Hadronic-vacuum-polarization contribution to the muon's anomalous magnetic moment from four-flavor lattice QCD

Abstract

We calculate the contribution to the muon anomalous magnetic moment hadronic vacuum polarization from the connected diagrams of up and down quarks, omitting electromagnetism. We employ QCD gauge-field configurations with dynamical $u$, $d$, $s$, and $c$ quarks and the physical pion mass, and analyze five ensembles with lattice spacings ranging from a~0.06-0.15 fm. The up- and down-quark masses in our simulations have equal masses $m_l$. We obtain, in this world where all pions have the mass of the $\pi^0$, $10^{10} a_\mu^{ll}({\rm conn.}) = 630.1(8.3)$, in agreement with independent lattice-QCD calculations. We then combine this value with published lattice-QCD results for the connected contributions from strange, charm, and bottom quarks, and an estimate of the uncertainty due to the fact that our calculation does not include strong-isospin breaking, electromagnetism, or contributions from quark-disconnected diagrams. We obtain for the total order $(\alpha^2)$ hadronic-vacuum polarization to the muon's anomalous magnetic moment $10^{10} a_\mu^{\rm HVP,LO} = 691(8)_{u,d}(1)_{s,c,b} (13)_{\rm other}$, where the errors are from the light-quark connected contribution, heavy-flavor connected contributions, and omitted effects listed above, respectively. Our result agrees with both $ab-initio$ lattice-QCD calculations and phenomenological determinations from experimental $e^+e^-$-scattering data. It is 1.7$\sigma$ below the "no new physics" value of the hadronic-vacuum-polarization contribution inferred from combining the BNL E821 measurement of $a_\mu$ with theoretical calculations of the other contributions.