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Final Activity Report Summary - LATSTRONGCP (Lattice calculation of the electric dipole moment of the neutron)

The strong force of nature controls the physics of particles like the proton and neutron that make up the atomic nucleus. It also controls the physics of a myriad of other particles, generically called hadrons, produced in high energy collisions at particle accelerators like CERN. The strong force acts at a fundamental level between the constituents of the proton and neutron and the other hadrons, called quarks. Quarks, however, can never be seen as free particles, so all the experimental information we can get comes from studies of hadrons in particle detectors. Understanding how the strong force works then requires matching theoretical results for the physics of hadrons with experiment and this requires very good control of the theoretical calculations.

The theory of the strong force is called Quantum chromodynamics (QCD) and is very hard to solve. The only practical way to calculate the properties of hadrons from it is by a numerical method called lattice QCD. This method relies on splitting space-time up into a grid of points (a lattice) and then solving the equations of QCD on this grid. As well as quarks, the equations of QCD also contain particles called gluons that transmit the strong force between the quarks. For an accurate solution of QCD, we need to handle accurately the equations for the quarks and for the gluons and for the interactions between them. In recent years, it has become possible to do this much more accurately than in the past because we now have the computer power to handle the so-called sea quarks, generated by energy fluctuations in the vacuum. These produce 10 % or so effects in the physics of hadrons and so accurate calculations must include them. The most costly part of a lattice QCD calculation is then generating configurations of gluon fields on the space-time lattice that include the effect of sea quarks.

There are several different ways to handle the quarks and gluons. This project has concentrated on one of the most theoretically attractive but numerically expensive methods. It has turned out to be extremely challenging even with the computer power of a specially designed computer on which the fellow is an expert. She has concentrated on developing methods and code required for the calculation of quantities, such as decay constants, that can be compared to experiment. She has also transferred her expertise on the computer to younger members of the collaboration in the United Kingdom. Results are now starting to appear on the masses and properties of various hadrons. A key result, to appear shortly, is the mixing rate for neutral K mesons, which is very important for understanding the self-consistency of the Standard Model of particle physics with respect to violations of matter-antimatter symmetry.

An important output has been that the gluon field configurations generated in this project have been made publicly available. Other members of the community can do their own physics analyses on them, without requiring the very large amounts of computer time needed to generate them.

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