October 1, 2013

Theoretical Physicists Still Unraveling Big Bang

Tiffany Trader

It’s one of the most essential questions, which speaks to the very fact of our existence: why is the universe made of matter?

Researchers at Brookhaven National Laboratory are attempting to determine why the early universe ended up with an excess of matter. Without that excess, the matter and antimatter created by the Big Bang would have cancelled each other out, leaving the universe devoid of matter. Imagine a world that contains nothing but light, no planets, no stars, no people. Theoretical physicists have long suspected there was a way to solve for this imbalance, and by doing so, shed light on our very existence. They’ve spent the last 50 years attempting to unravel this fundamental riddle.

“The fact that we have a universe made of matter strongly suggests that there is some violation of symmetry,” said Taku Izubuchi, a theoretical physicist at the US Department of Energy’s (DOE) Brookhaven National Laboratory.

This asymmetry is called charge conjugation-parity (CP) violation. It occurs when “certain subatomic interactions happen differently if viewed in a mirror (violating parity) or when particles and their oppositely charged antiparticles swap each other (violating charge conjugation symmetry).”

Scientists at Brookhaven discovered evidence of this symmetry “switch-up” in experiments conducted in 1964 at the Alternating Gradient Synchrotron, with additional evidence coming from experiments at CERN.

The work led to a Nobel Prize for the researchers, who were able to observe the decay of a subatomic particle, known as a kaon, into two other particles called pions. These particles are further composed of quarks. But that’s as far as the research went: understanding kaon decay in terms of its quark composition was the next horizon.

The next step was for theoretical physicists to develop a theory to explain this kaon decay process – a mathematical description that could calculate how frequently it happens and whether it would help explain the fundamental matter imbalance in the universe. “Our results will serve as a tough test for our current understanding of particle physics,” Izubuchi said.

The work belongs to a field called Quantum Chromodynamics, or QCD – which comprises a multitude of variables and possible values for those variables. The necessary computational tools only recently became sophisticated enough to handle such advanced calculations. Currently theoretical physicists are performing kaon calculations using the QCDOC supercomputer at Brookhaven.

Still even with the best-in-class supercomputers, the problem would have taken many years if not for a new efficient algorithm developed by the Brookhaven group in late 2012. “The algorithm…divides the whole calculation into a ‘difficult’ but small piece and an ‘easier’ large piece, and devotes more computation time to the latter part to save the total computation required,” explains Izubuchi.

“It accelerates the speed of the computations by a factor of ten or more. This very simple idea of dividing the calculation into two pieces actually helped to reduce the statistical error of the computation by a lot,” he adds.

So did the theorists achieve their long-sought answer? It’s a matter of yes and no. The calculated strength of the weak interaction only partially accounts for the matter antimatter asymmetry after the Big Bang, according to Izubuchi.

He adds: “We cannot explain why the universe is matter-rich based solely on the amount of CP violation that this kaon decay accounts for. So there may be other sources of CP violation other than the weak interaction that would be revealed if a discrepancy were found between our calculation and the experimental results.”

This research is part of DOE’s Scientific Discovery through Advanced Computing program “Searching for Physics Beyond the Standard Model: Strongly-Coupled Field Theories at the Intensity and Energy Frontiers,” supported by the DOE Office of Science. The project relied on a number of Blue Gene/Q systems in labs around the world as well as PC cluster machines at Fermi National Accelerator Laboratory and at RIKEN.

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