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Why is the Universe Dominated by Matter, Not Anti-Matter? |

A collaboration with major participation by physicists at the University of Wisconsin-Madison has made a precise measurement of elusive, nearly massless particles, and obtained a crucial hint as to why the universe is dominated by matter, 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 kilometers north of Hong Kong. For the measurement of anti-neutrinos it 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.

“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 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.”

Heeger and his group at UW-Madison have been responsible for much of the design and development of the anti-neutrino detectors at Daya Bay. Jeff Cherwinka, from the university’s Physical Sciences Laboratory in Stoughton, Wis. is chief engineer of the experiment and has overseen much of the detector assembly and installation. The construction of the experiment was completed this fall and data-taking started in October using the full set of anti-neutrino detectors. continue reading

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