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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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South Pole telescope nears completion

IceCube at the South Pole

An overview of the South Pole, with the US Amundsen-Scott South Pole Station to the left of the runway and the IceCube facility to the right. Photo by Forest Banks

  • IceCube telescope aims to detect neutrinos
  • Network of under-ice detectors, 1 cubic km in volume
  • International effort; due for completion this month

A unique kind of telescope is about to be completed, buried deep beneath the ice under the US Amundsen-Scott South Pole Station.

Called the IceCube Neutrino Observatory, it records the rare collisions of neutrinos, elusive sub-atomic particles, with the atomic nuclei of the water frozen into ice.

Neutrinos come from the Sun, from cosmic rays interacting with the Earth’s atmosphere, and from dramatic astronomical sources such as exploding stars in the Milky Way and other distant galaxies.

Trillions of neutrinos stream through the human body at any given moment, but they rarely interact with regular matter, and researchers want to know more about them and where they come from.

Diagram of IceCube

What IceCube looks like under the ice—strings of special detectors in an array measuring one cubic kilometre in volume. Courtesy IceCube.

IceCube is the world’s largest neutrino detector, measuring a cubic kilometre in volume. The size of the detector is important because it increases the number of potential collisions that can be observed, making neutrino astrophysics a reality. The observatory is slated for completion in December 2010.

Astronomy under the ice

Since 2004, the USA, Belgium, Germany and Sweden have been building the detector in the continental ice sheet that covers Antarctica to a depth of almost three kilometres in places.

A powerful hot-water drill creates holes almost 2.5 kilometres deep into the ice. These holes house strings of digital optical modules that detect the interactions of the neutrinos with the ice.

Seven holes remained to be drilled in December 2010, which will bring the total to 86 strings.

Even now, the IceCube detector records several tens of thousands of neutrino interactions every year. The detector records one terabyte of data (more than 1,000 gigabytes) every day, and over a petabyte of data (quadrillion bytes) per year. Data is meticulously examined for evidence of neutrino events.

International collaboration

While the Observatory is managed by the University of Wisconsin-Madison and primarily funded by the US National Science Foundation, Germany, Belgium and Sweden contributed to its construction.

Jessica Hodges with an IceCube digital optical module

Jessica Hodges, IceCube physics graduate student, pictured with one of the optical detector modules. Photo by Glenn Grant / National Science Foundation.

More than 250 scientists from 36 institutions in the USA, the partner countries, and elsewhere are now analysing the data collected by the observatory.

“The IceCube detector is a superb example of the kind of exciting ‘big science’ at the frontiers of knowledge that is ideally suited for support by the U.S. Antarctic Program, precisely because it could be built nowhere else in the world but in the Antarctic ice sheet,” said Karl A. Erb, director of NSF’s Office of Polar Programs (OPP).

Through OPP, NSF manages the US Antarctic Program, which coordinates all U.S. research on the southernmost continent and surrounding oceans.

“What’s more,” he added, “although the IceCube project is primarily funded by the National Science Foundation, it exemplifies a modern trend in the increasingly complex and multi-disciplinary scientific world; large-scale projects like the IceCube detector are too complex to be effectively mounted by one nation alone, but also require the scientific and logistical expertise of many nations acting together to produce scientifically significant results.”

Adapted from information issued by NSF / University of Wisconsin-Madison.

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