| Abstract
| - Aims. We aim to validate the Einstein equivalence principle (local position invariance) by limiting the fractional changes in the electron-to-proton mass ratio, μ = me/ mp, measured in Galactic plane objects. Methods. High-resolution spectral observations of dark clouds in the inversion line of NH 3(1, 1) and pure rotational lines of other molecules (the so-called ammonia method) were performed at the Medicina 32-m and the Effelsberg 100-m radio telescopes to measure the radial velocity offsets, Δ RV = Vrot − Vinv, between the rotational and inversion transitions, which have different sensitivities to the value of μ. Results. In our previous observations (2008-2010), a mean offset of ⟨Δ RV⟩ = 0.027 ± 0.010 km s -1 (3 σ confidence level (C.L.)) was measured. To test for possible hidden errors, we carried out additional observations of a sample of molecular cores in 2010-2013. As a result, a systematic error with an amplitude ~0.02 km s -1 in the radial velocities was revealed. The averaged offset between the radial velocities of the rotational transitions of HC 3N(2-1), HC 5N(9-8), HC 7N(16-15), HC 7N(21-20), and HC 7N(23-22), and the inversion transition of NH 3(1, 1) is ⟨Δ RV⟩ = 0.003 ± 0.018 km s -1 (3 σ C.L.). This value, when interpreted in terms of Δ μ/ μ = ( μobs − μlab)/ μlab, constraints the μ-variation at the level of Δ μ/ μ < 2 × 10 -8 (3 σ C.L.), which is the most stringent limit on the fractional changes in μ based on astronomical observations.
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