Probing the electron-to-proton mass ratio gradient in the Milky Way with
  class I methanol masers
  We estimate limits on non-universal coupling of hypothetical hidden fields to
standard matter by evaluating the fractional changes in the electron-to-proton
mass ratio, mu = m_e/m_p, based on observations of ClassI methanol masers
distributed in the Milky Way disk over the range of the galactocentric
distances 4 < R < 12 kpc. The velocity offsets DeltaV = V44 - V95 measured
between the 44 and 95 GHz methanol lines provide, so far, one of the most
stringent constraints on the spatial gradient k_mu = d(Delta mu/mu)/dR <
2x10^-9 kpc-1 and the upper limit on Delta mu/mu < 2x10^-8, where Delta mu/mu =
(mu_obs-mu_lab)/mu_lab. We also find that the offsets DeltaV are clustered into
two groups which are separated by 0.022 +/- 0.003 km/s (1sigma C.L.). The
grouping is most probably due to the dominance of different hyperfine
transitions in the 44 and 95 GHz methanol maser emission. Which transition
becomes favored is determined by an alignment (polarization) of the nuclear
spins of the four hydrogen atoms in the methanol molecule. This result confirms
that there are preferred hyperfine transitions involved in the methanol maser
action.
