Isotope Shift Spectroscopy in Mercury Vapors: A Promising Alternative to Ytterbium for New Physics Search

Isotope shift metrology in the deep-UV region has been performed for all bosonic isotopes of mercury with a zero nuclear spin, using the technique of frequency-comb referenced, wavelength-modulated, saturated absorption spectroscopy. The absolute center frequencies of the 6s^{2} ^{1}S_{0}→6s6p ^{3}P_{1} transition have been measured with precision in the range of 2.5-5.9 10^{-12} in temperature-stabilized Hg vapor samples with natural abundances. Frequency shifts in four isotope pairs have been determined with unprecedented accuracy, the global uncertainty being improved by a factor greater than 20 with respect to the best experimental data of the past literature. Our dataset, when combined with previous measurements on the 6s6p ^{3}P_{2}→6s7s ^{3}S_{1} transition at 546 nm, allows us to build a King plot that reveals a nonlinearity with a statistical significance of 4.6σ.