We have pursued the first ever Quantum Chromodynamics (QCD), or strong force, lattice simulations of quarks and gluons with continuum limit results from chiral lattice quarks at their physical masses. Only left handed quarks couple to the electroweak forces. Our chiral approach reproduces the handedness symmetry of continuum QCD and is particularly good for determining the rates of weak decay processes involving left handed quark operators. This preference for left handed over right handed shows that the weak forces surprisingly break reflection (or parity) symmetry. Our results confirm the strong force as the confining theory of hadrons (particles made from quarks). The mass spectrum and pion decay constant agree with experimental determinations at the fraction of a percent level, and allow determination of the Vus quark flavor mixing parameter of the standard model. These and the following achievements are crucial ingredients in interpreting the results from the large experimental facilities (e.g. at CERN).
Figure 1. The figure above left shows the fit to simulation data for the decay constant from arXiv: 1411.7017 and right the action density for one of the gauge fields generated on the DiRAC facility.
Quark Masses:
We have determined the up/down and strange quark masses – fundamental constants of nature – to a fraction of a percent.
Neutral Kaon oscillations:
In the Standard Model (SM) of particle physics, weak interactions allow quarks to change from one type or flavour to another in a highly constrained pattern. Quark flavour-changing processes with no accompanying change in electric charge are very rare and present a small background against which to search for new physics. The SM allows electrically-neutral kaons to oscillate into their own antiparticles in a way which breaks the combined symmetry of particle-antiparticle exchange (charge) and mirror reflection (parity), known as CP symmetry. CP violation is essential to enable the dominance of matter over antimatter in the Universe and was first discovered in the neutral kaon system. We have calculated the neutral Kaon oscillation amplitude BK to 0.3% accuracy.
Algorithms:
Vigorous activity by UKQCD has leveraged STFC investment in DiRAC with around a 200x speedup from code and algorithmic improvements:
- UKQCD scientists worked for five years with IBM research to help design the BlueGene/Q chip (being responsible for 8% of the chip area);
- UKQCD have developed new multi-grid algorithms that adaptively accelerate the calculation of quark propagators;
- A volume averaging technique developed by RBC-UKQCD, known as AMA, has lowered our statistical errors to around the 0.2% level.
