Nejc Košnik (Ljubljana):

Scalar leptoquarks and fD_s puzzle

Abstract:

Motivated by the disagreement between experimental and lattice QCD results on the D_s 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 SU(5) GUT model these leptoquark states, contained in the 45-dimensional Higgs representation, could be safe against proton decay bounds. Using the current experimental measurements in tau, kaon and charm sectors, we find that scalar leptoquarks cannot naturally explain the D_s → μ ν and D_s → τ ν decay widths simultaneously. While any contributions of the triplet leptoquarks are already excluded, the singlets could only contribute significantly to the D_s → τ ν width. Finally, a moderate improvement of the experimental upper bound on the D⁰ → μ⁺ μ^- decay width could exclude the doublet contribution to the D_s → μ ν, 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.