One of the biggest mysteries of the universe is closer to being solved after four research groups reported new levels of sensitivity in massive experiments searching for a rare radioactive decay that would prove neutrinos are an entirely new class of particle.
The four experiments are searching for neutrinoless double-beta decay, in which two neutrinos cancel each other out, indicating that they are their own anti-particles – making them what are known as majorana particles.
No other fundamental particle behaves in this way, so the discovery of neutrinoless double-beta decay would challenge current understanding of how the universe was created, says Austin McDonald, a physicist at University of Texas, US, who works on a Spanish project called Neutrino Experiment with a Xenon TPC (NEXT-10).
“Discovery of neutrinoless double beta decay would be a major event in particle physics, on a par with, for example, the discovery of the Higgs boson,” he says.
Each of the experiments is studying a single radioactive isotope that undergoes “ordinary” double-beta decay – a process that emits two electrons and two neutrinos, which are at least a million times smaller.
Neutrinoless double-beta decay is a rare variant in which the neutrinos do not appear, but cancel each other out. Results from all four experiments suggest that to see it, you’d need to watch a single atom for, on average, 1026 years – about 10 quadrillion times the life of the universe.
The researchers’ approach to catching one of these events is to assemble over 1025 atoms – about a tonne of the radioactive isotope in question, and surround it with sensitive detectors. These have the resolution to distinguish between neutrinoless and normal double-beta decay, an energy difference of around one hundred-thousandth of a per cent. And with the likelihood of only a handful of events a year at most, the experiments need to be buried deep underground with hefty screening to block out cosmic rays and other background radiation.
The largest of these is the Cryogenic Underground Observatory for Rare Events (CUORE) at the Gran Sasso Laboratory in Italy, which searches for neutrinoless double-beta decay in the radioactive isotope tellurium-130. Their 988 tellurium oxide detectors contain just over 200 kilograms of the stuff, surrounded by lead from ancient Rome – used because it has extremely low background radioactivity.
A second experiment at Gran Sasso is the Germanium Detector Array (GERDA), searching for neutrinoless double-beta decay in detectors made of the isotope germanium-76, hanging in a four-metre high tank screened by liquid argon. Although they have only 38 kilograms of material, their screening gives them slightly better sensitivity than CUORE.
The MAJORANA Demonstrator (not an acronym, by the way) at Sanford Underground Research Facility in South Dakota, US, also studies germanium-76. The first phase of the experiment is 1.6 kilometres underground in a former mine shaft, and uses 40 kilograms of pure germanium crystals encased in lead. In the future it will merge with GERDA to create a larger germanium experiment.
Also in the US, in a 600-metre deep ancient salt formation, at the Waste Isolation Plant in New Mexico is the Enriched Xenon Observatory (EXO-200). EXO-200’s heart is liquid xenon, containing around 120 kilograms of the double-beta decaying isotope xenon-136 in a chamber of ultra-pure copper. The surrounding salt provides more screening.
Despite the combined efforts, none of the experiments has detected neutrinoless double-beta decay yet, but that doesn’t mean it doesn’t occur, says Oliviero Cremonesi from CUORE.
“Unfortunately, not observing does not mean it does not exist, but simply that at our sensitivity level we are not able to observe it – because it does not exist or simply because the half-life is longer.”
The most sensitive results reported come from GERDA, whose scientists calculate that half-life must be longer than 80 octillion years (8 x 1025 years), with the other three experiments within the same order of magnitude.
The groups are now preparing to assemble more massive experiments and trial more sensitive detection techniques. For example, in Spain, NEXT researchers are planning to expand their current 10-kilogram xenon-136 experiment to 100 kilograms, and are pioneering a new method of measuring the rare events, by capturing tiny flashes of fluorescence from the product of xenon decay called barium-136.
But the groups face the possibility that the sought-after reaction never occurs, that neutrinos are not their own anti-particles and instead behave like all the other fundamental particles, which all have a separate antimatter counterpart.
If that is the case, it leaves a major mystery surrounding antimatter unsolved.
English physicist Paul Dirac in 1928 predicted that each particle has a mirror-image anti-particle, with opposite characteristics, such as charge and symmetry. These particles – for example, antiprotons and antielectrons (known as positrons) – are now familiar from radioactive decays and reactions produced in particle accelerators such as the Large Hadron Collider.
But Dirac’s theory is beautifully symmetric. In fact, it is too symmetric, because it predicts that during the Big Bang, equal amounts of matter and antimatter should have been created, which should have all then immediately paired up and disappeared in myriad puffs of energy, leaving no matter at all.
But for some reason at the beginning of the universe more matter than antimatter was created. What antimatter there was quickly paired up with matter and disappeared: the stars and planets we see today are what was left over. But scientists are scratching their heads as to why matter dominated. All the experiments to date seem to support the symmetrical creation theory.
Neutrinoless double-beta decay could hold the answer. Neutrinos have no charge, and so it’s possible they could be their own anti-particle. In other words, two neutrinos could cancel each other out – the tell-tale glimpse for which the experiments around the globe are looking.
The concept that a particle could be its own anti-particle was named after the Italian physicist Ettore Majorana, who came up with the hypothesis in 1937.
Should neutrinos turn out to be majorana particles capable of cancelling themselves out, in contrast to the quarks and electrons, then it would be a departure from the symmetry of Dirac’s equation and would be a major clue why the universe of matter exists.