Probing the electron-to-proton mass ratio gradient in the Milky Way with class I methanol masers

TL;DR

This study uses methanol maser observations across the Milky Way to set stringent limits on spatial variations of the electron-to-proton mass ratio, testing fundamental physics and hyperfine transition effects.

Contribution

It provides the first tight constraints on the galactic gradient of the electron-to-proton mass ratio using methanol masers and reveals hyperfine transition dominance in maser emission.

Findings

Upper limit on the mass ratio gradient: < 2x10^-9 kpc^-1

Upper limit on fractional change in mu: < 2x10^-8

Identification of hyperfine transition dominance in maser lines

Abstract

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.