title: A search for varying fundamental constants using hertz-level frequency measurements of cold CH molecules.
abstract: Many modern theories predict that the fundamental constants depend on time, position or the local density of matter. Here we develop a spectroscopic method for pulsed beams of cold molecules, and use it to measure the frequencies of microwave transitions in CH with accuracy down to 3 Hz. By comparing these frequencies with those measured from sources of CH in the Milky Way, we test the hypothesis that fundamental constants may differ between the high- and low-density environments of the Earth and the interstellar medium. For the fine structure constant we find Δα/α=(0.3 ± 1.1) × 10⁻⁷, the strongest limit to date on such a variation of α. For the electron-to-proton mass ratio we find Δμ/μ=(-0.7 ± 2.2) × 10⁻⁷. We suggest how dedicated astrophysical measurements can improve these constraints further and can also constrain temporal variation of the constants.
authors.affiliation: Centre for Cold Matter, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, UK.
authors.forename: S
authors.initials: S
authors.lastname: Truppe
authors.forename: R J
authors.initials: RJ
authors.lastname: Hendricks
authors.forename: S K
authors.initials: SK
authors.lastname: Tokunaga
authors.forename: H J
authors.initials: HJ
authors.lastname: Lewandowski
authors.forename: M G
authors.initials: MG
authors.lastname: Kozlov
authors.forename: Christian
authors.initials: C
authors.lastname: Henkel
authors.forename: E A
authors.initials: EA
authors.lastname: Hinds
authors.forename: M R
authors.initials: MR
authors.lastname: Tarbutt