2010A&A...512A..44L

We probe the dependence of the electron-to-proton mass ratio, µ=m_e/m_p, on the ambient matter density by means of radio astronomical observations. The ammonia method, which has been proposed to explore the electron-to-proton mass ratio, is applied to nearby dark clouds in the Milky Way. This ratio, which is measured in different physical environments of high (terrestrial) and low (interstellar) densities of baryonic matter is supposed to vary in chameleon-like scalar field models, which predict strong dependences of both masses and coupling constant on the local matter density. High resolution spectral observations of molecular cores in lines of NH₃ (J,K)=(1,1), HC₃N J=2-1, and N₂H⁺ J=1-0 were performed at three radio telescopes to measure the radial velocity offsets, ΔV=V_rot-V_inv, between the inversion transition of NH₃ (1,1) and the rotational transitions of other molecules with different sensitivities to the parameter Δµ/µ=(µ_obs-µ_lab)/µ_lab. The measured values of ΔV exhibit a statistically significant velocity offset of 23±4_stat±3_sysm/s. When interpreted in terms of the electron-to-proton mass ratio variation, this infers that Δµ/µ=(2.2±0.4_stat±0.3_sys)x10^–8. If only a conservative upper bound is considered, then the maximum offset between ammonia and the other molecules is |ΔV|≤30m/s. This provides the most accurate reference point at z=0 for Δµ/µ of |Δµ/µ|≤3x10^–8.