Aims. We probe the dependence of theelectron-to-proton mass ratio,,on the ambient matter density by means of radio astronomicalobservations. Methods. The ammonia method, which has been proposedto explore the electron-to-proton mass ratio, is applied to nearby darkclouds in the Milky Way. This ratio, which is measured in differentphysical environments of high (terrestrial) and low (interstellar)densities of baryonic matter is supposed to vary in chameleon-likescalar field models, which predict strong dependences of both massesand coupling constant on the local matter density. High resolutionspectral observations of molecular cores in lines of NH3(J,K)= (1,1), HC3NJ = 2-1, and N2H+J = 1-0 were performed at three radio telescopes tomeasure the radial velocity offsets,,between the inversion transition of NH3 (1,1)and the rotational transitions of other molecules with differentsensitivities to the parameter.Results. The measured values of exhibit a statistically significant velocity offset ofms-1.When interpreted in terms of the electron-to-proton mass ratiovariation, this infers that=.If only a conservative upper bound is considered, then the maximumoffset between ammonia and the other molecules isms-1.This provides the most accurate reference point at z= 0 for of .Key words: line: profiles - ISM: molecules- radio lines: ISM - techniques:radial velocities
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