The Standard Model (SM) of Elementary Particles has been extremely successful in correctly predicting and describing the properties of elementary particles studied at experimental facilities all around the world. The SM itself covers particles like quarks and electrons, for instance, and their anti-particles. While both particles and anti-particles behave largely in the same way, the SM allows for tiny differences in their behaviour.

This is however extremely important since it is an essential ingredient for being able to describe the abundance of matter over anti-matter observed in our universe. One of the big questions in modern particle physics is therefore, whether the SM correctly describes the observed symmetry breaking, or whether new, not yet known and undiscovered physics is driving it.

DiRAC resources have now enabled a detailed study of the symmetry breaking by considering the decay of kaons into two pions, as studied experimentally at CERN and Fermilab two decades ago. Both particles are mesons, i.e. they consist of a quark and an anti-quark, respectively. Researchers from Brookhaven National Laboratory, CERN, Columbia University, the University of Connecticut, the University of Edinburgh, the Massachusetts Institute of Technology, the University of Regensburg and the University of Southampton (RBC/UKQCD Collaboration) set out to compute the amount of symmetry violation present in the SM by means of large-scale computer simulations of Lattice Quantum Chromodynamics (QCD). QCD is the part of the SM that describes the strong interactions of quarks (and anti-quarks) via the exchange of force carriers called gluons. The group of researchers have developed the required theoretical understanding and the numerical and algorithmic techniques needed to complete their ambitious and state-of-the-art program.

In Lattice Quantum Chromodynamics one performs the calculation by constructing a discrete four-dimensional space-time grid (the lattice) on which one numerically solves the QCD equations of motion. Such lattice QCD simulations are the only known method to address the above questions without having to rely on ad hoc assumptions. This type of computation crucially relies on access to the world’s fastest parallel supercomputers like DiRAC’s Extreme Scaling system.

To date RBC/UKQCD’s prediction is the only one of its kind and therefore highly anticipated. At the level of precision achieved in their calculation the result agrees with the experimental measurement. On the one hand this result is yet another piece of evidence for the unparalleled success of the SM as the theory of elementary particle physics. On the other hand, we expect the SM to be incomplete in describing all aspects of particle physics and CP violation remains an excellent place to look for limitations of the SM. The Collaboration is therefore now working on ways to improve the precision of their result.

**Bibliography:**

- “Direct CP violation and the ΔI=1/2 rule in K→ππ decay from the Standard Model”, arXiv: 2004:09440
- “Standard Model Prediction for Direct CP Violation in K → ππ Decay”
*Phys.Rev.Lett.*115 (2015) 21, 212001, arXiv:1505.07863