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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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Matter vs antimatter

Nebulae in space

The Big Bang model suggests equal quantities of matter and antimatter should have formed, but the antimatter is nowhere to be seen out in space. A new experiment gives clues as to why matter has come to dominate.

  • Big Bang model suggests equal amounts of matter and antimatter
  • But only matter is seen widespread throughout the universe
  • New experiment gives clues as to why matter is dominant

A large collaboration of physicists working at the Fermilab Tevatron particle collider in the USA has found evidence for a possible explanation for the prevalence of matter over antimatter in the universe.

They found that smashing protons together in their experiment produced short-lived particles called “B mesons” that almost immediately broke down into debris that included slightly more matter than antimatter.

When particles of matter and antimatter collide, they annihilate each other and release energy. If there is slightly more matter to begin with, then all the antimatter will be annihilated, leaving a small excess of matter.

This sort of matter/antimatter asymmetry accounts for the fact that just about all the material in the universe is made of the normal matter we’re familiar with.

The Fermilab Tevatron

The Fermilab Tevatron (circular feature in background) is used to smash atomic particles together at high speed.

The Big Bang model suggests that equal amounts of matter and antimatter should have been formed, but the antimatter seems to have disappeared—everything in the universe seems to be made of normal matter. Why?

Physicists have long known about processes described by current physics theory that would produce tiny excesses of matter, but the amounts the theories predict are far smaller than necessary to create the universe we observe.

The Tevatron experiments suggest that we are on the verge of accounting for the quantities of matter that exist today.

But the truly exciting implication is that the experiment implies that there is new physics, beyond the widely accepted Standard Model, that must be at work.

If that’s the case, major scientific developments lie ahead.

The results emerge from a complicated and challenging analysis, and have yet to be confirmed by other experiments.

If the matter/antimatter imbalance holds up under the scrutiny of researchers at the Large Hadron Collider in Europe and competing research groups at Fermilab, it will likely stand as one of the most significant milestones in high-energy physics, according to Roy Briere of Carnegie Mellon University in Pittsburgh.

The results have been published in the journals Physical Review Letters and Physical Review D.

Adapted from information issued by APS / NASA / ESA / Orsola De Marco (Macquarie University).

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Where is the antimatter?

The Alpha Magnetic Spectrometer is a particle physics experiment module that is to be mounted on the International Space Station. It is designed to search for various types of unusual matter by measuring cosmic rays. Its experiments will help researchers study the formation of the Universe and search for evidence of dark matter and antimatter.

Final testing is being completed at ESA’s European Space Research and Technology Centre (ESTEC) facility in the Netherlands and delivery to the Kennedy Space Center in Florida is expected in early September 2010.

Launch is targeted for February 2011 on space shuttle Endeavour flight STS-134, the last flight in the shuttle programme.

Adapted from information issued by ESA / Wikipedia.

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