Keck telescope constraint on cosmological variation of the
  proton-to-electron mass ratio
  Molecular transitions recently discovered at redshift z_abs=2.059 toward the
bright background quasar J2123-0050 are analysed to limit cosmological
variation in the proton-to-electron mass ratio, mu=m_p/m_e. Observed with the
Keck telescope, the optical echelle spectrum has the highest resolving power
and largest number (86) of H_2 transitions in such analyses so far. Also,
(seven) HD transitions are used for the first time to constrain mu-variation.
These factors, and an analysis employing the fewest possible free parameters,
strongly constrain mu's relative deviation from the current laboratory value:
dmu/mu =(+5.6+/-5.5_stat+/-2.9_sys)x10^{-6}, indicating an insignificantly
larger mu in the absorber. This is the first Keck result to complement recent
null constraints from three systems at z_abs>2.5 observed with the Very Large
Telescope. The main possible systematic errors stem from wavelength calibration
uncertainties. In particular, distortions in the wavelength solution on echelle
order scales are estimated to contribute approximately half the total
systematic error component, but our estimate is model dependent and may
therefore under or overestimate the real effect, if present.
  To assist future mu-variation analyses of this kind, and other astrophysical
studies of H_2 in general, we provide a compilation of the most precise
laboratory wavelengths and calculated parameters important for absorption-line
work with H_2 transitions redwards of the hydrogen Lyman limit.
