Searching for chameleon-like scalar fields with the ammonia method^*

¹ INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
² Ioffe Physical - Technical Institute, Polytekhnicheskaya Str. 26, 194021 St. Petersburg, Russia e-mail: lev@astro.ioffe.rssi.ru
³ Institute for Applied Physics, Uljanov Str. 46, 603950 Nizhny Novgorod, Russia
⁴ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Institute of Astronomy, The University of Tokyo, Osawa, Mitaka, Tokyo 181-0015, Japan

Received: 29 July 2009
Accepted: 18 November 2009

Abstract

Aims. We probe the dependence of the electron-to-proton mass ratio, μ = m_e/m_p, on the ambient matter density by means of radio astronomical observations.

Methods. The ammonia method, which has been proposed to explore the electron-to-proton mass ratio, is applied to nearby dark clouds in the Milky Way. This ratio, which is measured in different physical environments of high (terrestrial) and low (interstellar) densities of baryonic matter is supposed to vary in chameleon-like scalar field models, which predict strong dependences of both masses and coupling constant on the local matter density. High resolution spectral observations of molecular cores in lines of NH₃ (J,K) = (1,1), N J = 2-1, and J = 1-0 were performed at three radio telescopes to measure the radial velocity offsets, ≡ V_rot - V_inv, between the inversion transition of (1,1) and the rotational transitions of other molecules with different sensitivities to the parameter ≡ /.

Results. The measured values of exhibit a statistically significant velocity offset of 23±± m s. When interpreted in terms of the electron-to-proton mass ratio variation, this infers that = (2.2±±) × 10^-8. If only a conservative upper bound is considered, then the maximum offset between ammonia and the other molecules is ≤ 30 m s. This provides the most accurate reference point at z = 0 for  of ≤ 3×10^-8.