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Current and future constraints on Bekenstein-type models for varying couplings

Astrophysical tests of the stability of dimensionless fundamental couplings, such as the fine-structure constant $α$ and the proton-to-electron mass ratio $μ$, are an optimal probe of new physics. There is a growing interest in these tests, following indications of possible spacetime variations at the few parts per million level. Here we make use of the latest astrophysical measurements, combined with background cosmological observations, to obtain improved constraints on Bekenstein-type models for the evolution of both couplings. These are arguably the simplest models allowing for $α$ and $μ$ variations, and are characterized by a single free dimensionless parameter, $ζ$, describing the coupling of the underlying dynamical degree of freedom to the electromagnetic sector. In the former case we find that this parameter is constrained to be $|ζ_α|<4.8\times10^{-6}$ (improving previous constraints by a factor of 6), while in the latter (which we quantitatively compare to astrophysical measurements for the first time) we find $ζ_μ=(2.7\pm3.1)\times10^{-7}$; both of these are at the $99.7\%$ confidence level. For $ζ_α$ this constraint is about 20 times stronger than the one obtained from local Weak Equivalence Principle tests, while for $ζ_μ$ it is about 2 orders of magnitude weaker. We also discuss the improvements on these constraints to be expected from the forthcoming ESPRESSO and ELT-HIRES spectrographs, conservatively finding a factor around 5 for the former and around 50 for the latter.