Knowledge Bank

The $A1\Pi -X1\Sigma +$ 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$\mathrm{<mrow>\prime 3</mrow>}\Sigma +$ 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 $\Delta \mu /\mu$.