THE CO A-X SYSTEM FOR CONSTRAINING COSMOLOGICAL DRIFT OF THE PROTON-ELECTRON MASS RATIO

Abstract

The $\textrm{A}^1\Pi-\textrm{X}^1\Sigma^+$ band system of carbon monoxide, which has been detected in six highly redshifted galaxies ($z=1.6-2.7$), is identified as a novel probe method to search for possible variations of the proton-electron mass ratio ($\mu$) on cosmological time scales. Laboratory wavelengths of the spectral lines of the A-X ($v$,0) bands for $v=0-9$ have been determined at an accuracy of $\Delta\lambda/\lambda=1.5 \times 10^{-7}$ through VUV Fourier-transform absorption spectroscopy, providing a comprehensive and accurate zero-redshift data set. Two-photon Doppler-free laser spectroscopy has been applied for the (0,0) and (1,0) bands, achieving $3 \times 10^{-8}$ accuracy level, verifying the absorption data. Accurate sensitivity coefficients $K_{\mu}$ for a varying $\mu$ have been calculated for the CO A-X bands, so that an operational method results to search for $\mu$-variation. The data from both experiments were used to perform an improved analysis of the perturbations in the A$^1\Pi$, $v=0$ and 1 levels by vibrational levels in the D$^1\Delta$, I$^1\Sigma^-$, e$^3\Sigma^-$, d$^3\Delta$, and a$'^3\Sigma^+$ states. The object Q1237+064 will be observed in May 2013, aiming for a good signal-to-noise spectrum containing the CO A-X bands as well as the H$_2$ Lyman and Werner bands. This should result in an accurate and robust constraint on $\Delta\mu/\mu$.