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TODAY’S universe is only half the place it used to be. Admittedly there is still a lot of it around, locked up in dust and gas, but that’s only half the story – quite literally. Over 13 billion years ago, the big bang forged equal amounts of matter and antimatter. Take a look around today though, and you’ll see that all the antimatter has vanished.

It’s lucky for us that it has. According to our best theories, all the matter and antimatter should have annihilated each other in a puff of radiation. The universe should be filled with pure light, not planets and stars and people. To create the universe we see today, matter somehow gained the upper hand over antimatter a fraction of a second after the big bang. But how?

Five shiny germanium cylinders sitting in a cavern under a mountain in central Italy hold the answer, or at least the physicists in charge of them believe they do. Hans Klapdor-Kleingrothaus and his colleagues from the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, have been monitoring the cylinders day and night for over a decade in the hope of spotting a form of radioactive decay that, according to all received wisdom, should not exist. And earlier this year they claimed to have made nearly 30 sightings of it. “If confirmed, this discovery could be worth a Nobel prize,” says Petr Vogel, a nuclear physicist at the California Institute of Technology in Pasadena.

What the Heidelberg group claim to have seen is a peculiar form of beta decay. In normal beta decay, a neutron inside an atomic nucleus spontaneously transforms into a proton, spitting out an electron and a particle called an antineutrino in the process. A handful of rare isotopes go one better, undergoing double-beta decay, where two neutrons inside the same nucleus decay at the same time, coughing up two electrons and two antineutrinos.

Physicists have been observing double-beta decay in experiments for nearly 20 years. But Klapdor-Kleingrothaus and colleagues say they have seen a version of it where no antineutrinos are produced at all. The process was predicted as long ago as 1937 by an Italian physicist called Ettore Majorana, but was dismissed as impossible.

One of the reasons this neutrinoless double-beta (NDB) decay is forbidden by our standard picture of matter and forces is because it fails to conserve a quantity known as the lepton number, one of the key checks and balances that the universe appears to follow. Electrons and neutrinos are leptons, and are assigned a value of 1. Their antimatter counterparts, also leptons, have a value of −1, and protons and neutrons have a value of 0.

Physicists have found that if you look at any reaction, such as an electron smashing into a proton or beta decay of a radioactive element, the lepton number after the reaction is always the same as it was before. But in NDB decay, the number changes from 0 before the decay to 2 after it, thereby violating the law.

Originally a collaboration between physicists from Heidelberg and the Kurchatov Institute in Moscow, Russia, the experiment observing the germanium cylinders has been running since 1990 at the Gran Sasso laboratory, 120 kilometres east of Rome. The idea behind the experiment is simple enough. The cylinders are made from germanium-76, a natural isotope of germanium, known to undergo double-beta decay. If one of the germanium-76 nuclei instead undergoes NDB decay, it will transform into a selenium nucleus and spit out two electrons carrying a well-defined amount of energy without any antineutrinos (see Graphic). Germanium is also a semiconductor, so the surrounding germanium atoms can absorb this energy and produce a measurable electric current.

To claim a discovery, the team has to see several unambiguous electrical signals. But from the start the researchers knew they would have their work cut out looking for something that isn’t supposed to exist. The trouble is, NDB decay isn’t the only process that leads to a current in germanium. Cosmic rays zipping through the Earth’s atmosphere produce showers of particles that can pass through the germanium cylinders and generate a current. Then there is the natural radioactivity of the surrounding rock and other materials in the laboratory. This background radiation is so abundant that it could completely swamp any other signal.

To boost their chances of recording NDB decay, Klapdor-Kleingrothaus and colleagues use large blocks of enriched germanium whose proportion of germanium-76 has been boosted from the normal 8 per cent to 80 per cent. And to block out the natural radioactivity, they keep the cylinders in a lead-lined steel box. Meanwhile the 1400 metres of rock above the laboratory shields the experiment from cosmic rays.

Of course, that still leaves some residual radioactivity from the materials inside the box, as well as the more common version of double-beta decay in the germanium itself. But in principle the current produced by NDB decay should be distinguishable from anything elicited by this remaining radioactivity. The strength of the current reflects how much energy is dumped in the germanium, so all the researchers need to do is count the number of signals they see with that particular current.

Three years ago, Klapdor-Kleingrothaus’s group published their results in the journal Modern Physics Letters A (vol 16, p 2409). They claimed to have seen 15 NDB decays in 10 years of measurements. The Heidelberg team believed their discovery would rock the foundations of physics.

But the announcement was greeted with howls of protest. While most physicists regarded the measurements made by the Gran Sasso experiment as the best in the world, they viewed the analysis carried out by Klapdor-Kleingrothaus’s team as seriously flawed. Steven Elliott of the University of Washington in Seattle and 25 other researchers from around the world even sent a letter to Modern Physics Letters A raising several problems with the claim.

The critics’ main concern was that the Heidelberg team had underestimated the number of anonymous background processes taking place within the germanium. Elliott and his colleagues concluded that the team had probably mistaken a statistical fluctuation in these processes for a real NDB decay signal.

It didn’t take long for the mood to turn ugly. At first Klapdor-Kleingrothaus simply dismissed the critics’ concerns. “A lot of what he said was ‘that’s not true’, without any supporting evidence as to why,” says Elliott.

Criticism of the Heidelberg claim also came from closer to home. The physicists from the Kurchatov Institute, who had provided the enriched germanium from the Soviet atomic energy programme, quit the collaboration in 2001. They presented their own analysis at a conference in Russia last June, saying that only two of the five germanium blocks showed any discernable signal. Even worse, they concluded that this “cannot be considered as any evidence of neutrinoless double-beta decay”.

Yet the Heidelberg team has remained defiant. Klapdor-Kleingrothaus has responded to most of Elliott’s criticisms, in particular the doubts about the background processes. And in February, the group published a second claim based on an extra three years’ worth of measurements (Physics Letters B, vol 586, p 198). Improvements in the way they collect and analyse the signals, they say, has boosted the number of NDB decays from 15 to 29. This raises the chance of the signal being real, rather than a fluke, from 97 per cent to 99.997 per cent. “Our new paper shows that our critics are wrong,” says Klapdor-Kleingrothaus. “There is no disagreement because these people accept that what they said initially was not right.”

If the results stand up, then neutrinoless double-beta decay could be the start of something big. This oddball process would make the neutrino unique among the fundamental particles of matter. Physicists believe that the interactions and decays of matter and antimatter follow a set of specific laws, including the conservation of energy and lepton number. These laws say that swapping matter for antimatter is equivalent to time running backwards. Rewinding the film of a neutron spitting out an antineutrino would show the same neutron absorbing a neutrino. But in NDB decay, the antineutrino produced by a decaying neutron is immediately absorbed by another neutron, which breaks all the rules.

Cosmic changelings

But there is a loophole: if the ejected antineutrino turns into a neutrino before it reaches the second neutron, then it can be absorbed without breaking the law. The only way this can happen is if neutrinos and antineutrinos are, in fact, identical.

This would set these ghostly particles apart from other types of matter and antimatter. Take positrons, for instance, the antimatter nemesis of electrons. They weigh the same as electrons and experience the same force, but they are distinct particles whose attributes, such as electric charge and lepton number, are reversed. Certainly they do not suffer from the same identity crisis that Majorana predicted for neutrinos.

The Heidelberg findings could help to explain why the universe is full of matter and not antimatter, says Sacha Davidson, a theoretical physicist at the University of Durham in the UK. Not only do they point to neutrinos’ changeling behaviour, they also predict the existence of an extremely heavy form of neutrino at high temperatures.

In the aftermath of the big bang fireball, the universe would have been an inferno. Much of this energy could have been stored in pairs of supermassive neutrinos. If Majorana’s theory is correct, these neutrinos would repeatedly flip into supermassive antineutrinos and back again. As the universe expanded and cooled, they decayed into their lighter counterparts.

But that’s not the end of the story. In the 1960s, Soviet physicist Andrei Sakharov suggested that a minuscule difference in the rate of decay of matter and antimatter could tip the balance in favour of matter over antimatter (91av, 6 February 1999, p 26). This process, which is known as CP violation, has been seen in experiments, most recently last month by the BaBar experiment at the Stanford Linear Accelerator Center in California. But the bias seen there is not big enough on its own to explain why the antimatter created in the big bang vanished.

Neutrinos could come to the rescue. Davidson says CP violation would have upset the balance of neutrinos and antineutrinos, leaving a very slight excess of antineutrinos shortly after the big bang.

So how does the imbalance between neutrinos and antineutrinos explain why our world is made up of protons and neutrons, rather than their antimatter rivals? According to Davidson, in its first microseconds of existence the universe contained massive particles reminiscent of the pill-chomping computer-game character Pacman. For every neutrino that Pacman ate, he gobbled a proton or a neutron. Likewise every antineutrino was washed down with an antiproton or antineutron. With an initial excess of antineutrinos, Pacman would have guzzled more antimatter than matter. Any antimatter left over would have interacted with matter in a flash of light. “We are left with universe that contains more protons and neutrons than their antimatter partners,” says Davidson.

Elliott concedes that the latest work by the Heidelberg group is a big improvement on the first paper. But it remains controversial. “It’s not totally clear that a discovery has been made,” says Vogel. “Klapdor-Kleingrothaus is not obviously wrong, but he is also not obviously right.” He and his colleagues refuse to endorse the result until someone else sees the effect in a different experiment.

Ron Brodzinski at the Pacific Northwest National Laboratory in Richland, Washington, insists that the Heidelberg group’s analysis is flawed. He still doubts whether the background radioactivity was handled correctly.

Not everyone takes such a tough line, however. “If you ask around you’ll find people who have particular complaints about the work,” says Elliott, “but no one has produced a smoking gun to say it is definitely wrong. A thinking person could argue for or against the supposed signal.”

In the light of all this uncertainty, Elliott says it is vital to carry out independent searches for NDB decay. But other experiments are unlikely to gather enough measurements to prove or disprove the claim soon, according to Frank Avignone of the University of South Carolina in Columbia. Such proof, he says, is likely to come from a new generation of experiments using hundreds, rather than tens, of kilograms of germanium. And there are ambitious plans to build even larger detectors weighing around a tonne by 2010.

Even though he believes his group is right, Klapdor-Kleingrothaus would also welcome more results. “The consequences of what we have found are so huge that you need to see the effect with another experiment,” he says. “But it is only for this reason, and not because we are wrong.”