Carbon monoxide ( CO ) absorption in the sub-damped Lyman- \alpha absorber at redshift z _ { \textrm { \scriptsize abs } } \simeq 2.69 , toward the background quasar SDSS J123714.60+064759.5 ( J1237+0647 ) , was investigated for the first time in order to search for a possible variation of the proton-to-electron mass ratio , \mu , over a cosmological time-scale .
The observations were performed with the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of 40 per 2.5 km s ^ { -1 } per pixel at \sim 5000 Å. Thirteen CO vibrational bands in this absorber are detected : the A ^ { 1 } \Pi - X ^ { 1 } \Sigma ^ { + } ( \nu ^ { \prime } ,0 ) for \nu ^ { \prime } = 0 - 8 , B ^ { 1 } \Sigma ^ { + } - X ^ { 1 } \Sigma ^ { + } ( 0,0 ) , C ^ { 1 } \Sigma ^ { + } - X ^ { 1 } \Sigma ^ { + } ( 0,0 ) , and E ^ { 1 } \Pi - X ^ { 1 } \Sigma ^ { + } ( 0,0 ) singlet-singlet bands and the d ^ { 3 } \Delta - X ^ { 1 } \Sigma ^ { + } ( 5,0 ) singlet-triplet band .
An updated database including the most precise molecular inputs needed for a \mu -variation analysis is presented for rotational levels J = 0 - 5 , consisting of transition wavelengths , oscillator strengths , natural lifetime damping parameters , and sensitivity coefficients to a variation of the proton-to-electron mass ratio .
A comprehensive fitting method was used to fit all the CO bands at once and an independent constraint of \Delta \mu / \mu = ( 0.7 \pm 1.6 _ { stat } \pm 0.5 _ { syst } ) \times 10 ^ { -5 } was derived from CO only .
A combined analysis using both molecular hydrogen and CO in the same J1237+0647 absorber returned a final constraint on the relative variation of \Delta \mu / \mu = ( -5.6 \pm 5.6 _ { stat } \pm 3.1 _ { syst } ) \times 10 ^ { -6 } , which is consistent with no variation over a look-back time of \sim 11.4 Gyrs .