LAUSR

We consider effects of coloured scalar mediators in decays c → u invisibles. In particular, in these processes, as invisibles, we consider massive right-handed fermions. The coloured scalar S̄1 ≡ (3̄, 1,−2/3), due to its coupling to weak singlets up-quarks and invisible right-handed fermions (χ), is particularly interesting. Then, we consider R̃2 ≡ (3̄, 2, 1/6), which as a weak doublet is a subject of severe low-energy constraints. The χ mass is considered in the range (mK − mπ)/2 ≤ mχ ≤ (mD −mπ)/2. We determine branching ratios for D → χχ̄, D → χχ̄γ and D → πχχ for several χ masses, using most constraining bounds. For S̄1, the most constraining is D − D̄ mixing, while in the case of R̃2 the strongest constraint comes from B → K / E. We find in decays mediated by S̄1 that branching ratios can be B(D → χχ̄) < 10−8 for mχ = 0.8 GeV, B(D → χχ̄γ) ∼ 10−8 for mχ = 0.18 GeV, while B(D → πχχ̄) can reach ∼ 10−8 for mχ = 0.18 GeV. In the case of R̃2 these decay rates are very suppressed. We find that future tau-charm factories and Belle II experiments offer good opportunities to search for such processes. Both S̄1 and R̃2 might have masses within LHC reach.