Cosmic sleuths on the case

It's not every day you get to help determine a fundamental constant of nature and a new phenomenon in particle physics. But an international team of scientists believes they have done just that, at an underground site near Daya Bay's nuclear plants.

A team of scientists from Hong Kong, the mainland and other countries - of which I am a member - has measured subtle changes in the behaviour of one of nature's most elusive particle: the neutrino.

This information is a key that will help researchers unlock one of the universe's great mysteries: why the universe is dominated by matter rather than balanced equally with antimatter.

Before you finish reading this sentence, hundreds of trillions of neutrinos produced in the centre of the sun will have penetrated your body without leaving any trace. Among the elementary particles - basic building blocks of all matter - neutrinos have always been the most mysterious.

They carry some mass and energy, but they barely interact at all; to a neutrino, you, I, the earth and even the sun are all transparent.

One of the Daya Bay Reactor neutrino experiment's detectors, with 20 tonnes of detecting materials, managed to 'catch' only about 700 neutrinos per day out of the vast flood - 10 to the power of 22 -that was passing through it from the reactors.

So neutrinos are truly ghost particles. But they must be there for a reason.

Many scientists are betting that these ghost particles hold the key to solving some of the most important outstanding problems in cosmology and particle physics. Perhaps the most intriguing one is why there is any matter left in the universe today, if the Big Bang Theory is correct.

According to the Big Bang theory, the universe began as a hot and dense fireball about 13.7 billion years ago, when space-time and energy-matter were all born together. The subsequent expansion of the universe has continued until today and has cooled and diluted the primordial fireball so much that the universe has become cold and dark.

Now we have only the cosmological microwave background - remnant radiation from the Big Bang - to remind us of the universe's violent birth.

The Big Bang theory has been extremely successful in many ways, explaining many mysteries of our universe. It has become the standard model of cosmology in the scientific community and with the public.

But there is a serious problem in the Big Bang theory. After explaining how antimatter got into the universe, it fails to tell us what happened to the stuff. Somehow, most if not all of the antimatter has disappeared.

We have known about antimatter for about 80 years. In theory, there is an antiparticle that corresponds to every kind of particle in existence: they possess identical mass but opposite electrical charges.

For example, the antiparticle of an electron is a positron, which has exactly the same mass but with the charges flipped.

When they meet, a particle and its antiparticle may annihilate each other, turning their masses into energy according to Einstein's famous formula E=mc2.

Symmetry is the key to particles and antiparticles. If the universe started off as pure energy that was converted into matter, then particles and antiparticles should be created in perfect symmetry. Every electron should be accompanied by a positron, and we should find as much antimatter as matter in the universe.

But astronomers have had to look hard to find any trace of antimatter anywhere in the universe. Everywhere, matter dominates. Why is there so much of the one and so little of the other?

Modern physics knows of no mechanism to pull apart matter from antimatter - in fact, they generally attract each other owing to their opposite charges. So we should be witnessing the continued annihilations that occur when matter and antimatter meet, even in our own Milky Way galaxy. They are readily observable by the bursts of gamma rays they emit.

So, what could have upset the universe's symmetry between matter and antimatter?

If we can understand the source of this asymmetry, it may show us what happened to all the antimatter. If matter and antimatter exist in different quantities in the universe, each must be governed by a different reaction rate.

For years, physicists have sought evidence of this. But first they had to learn how to recognise the evidence for asymmetry, and this brings us to the quest for answers in Daya Bay.

Physicists there were able to measure how often electron neutrinos transformed into other kinds, or 'flavours' of neutrinos - a process physicists call oscillation.

This oscillation is a face-changing trick. There are three basic neutrino types - electron, muon, and tau neutrinos - and a freely travelling neutrino continuously transform from one type to another.

Think of a rotating coin as an analogy. At some moments you see the head, at some others the tail, and in general you see some a combination of the two faces. Imagine a rotating three-faced coin and you have a good picture of neutrino oscillation.

An electron-type neutrino will oscillate into a muon-type neutrino, and so will the corresponding antineutrinos. If their rates differ, that's a direct indication of asymmetry - or CP violation as physicists call it.

The catch is that the oscillation depends on two parameters: one is known by the Greek letter delta, and the other was undetermined until about two weeks ago.

This is where the Daya Bay Reactor Neutrino Oscillation Experiment comes in. The experiment is well-positioned for a precise measurement of the second parameter because it is close to some of the world's most powerful nuclear reactors - the Daya Bay and Ling Ao nuclear power reactors, located some 55 kilometres from Hong Kong.

Further, the Daya Bay experiment used virtually identical massive detectors in three experimental halls deep under the adjacent mountains, which provide natural shielding from cosmic rays, to improve accuracy.

From last December to last month, the Daya Bay team observed tens of thousands of interactions of electron antineutrinos in its detectors. The missing parameter - which is called theta_13 - was the measure of how many electron neutrinos oscillated into other kinds of neutrinos.

The copious data allowed for measurement of theta_13 with unmatched precision. On March 8, the Daya Bay collaboration announced a surprisingly large value for theta_13, which corresponds to the disappearance of 6 per cent of electron antineutrinos on their journey between the reactors and far detectors.

The result marks the most important discovery in elementary particle physics from China so far. The parameter itself is important in its own right, providing clues to a deeper understanding of elementary particles.

Its large value also boosts the prospects for future experiments to measure neutrino CP violation, perhaps solving the mystery of why matter dominates in the universe.

Chu Ming-chung is professor of physics at Chinese University of Hong Kong and a leader of the Hong Kong team in the neutrino experiment at the Daya Bay nuclear power plants

13.7b

The number of years that have elapsed since the Big Bang, the awesome explosion that scientists think created the universe