A long-standing controversy among particle physicists looks to be settled—in the less exciting way—thanks to new data from an ultrasensitive particle detector deep underground. Physicists operating the GERmanium Detector Array (GERDA) 1400 meters down in Italy's Gran Sasso National Laboratory say that they see no signs of a hypothesized type of nuclear decay called neutrinoless double-beta decay that, were it conclusively observed, would almost certainly merit a Nobel Prize. The new results stick a pin in a claim made by a rival group in 2001.
The search for neutrinoless double-beta decay is the sort of thing that might make you wonder what particle physicists are on about. Ordinary beta decay occurs when a neutron in a nucleus turns into a proton while spitting out an electron and an antineutrino—a chargeless, almost massless particle that only interacts extremely weakly with ordinary matter. Some kinds of nuclei are also known to undergo a much rarer process known as double-beta decay, in which two neutrons convert into two protons simultaneously, simultaneously emitting two electrons and two antineutrinos. However, theorists have also predicted an even rarer form of double-beta decay in which two neutrons decay into two protons and two electrons, but no antineutrinos.
Why on Earth would that be interesting? Well, it all gets into the identity politics of neutrinos. It's possible to trigger a nuclear reaction much like beta decay in which instead of emitting an antineutrino, a neutron absorbs a neutrino and then turns into a proton and an electron. So neutrinoless double-beta decay would be just like teaming a normal beta decay with this triggered reaction—if the antineutrino emitted in the regular beta decay is absorbed as a neutrino in the additional nuclear interaction. That is, neutrinoless double-beta decay can take place only if the neutrino is its own antiparticle. And if that were true, then the neutrino would be the only matter particle for which that weird blending of matter and antimatter would hold. (The photon, which makes up light, not matter, is already its own antiparticle.)
That's why physicists sat up and paid attention in 2001 when Hans Volker Klapdor-Kleingrothaus and colleagues at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, claimed to have observed such decay. In their "Heidelberg-Moscow" experiment at Gran Sasso, they studied 11.5 kilograms of germanium enriched with the nucleus germanium-76, one of the few nuclei with the right numbers of protons and neutrons to possibly undergo the decay. Watching the material for 13 years, Klapdor-Kleingrothaus and colleagues claimed to have seen a clear excess of events coming from the decay. However, many other physicists disputed their claim, arguing that the Heidelberg team had not done enough to rule out ordinary radioactive decay as the cause of their putative signal.
GERDA researchers aimed to put the claim to the test. Comprising physicists from 19 research institutes and universities across Europe, the team deployed a detector consisting of 18 kilograms of germanium and ran for far less time, from November 2011 to May 2013. But the detector's superior construction enabled researchers to better filter out background radiation—which was already reduced by building the detector underground—and the device quickly achieved a higher sensitivity to the rare decay.
GERDA sees no evidence for neutrinoless double-beta decay. In a seminar at Gran Sasso's surface facilities today, Stefan Schönert, a physicist at the Technical University of Munich in Germany and spokesperson for the collaboration, explained that the experimenters measure the energies of the twin electrons and see three events in the energy range in which Klapdor-Kleingrothaus and colleagues claimed an excess. That's almost exactly what would be expected from background events alone, Schönert reported. "The [Heidelberg] claim is refuted with high probability," he says.
The latest results are "very exciting," says Steven Elliott, a neutrino physicist at the Los Alamos National Laboratory in New Mexico, but they may not quite provide the final word on the matter. "The data indicate that the past claim is very unlikely to be correct," he says. "But there might still be some wiggle room due to the low statistics involved."
Of course, the result doesn't prove that such decay is impossible, just that it happens more rarely than can be measured so far. Whereas the team led by Klapdor-Kleingrothaus claimed to have observed the rare decay with a half-life of 1.2x1025 years, the new results, Schönert explained, implied that the half life instead can be no lower than 2.1x1025 years. That's a mere million billion times the age of the universe. So the GERDA team will continue its search. Later this year, the researchers will start taking data with new detector modules that roughly double the amount of germanium-76 and will further reduce background levels by a factor of 10. Ultimately, it may take a detector weighing a metric ton to detect the rare decay—if it's there.