We present precision spectroscopy and isotope shift measurement of the ${{}^{1}S}_0\leftrightarrow {{}^{3}P}_2$ clock transition in neutral ytterbium (Yb) atoms. By revealing a magic wavelength at 905.4(2) nm, we successfully achieve the atomic spectrum narrower than 100 Hz. The interleaved clock operation between isotopes allows us to determine isotope shifts of four bosonic isotope pairs at Hz-level uncertainties, which is combined with those of other four ultranarrow transitions in Yb and ${\mathrm{Yb}}^{+}$ to construct the King plot. We show that a rich dataset spanning multiple transitions enables a test of a restricted isotope-shift ansatz comprising the leading mass and field shifts, one effective factorized higher-order standard model contribution, and a new-particle contribution. Within this ansatz, the five-transition data are inconsistent with the new-particle contribution serving as the sole additional contribution over the mass ranges $m_{\varphi }\le 6.5\mathrm{keV}$ and $18\mathrm{keV}\le m_{\varphi }<100\mathrm{keV}$, while the previous works rely on another bound set by the neutron scattering and $(g-2{)}_e$ measurements. This Letter paves the way for the effective use of precision isotope shift data in the King plot analysis and stimulates further measurements in Yb and other elements.

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• Atomic, optical & lattice clocks
• Cold gases in optical lattices
• Hypothetical particle physics models
• Isotope shift
• Atoms
• Hypothetical particles
• Precision measurements
• Spectroscopy