New calculation refines comparison of matter with antimatter

An international collaboration of theoretical physicists, including Professor Chris Sachrajda from the University of Southampton, has published a new calculation that bolsters efforts to explain of the predominance of matter over antimatter in the universe.

The collaboration, known as RBC-UKQCD, includes scientists from the Brookhaven National Laboratory, CERN (the European particle physics laboratory), Columbia University, the University of Connecticut, the University of Edinburgh, the Massachusetts Institute of Technology, the University of Regensburg as well as Southampton.

Their results, published this month in the journal Physical Review D, have been highlighted as an Editor's Suggestion.

Professor Sachrajda, of the Southampton High Energy Physics theory group, explains: "Our current understanding is that the universe was created with almost equal amounts of matter and antimatter so that a difference between their interactions is required to have tipped the balance to favour matter over antimatter.

"Such tiny differences, called 'CP-violation', are a natural feature of the Standard Model of Particle Physics, and the aim of this project was to compute their effect in the decays of subatomic particles called kaons into two pions. Understanding these decays quantitatively and comparing the predictions with the experimental measurements made two decades ago at CERN and the Fermi National Accelerator Laboratory gives scientists a way to test our understanding of 'direct CP-violation'."

The results proved to be consistent with experimental measurements, thus significantly constraining models of 'new physics' attempting to explain phenomena such as dark matter which require physics beyond the standard model.

Professor Norman Christ, of Columbia University, says: "Any differences in matter and antimatter that have been observed to date are far too weak to explain the predominance of matter found in our current universe. Finding a significant discrepancy between an experimental observation and predictions based on the Standard Model would potentially point the way to new mechanisms of particle interactions that lie beyond our current understanding and which we hope to find to help to explain this imbalance."

To this end, following this pioneering work, the focus in the next two-to-three years will be to significantly improve the precision of the computations with the potential of revealing any sources of matter/antimatter asymmetry lying beyond the current theory's description of our world.

The final result depends on calculations performed in 2012 and 2015 which constituted the theses of Southampton PhD students Dr Elaine Goode and Dr Tadeusz Janowski.

The two pions into which the kaon decays can be in either of two channels; in one the interactions between the two pions are attractive and in the other repulsive. The earlier calculations were of the decays into the repulsive channel and the 2012 paper won the Kenneth Wilson Lattice Award for 'excellence in Lattice Field Theory'. The present calculation studies the decays into the attractive channel which required very significant additional theoretical and computational developments.

The difficulty of the calculation is due to the quarks, the constituents of the kaon and pions, interacting through the strong nuclear force. These strong interactions cannot be computed analytically and so to conquer the challenge the theorists used lattice Quantum Chromodynamics. This technique 'places' the particles on a space-time lattice of three spatial dimensions plus time.

Nevertheless, the computation required integrating 67 million degrees of freedom, performed using cutting-edge supercomputers, located in the USA, Japan as well as in the UK.