The charm-quark contribution to light-by-light scattering in the muon from lattice QCD

Area of Science:

• Theoretical Physics
• Quantum Chromodynamics (QCD)

Background:

• The muon anomalous magnetic moment (g-2) is a key observable in particle physics.
• Hadronic light-by-light scattering is a significant theoretical uncertainty in the muon g-2 calculation.
• The charm quark contribution to this process is not precisely known.

Purpose of the Study:

• To compute the hadronic light-by-light scattering contribution from the charm quark to the muon anomalous magnetic moment.
• To provide a non-perturbative, lattice QCD-based determination of this contribution.
• To reduce theoretical uncertainties in the Standard Model prediction of the muon g-2.

Main Methods:

• Lattice Quantum Chromodynamics (LQCD) simulations.
• Ensembles with dynamical up, down, and strange quarks at the SU(3) symmetric point.
• Combined charm-mass and continuum extrapolation, utilizing lighter-than-physical charm masses and fine lattice spacings.

Main Results:

• A final value for the charm quark contribution to the muon g-2 was obtained.
• Cutoff effects were found to be sizeable, necessitating specific extrapolation techniques.
• The mixed charm-light disconnected contraction contributed a small negative amount.

Conclusions:

• The computed value is consistent with estimates from simpler charm quark loop models.
• The result is free from perturbative scheme dependence on the charm mass.
• This lattice QCD calculation offers a robust determination of the charm contribution to the muon g-2.