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Where is all the antimatter?

The interior of the Daya Bay neutrino experiment

Interior of the Daya Bay neutrino experiment. The ‘bumps’ lining the wall are sensitive detectors that pick up the flash of light when an anti-neutrino is found.

A PRECISE MEASUREMENT of elusive, nearly massless particles, has provided a crucial hint as to why the universe is dominated by matter, and not by its close relative, anti-matter.

The particles, called anti-neutrinos, were detected at the underground Daya Bay experiment, located near a nuclear reactor in China, 55 kilometres north of Hong Kong.

For the measurement, made in 2012, the Daya Bay collaboration has been named runner-up for breakthrough of the year from Science magazine.

Anti-particles are almost identical twins of sub-atomic particles (electrons, protons and neutrons) that make up our world. When an electron encounters an anti-electron, for example, both are annihilated in a burst of energy. Failure to see these bursts in the universe tells physicists that anti-matter is vanishingly rare, and that matter rules the roost in today’s universe.

Neutrino – the last hope?

“At the beginning of time, in the Big Bang, a soup of particles and anti-particles was created, but somehow an imbalance came about,” says Karsten Heeger, a professor of physics at the University of Wisconsin-Madison (UW-Madison). “All the studies that have been done have not found enough difference between particles and anti-particles to explain the dominance of matter over anti-matter.”

But the neutrino, an extremely abundant but almost massless particle, may have the right properties, and may even be its own anti-particle, Heeger says. “And that’s why physicists have put their last hope on the neutrino to explain the absence of anti-matter in the universe.”

Daya Bay pool holding four anti-neutrino detectors

A pool holding four anti-neutrino detectors begins filling with ultra-pure water in September, 2012 at the Daya Bay Neutrino experiment. The experiment is helping to explain why the universe contains virtually no anti-matter.

A fertile source

Reactors, Heeger says, are a fertile source of anti-neutrinos, and measuring how neutrinos change during their short flights from the reactor to the detector, gives a basis for calculating a quantity called the “mixing angle,” the probability of transformation from one flavour into another.

The measurement of the Daya Bay experiment, released in March 2012, even before the last set of detectors was installed, showed a surprisingly large angle, Heeger says. “People thought the angle might be really tiny, so we built an experiment that was 10 times as sensitive as we ended up needing.

As expected, Science‘s breakthrough of the year was the detection of the Higgs boson, an elusive sub-atomic particle that completes the “particle zoo” predicted by the standard model of physics.

Adapted from information issued by University of Wisconsin-Madison. Daya Bay photo courtesy Roy Kaltschmidt, LBNL. Detector image by Roy Kaltschmidt, Berkeley Lab Public Affairs).

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