Indication of the Electron-to-Proton Mass Ratio Variation in the Galaxy

Abstract

Near (~100 pc) and far (~8.7 kpc) relative to the Galactic center, the molecular clouds SgrB2(N) and Orion-KL exhibit different values of the fundamental physical constant μ = m_e/m_p, which is the electron-to-proton mass ratio. Measured frequency difference between the emission lines of methanol (CH₃OH), \({{J}_{{{{K}_{u}}}}}\) → \({{J}_{{{{K}_{\ell }}}}}\) = 6₃ → 5₂A⁺ 542 000.981 MHz, 6₃ → 5₂A^– 542 081.936 MHz, and 8₀ → 7_–1E 543 076.194 MHz, observed with the space observatory Herschel toward SgrB2(N) and Orion-KL corresponds to (Sgr-Ori): Δμ/μ = (‒3.7 ± 0.5) × 10^–7 (1σ C.L.). At the same time, comparison of the same methanol lines in Orion-KL with laboratory frequencies shows no significant changes in μ (Ori-lab): Δμ/μ = (–0.5 ± 0.6) × 10^–7, while a comparison between SgrB2(N) and laboratory lines indicates a lower value of μ near the Galactic center (Sgr-lab): Δμ/μ = (–4.2 ± 0.7) × 10^–7. The reduced value of μ in SgrB2(N) is not explained by known systematic effects and requires further investigation.