We report a precise measurement of the isotope shifts in the $4{}^{2}S_{1/2}\to 3{}^{2}D_{3/2}$ electric quadrupole transition at 732 nm in ${}^{42,44,48}\mathrm{Ca}^{+}$ relative to ${}^{40}\mathrm{Ca}^{+}$ via high-resolution laser spectroscopy of cotrapped ions, finding measured shifts of $2775392374.8\left(6.0\right), 5347679835.1\left(5.9\right)$, and $10003129115.1\left(5.7\right)$ Hz between ${}^{42,44,48}\mathrm{Ca}^{+}$ and ${}^{40}\mathrm{Ca}^{+}$, respectively. When combined with prior measurements on the $4{}^{2}S_{1/2}\to 3{}^2{D}_{5/2}$ transition [F. W. Knollmann et al., Phys. Rev. A 100, 022514 (2019)], a King plot analysis shows the data to be consistent with linearity below the level of parts per billion. This observed linearity, which is free of nuclear systematics, improves the previous isotope-shift-based limits of ${\mathrm{Ca}}^{+}$ for couplings of a beyond the standard model scalar boson to electrons and neutrons by a factor of 3. Our new limit excludes part of the coupling range remaining for a new physics interpretation after accounting for one higher-order nuclear term in the nonlinear King plot of Yb/${\mathrm{Yb}}^{+}$.

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• Electron correlation calculations for atoms & ions
• Electronic structure of atoms & molecules
• Trapped ions
• Optical spectroscopy