  The charge radius of the proton has been measured in scattering and
spectroscopy experiments using both electronic and muonic probes. The
electronic and muonic measurements are discrepant at $5\sigma$, giving rise to
what is known as the proton radius puzzle.
  With the goal of resolving this, we introduce a novel method of using lattice
QCD to determine the isovector charge radius -- defined as the slope of the
electric form factor at zero four-momentum transfer -- by introducing a mass
splitting between the up and down quarks. This allows us to access timelike
four-momentum transfers as well as spacelike ones, leading to potentially
higher accuracy in determining the form factor slope at $Q^2 = 0$ by
interpolation. In this preliminary study, we find a Dirac isovector radius
squared of $0.320 \pm 0.074$ fm$^2$ at quark masses corresponding to $m_\pi =
450$ MeV. We compare the feasibility of this method with other approaches of
determining the proton charge radius from lattice QCD.
