CUHK Passions and Pursuits

6 I t is through the examination of the smallest parts of the universe that we may come to understand its biggest secrets, according to the work of CUHK physicist Chu Ming-chung . Professor Chu has devoted his life to the study of neutrinos, some of the least understood subatomic particles on the planet. Thanks to ground-breaking work at the Daya Bay Nuclear Power Plant, he and his team discovered that neutrinos transform in a way that helps explain the evolution of the universe. Professor Chu is the principal investigator of the Hong Kong team involved in the Daya Bay Collaboration, which has established that neutrinos ‘oscillate’ or change form. There are three kinds of neutrino—electron, muon and tau—and his research proves that the electron form of neutrino can transform into both the muon and the tau forms. Neutrinos and their counterpart, anti-neutrinos, were some of the first particles emitted after the Big Bang, and are continually produced within stars. Anti-neutrinos are plentiful in nuclear reactions, hence the experiment is located near Daya Bay power plant’s six reactors. Measuring the anti-neutrinos also provides data about neutrinos, since scientists believe the two types of particle have near-identical physical traits. After correcting for distance, the Daya Bay study showed that 8.4% of the anti-neutrinos were vanishing unexpectedly by the time they reached the detectors, demonstrating that they had transformed. Anti-neutrinos travel almost at the speed of light and respond only weakly to gravity, given their tiny mass, and so are incredibly hard to detect. Through tens of thousands of calculations, the scientists established the precise amount of ‘leakage’ as the anti-neutrinos change. What’s more, the oscillation indicates that the three forms have different weights, establishing that neutrinos themselves have mass. The ‘mass squared’ discrepancy established by the Daya Bay experiment is expressed as Δm 2 13 , while the rate of oscillation is known as theta one- three, or θ 13 . Meanwhile, the evidence that neutrinos transform from one type to another helps indicate the likely original forms of particle and matter created by the Big Bang. ‘We have shown that the neutrinos are part of the same entity, and we have shown quantitatively how much one transforms into another,’ Professor Chu says. ‘Now we are chasing back in history to trace the original of the universe when everything was unified.’ It was only in recent years that scientists established that neutrinos had mass at all. So further investigation into the θ 13 rate of neutrino oscillation established at Daya Bay could go some way to addressing that mystery. Professor Chu believes that the discovery of the neutrino oscillation at Daya Bay provides a greater understanding of particular physics that will hone the Grand Unification Theory. That is the overarching theory that unifies all fundamental interactions and fundamentally explains all life. The theory is constantly challenged and adapted. Measuring the θ 13 discrepancy helps rule out versions that don’t allow for that oscillation. So the discovery of theta one-three is an incremental step towards the construction of that theory. ‘I’m optimistic,’ Chu says. ‘But we’re probably less than halfway there.’ c c A full size anti-neutrino detector model at the underground Daya Bay Far Experimental Hall