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Physicists Wonder Where the Antimatter Went

Tue, 01/22/2013 - 8:34am
Chinese Academy of Sciences

3 Anti-Neutrino detectors at Hall Three of the Daya Bay Reactor Neutrino Experiment in China. Image: Chinese Academy of SciencesStudying the tiny and almost massless fundamental particles called neutrinos might turn out to have huge consequences for our understanding of what the universe is made of. The universe as astronomers observe is almost entirely made of matter with very little hints of antimatter in it. Scientists studying neutrinos in China may be able to tell us the reason behind this.

"Physicists have put their last hope on the neutrino to explain the absence of antimatter in the universe," says Karsten Heeger a professor of physics at UW-Madison.

In an international collaboration between Chinese and American physicists, The Daya Bay Reactor Neutrino Experiment is being conducted near a nuclear reactor in China, 55 kilometers north of Hong Kong. The experiment was built to study the difference between neutrinos and anti-neutrinos.

Physicists have recently come up with a surprising result of how neutrino and anti-neutrinos may be different in a fundamental manner which results in a universe void of antimatter. Their result hints that at the Big Bang an equal amount of matter and antimatter were created yet afterwards antimatter has slowly disappeared.

"This is really remarkable,” says Wenlong Zhan, vice president of the Chinese Academy of Sciences and head of the Chinese Physical Society. “We hoped for a positive result when we decided to fund the project, but we never imagined it could come so quickly.”

The result of the experiment was recognized by Science magazine as the second biggest breakthrough of the past year. The first breakthrough of 2012 was the detection of the Higgs boson at the Large Hadron Collider.

Neutrinos: tiny but potent

Just like electrons, neutrinos are fundamental particles too. You can find neutrinos in every part of the universe since the Big Bang and till now. They are in your backyard, at the center of the earth, they are produced via nuclear reactions in stars like our sun, and when the bigger stars die in supernovae. As they have a tiny mass, they move at nearly the speed of light and pass through matter unnoticed. As you are reading this article, thousands of neutrinos pass though your body.

Unlike electrons and protons, however, neutrinos come with no electric charge. This is why they are called neutrinos as they seemed at first like small neutrons. The Italian physicist Wolfgang Pauli first predicted its existence in 1930, and then 26 years later it was detected during an experiment in 1956.

At the early universe, neutrinos were produced in great numbers and despite their small mass they contribute to the total mass of the universe and affect its evolution, and expansion rate. Down here on earth, neutrinos are produced as cosmic rays interacting with atoms in the atmosphere and also geologically in the decays of radioactive atoms like uranium and thorium.

Little antimatter around

For every known particle out there, there is a partner anti-particle. They have similar mass but differ in charge and a few other properties. For an electron, we have the anti-electron also known as a positron, and for a proton there is the anti-proton. The moment a particle meets its partner anti-particle, they both turn into energy in the form of photons.

Yet every direction we turn our telescopes, almost everything we see in the universe is made up of matter not antimatter. Why is there so little antimatter around? This is one question that has been troubling cosmologists for a while now.

Well, there are two possible explanations for this dilemma. Either this was how the universe started at the beginning: more particles than anti-particles.

Or it all started with equal amounts of both, but an imbalance was built up sometime along the way because antimatter has different reactions and most of it was turned into other particles or energy.

"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 Heeger. “All the studies that have been done have not found enough difference between particles and anti-particles to explain the dominance of matter over antimatter.”

It turns out that the mystery of the matter/antimatter imbalance might be resolved by the result of the Daya Bay experiment which basically tells us that anti-neutrinos don’t behave in the exact same way as neutrinos.

Neutrinos transform into neutrinos

There was a major problem with neutrinos that was only resolved in 2002. The measured number of neutrinos reaching us from the sun was very little compared to the expected number. It turns out that neutrinos are more dynamic than previously thought and that they change their state or “flavor” as they travel through space.

Physicists have known for quite a while now that neutrinos come in three “flavors” (the electron, muon, and tau neutrinos). The resolution of the solar neutrino problem was to realize that all neutrinos constantly transform themselves from one flavor to another. This phenomenon of neutrino flavor mixing or oscillation may hold the key to solving the question of why antimatter is so scarce around us.

To detect the extremely delicate process of neutrino oscillation, the Daba Bay Neutrino Experiment observed how many anti-neutrinos survive some kilometers away from a nuclear reactor. The rate of mixing is quantified by three mixing angles (one for each pair of flavors), and what the experiment showed is that one of these angles is a little less than 9 degrees.

 The measurement of the Daya Bay experiment, released in early 2012 and refined a couple of weeks ago shows a surprisingly large angle. “People thought the angle might be really tiny, so we built an experiment that was 10 times as sensitive as we ended up needing,” Heeger says.

This result will open up the door to understanding more about the way neutrinos oscillate between their three flavors and to hopefully explain why one of the mixing angles is that large, and what this difference between neutrinos and their anti-particles means for cosmology.

"The neutrino community has been waiting for a long time for this parameter [the mixing angle] which will be used for planning experiments for next decade and beyond,” says Heeger.

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