Can scalar leptoquarks explain the fDs puzzle?

Motivated by the disagreement between experimental and lattice QCD results on the Ds decay constant we systematically reinvestigate role of leptoquarks in charm meson decays. We consider scalar leptoquarks that transform as a weak interaction triplet, doublet, or singlet in a model independent approach, and also argue that in a particular class of SU(5) GUT models these leptoquark states, contained in the 45-dimensional Higgs representation, could be safe against proton decay bounds. Using the current experimental measurements in τ, kaon and charm sectors, we find that scalar leptoquarks cannot naturally explain the Ds→μν and Ds→τν decay widths simultaneously. While any contributions of the triplet leptoquarks are already excluded, the singlets could only contribute significantly to the Ds→τν width. Finally, a moderate improvement of the experimental upper bound on the D0→μ+μ− decay width could exclude the doublet contribution to the Ds→μν, while present experimental data limits its mass to be below 1.4 TeV. Possible new signatures at present and near future experiments are also briefly discussed.