Limits on the spatial variations of the electron-to-proton mass ratio in the Galactic plane

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 NH3(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, ΔRV = 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 � ΔRV� = 0.027 ± 0.010 km s −1 (3σ 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 HC3N(2–1), HC5N(9–8), HC7N(16–15), HC7N(21–20), and HC7N(23–22), and the inversion transition of NH3(1, 1) is � ΔRV� = 0.003 ± 0.018 km s −1 (3σ C.L.). This value, when interpreted in terms of Δμ/μ = (μobs − μlab)/μlab, constraints the μ-variation at the level of Δμ/μ < 2 × 10 −8 (3σ C.L.), which is the most stringent limit on the fractional changes in μ