  We use six years of accurate hyperfine frequency comparison data of the dual
rubidium and caesium cold atom fountain FO2 at LNE-SYRTE to search for a
massive scalar dark matter candidate. Such a scalar field can induce harmonic
variations of the fine structure constant, of the mass of fermions and of the
quantum chromodynamic mass scale, which will directly impact the
rubidium/caesium hyperfine transition frequency ratio. We find no signal
consistent with a scalar dark matter candidate but provide improved constraints
on the coupling of the putative scalar field to standard matter. Our limits are
complementary to previous results that were only sensitive to the fine
structure constant, and improve them by more than an order of magnitude when
only a coupling to electromagnetism is assumed.
