NEW YORK: The world’s first precision measurement of an elusive parameter which describes the rate at which wispy subatomic neutrinos morph from one type to another has been determined.
Physicists have been chasing this measurement for over a decade, and the new result – submitted to Physical Review Letters by a multinational team of researchers at the Daya Bay Reactor Neutrino Experiment in China – opens the floodgates to new experiments that could explain the perplexing dominance of matter over antimatter in the universe.
“Our precise measurement will complete the understanding of the neutrino oscillation and pave the way for the future understanding of matter-antimatter asymmetry in the universe,” said Yifang Wang of China’s Institute of High Energy Physics and co-project manager of the Daya Bay experiment.
Discovered in the 1950s, neutrinos are tiny, ubiquitous, ghostlike particles that are virtually undetectable and outnumber protons and electrons by a million to one. They come in three types, or ‘flavours’ – electron, tau and muon neutrinos – each with corresponding antiparticles. They sprung into existence moments after the Big Bang and continue to be created today in the hearts of stars, in supernovae, and during radioactive decay.
Detecting them is tough: they are so incredibly slight that they were once thought to have no mass, they have no charge, and they are frustratingly reluctant to interact with matter. Flying through space at almost the speed of light, they pass through the Earth unhindered. Sixty million stream through your thumbnail unnoticed every second of the day.
But neutrinos aren’t only elusive; they’re indecisive too. They can oscillate back and forth between their three flavours, a process that is described by three ‘mixing angles’. Two of these were measured in the 60s and 90s respectively, but the final angle, known as θ13 (‘theta one three’) – which quantifies the rate at which electron neutrinos and electron antineutrinos transform into the other flavours – has evaded scientists until now.
“This is a new type of neutrino oscillation, and it is surprisingly large,” said Wang.
The Daya Bay team studied antineutrinos produced in six powerful nuclear cores at the Daya Bay plant in south China. Using six identical 20-tonne subterranean detectors, one set placed several hundred metres from the reactors and another set about two kilometres away, they were able to track the apparent disappearance of electron antineutrinos as they morphed into other flavours.
The scientists found that the number of electron antineutrinos dropped off by 6% between the two sets of detectors, which enabled them to determine a precise value of 8.8 degrees for θ13 in just under eight weeks of data-taking.
Where did all the antimatter go?
Now that the measurement is confirmed as non-zero, physicists can begin to look for a phenomenon called CP violation in neutrinos, which postulates that matter and antimatter are not always equal and opposite. If observed, it might finally solve the riddle of why we see an imbalance of the two in the universe today.
“A non-zero θ13 is pivotal to the future research in neutrino physics, astrophysics, and cosmology,” said Kam-Biu Luk of the Daya Bay experiment, the Lawrence Berkeley National Laboratory and the University of California at Berkeley.
A zero result would have ruled out CP violation in neutrinos, and a very small value would have made the phenomenon impossible to study. “We are thrilled to uncover a significantly larger effect [than expected],” said Luk. “It is as exciting as finding a nice gift in the box that you expect to be empty.”
Key neutrino measurement
The Daya Bay measurement follows an intense few months of results from teams around the world chasing the same goal. Last June, the T2K experiment in Japan measured the parameter using a different technique, and this was closely followed by a similar announcement from MINOS at Fermilab in Chicago. In November, France-based experiment Double-Chooz, who had previously limited the possible value of θ13 to less than 13 degrees, published an updated measurement.
But none of these experiments was able to produce a statistically significant result. The evidence in support of the new Daya Bay result suggests that there is only a one-in-several-million chance of it having been caused by a statistical anomaly.
“It’s impressive how quickly their result was obtained,” said physicist Matt Toups of Columbia University, who works on the competing Double Chooz experiment, adding: “For the larger community, it’s fantastic … it means that CP violation can be searched for on a 10 to 15 year timescale, with technology we already know can work.”