PI: Christine Davies
The muon, like its lighter cousin, the electron, has electric charge and spin and therefore a magnetic moment. The size of that moment and consequently its interaction with a magnetic field depends on the parameter,Β ππ , that relates its magnetic moment to its spin. ππ would take the value 2 in a world with classical B fields. In the real world, however, where the vacuum seethes with particle-antiparticle pairs that flicker in and out of existence, ππ receives a ~0.1% correction from the interaction of the muon with these βvirtual particlesβ. The anomalous magnetic moment of the muon,Β ππ, defined as half the difference of ππ from 2 is being measured to an accuracy of 2×10-10 (0.2ppm) by the Muon g-2 experiment at Fermilab, near Chicago. Theory calculations of ππ for comparison must include all possible interactions of the muon in the Standard Model. Those in which a photon radiated from the muon creates a quark-antiquark βbubbleβ contribute around 700 x 10-10. This leading-order hadronic vacuum polarisation (LOHVP) piece is critical to the Standard Model-experiment comparison. It can be calculated using lattice QCD or in a βdata-drivenβ approach (using e+e- cross-section data). Lattice QCD has recently become accurate enough to confront the data-driven values being used.Β
The LOHVP calculation requires an integral over all time, but we can split this into two at t1 and combine lattice QCD from t=0 to t=t1 with data-driven from t=t1 upwardsto t=infinity to test their compatibility. A t1-independent total should be obtained.Β Our recent results (arXiv:2410.23832) in Figure 2 show that previous data-driven values using the KNT19 dataset (in red) are not compatible with lattice QCD, but those using new data for π+π– β π+π– from CMD3 (in blue) are. The blue total (in agreement with a new value from BMW/DMZ in green) is then ~25×10-10 higher than the KNT19 purely data-driven value (leftmost red point).Β Β
This is one of several new lattice QCD results that point towards a larger LOHVP and one that gives a Standard ModelΒ value that agrees with the Muon g-2 experimental result (see review: arXiv:2503.03364) There is a lot more work still to be done to pin down this picture, but rather than hinting at new physics as had been thought, ππ may once again demonstrate the supremacy of the Standard Model!