New limit on a varying proton-to-electron mass ratio from
  high-resolution optical quasar spectra
  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 spectrum has the highest resolving power and
largest number (86) of H_2 transitions in such analyses so far. Also, (7) 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+/-2.7_sys)x10^{-6}. This is the first Keck result to
complement recent constraints from three systems at z_abs>2.5 observed with the
Very Large Telescope.
