιχειρεί να κάνει η νέα μελέτη, η οποία δημοσιεύεται στο κορυφαίο περιοδικό Science. Οι ερευνητές του Πανεπιστημίου του Ντελφτ στην Ολλανδία κατασκεύασαν ένα ειδικό τρανζίστορ, το οποίο περιλαμβάνει ένα εξαιρετικά λεπτό, ημιαγώγιμο καλώδιο που συνδέει δύο ηλεκτρόδια.

Η θεωρία προέβλεπε ότι, αν διοχετευτεί ηλεκτρικό ρεύμα σε αυτό το νανοκαλώδιο, υπό την επίδραση ενός μαγνητικού πεδίου, και κάτω από συγκεκριμένες συνθήκες, τα ηλεκτρόνια στο νανοκαλώδιο θα συμπεριφερθούν ως φερμιόνια Ματζοράνα. Αν πάλι η θεωρία δεν ευσταθεί, το ηλεκτρικό ρεύμα δεν θα έπρεπε καν να περάσει από το ημιαγώγιμο νανοκαλώδιο.

Πράγματι, οι ερευνητές κατέγραψαν τη ροή ηλεκτρονίων, ένδειξη ότι συμπεριφέρονται ως φερμιόνια Ματζοράνα.

Οι παρατηρήσεις αυτές αποτελούν ισχυρή ένδειξη για την ύπαρξη των παράξενων αυτών «σωματιδίων», ωστόσο το ρεύμα που καταγράφηκε ήταν 95% ασθενέστερο από το προβλεπόμενο, και παραμένει ασαφές αν αυτό οφείλεται σε τεχνικούς μόνο λόγους.

Κβαντικοί υπολογιστές, σκοτεινή ύλη

Αν τελικά η ύπαρξη των φερμιονίων Ματζοράνα επιβεβαιωθεί, αυτό θα έδινε τροφή για σκέψη και σε άλλους κλάδους της φυσικής.

Για παράδειγμα, τα φερμιόνια αυτά θα ήταν ιδανικά για την κατασκευή των λεγόμενων κβαντικών υπολογιστών, καθώς ένα ζευγάρι από φερμιόνια Μοτζοράνα θα «θυμόταν» την αρχική του κατάσταση ακόμα και αν χωριζόταν.

Επιπλέον, η επιβεβαίωση του Εττόρε Ματζοράνα θα αναπτέρωνε την αισιοδοξία των φυσικών που πιστεύουν ότι τα θεμελιώδη σωματίδια νετρίνα είναι στην πραγματικότητα φερμιόνια Ματζοράνα.

Θα αναπτέρωνε επίσης το ηθικό όσων πιστεύουν στην ύπαρξη του νιουτραλίνο, ενός θεωρητικού σωματιδίου που θα μπορούσε να είναι το συστατικό της μυστηριώδους «σκοτεινής ύλης», η οποία φαίνεται ότι γεμίζει το Σύμπαν αλλά δεν μπορεί να παρατηρηθεί άμεσα γιατί είναι αόρατη.