Quark-connected (left) and disconnected (right) diagrams contributing to the muon anomalous magnetic moment in QCD at O(α2) through the hadronic vacuum polarization. On the left, all QED vertices lie on a single quark loop while on the right the two quark loops are connected only by gluons.
To resolve the difference between lattice and data-driven theory values and compute the total, additional lattice calculations are needed. This team set to attain this goal through two independent groups, Fermilab/MILC and RBC/UKQCD.
The muon is an elementary particle identical to the ordinary electron except that it is about 200 times heavier. Its magnetic dipole moment is being measured at Fermilab and calculated by theorists world-wide to fantastic accuracy in a high-stakes test of the Standard Model (SM) of Particle Physics. To test the SM to a degree that allows discovery of physics beyond our current understanding of Nature’s laws, the theory errors on the contributions to the magnetic moment from the cloud of virtual quarks, anti-quarks, and gluons surrounding the muon during its brief lifetime must be reduced. These so-called hadronic contributions will be determined with improved precision in numerical simulations of Quantum Chromodynamics (QCD) known as latice QCD.
The theoretical calculation and measurement of the magnetic moment of the muon comprise one of the highest priorities of the DOE’s Office of High Energy Physics. Latice QCD calculations from many groups, using different formulations and methods, agree with each other but differ with longer-standing data-driven calculations, which calls into question the later’s disagreement with the SM. To resolve the difference between latice and data-driven theory values, the project aims to compute the hadronic contributions at the sub-percent level, and ultimately to reach the expected precision of the experiment, about one permille.