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What holds the universe together?

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Australia’s most powerful supercomputer, Vayu, at the NCI National Facility is engaged in an epic task of discovery, helping to define how elementary particles bind together to form our universe.

“This is about determining the fundamental laws of Nature by advancing our knowledge of the subatomic structure of the universe,” says Adelaide University’s Professor Derek Leinweber, whose team at the ARC Special Research Centre for the Subatomic Structure of Matter (CSSM) are using Vayu to probe the intricacies of quantum chromodynamics (QCD), the theory behind the so-called ‘strong force’ that binds the elemental building blocks of our universe together.

On a larger scale, this is the ‘nuclear’ force that binds protons and neutrons together to form the nucleus of an atom – and provides the basis for nuclear energy. On the much smaller, quantum scale being investigated by the CSSM team, it is the force that causes fundamental particles such as quarks and gluons – the smallest sort thought to exist – to form protons, neutrons and other more exotic particles.

“Quantum chromodynamics is the key fundamental quantum field theory of the Standard Model of the Universe,” Professor Leinweber says. “It describes how quarks and gluons interact to give rise to the mass of particles such as the proton and neutron – and so form ourselves and most of the visible world around us.”

The Strong Force is the most powerful of the four primary forces that govern the physics of the universe – 100 times stronger than the electromagnetic force and 1038 (100 billion, billion, billion, billion) times stronger than gravity. QCD is a fundamental mathematical model describing the innumerable ways the six different kinds of quarks attract one another, a process mediated by particles known as gluons, which come in eight varieties. Both types of fundamental particles carry special charges known as ‘colour’ (hence the term ‘chromodynamics’) which are at the origin of the strong force.

“We simulate the vast number of possible interactions on a space-time lattice using Vayu. This provides the only first-principles approach for revealing the properties of QCD and the manner in which it constructs the world around us,” he explains. “As the interactions are complex, we use scientific data visualisation techniques to understand the nature of these interactions.”  The lattice is a four dimensional grid used to describe the behaviour of quantum particles in space and time. The need for a very fine lattice spacing combined with a large physical volume means that the computational demands are immense.

“Lattice QCD is world-renowned for its colossal appetite for supercomputer power and as our project is among the most internationally competitive in this field, we need a great deal of computer power,” Professor Leinweber says. “This year we will use over 1.6 million CPU hours to create 130 Terabytes (130 million megabytes) of data describing how quarks propagate as they interact with other dynamical quarks.  This lays the foundation for determining the structure of matter in our universe.”

The work involves taking fundamental lattice-QCD theory that has been hammered out over recent years and then carrying out first-principles calculations to see what the theory predicts about particle behaviour.  The results are compared with experimental findings from international high energy physics facilities and create new knowledge on the essential mechanisms of quantum theory.

“This investigation merges new advances in lattice simulations of QCD, which provide access to the elusive light quark-mass regime, with proven techniques for revealing the electromagnetic structure of matter and the essence of QCD vacuum structure,” he says.

Already it has yielded world-first predictions about the internal structure of protons and the role of strange quarks within the proton.

“Supercomputing at a peak facility like the National Computational Infrastructure is all about enabling the unthinkable, making the impossible possible, doing now what otherwise cannot be done,” Professor Leinweber says.

“This is the final frontier, for us. Using the world-competitive supercomputer resources of NCI, we will reveal the manner in which quarks and gluons construct our universe.”

The work has important spinoffs for society and the economy.  Advanced medical scanners and therapies employ principles from quantum physics to detect and treat disease, while the computational skills it builds have valuable applications ranging from mining and information technology to capital markets and industrial scientific research.

“This research will maintain excellence and strength in an area where Australia has built an outstanding international reputation over the past decade. It will place Australia at the cutting edge of fundamental and computational science research and it will maintain and grow strong international links. It will produce Australian graduates and researchers who have worked in state-of-the-art science and high-performance computation,” Professor Leinweber says

“It will provide another piece in the age-old puzzle of how Nature really works.”

 

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