Can scalar leptoquarks explain the f_{D_s} puzzle?

TL;DR

This paper investigates whether scalar leptoquarks can resolve the discrepancy in the D_s decay constant measurements, finding that they cannot fully explain the data and constraining their possible contributions and masses.

Contribution

It provides a systematic, model-independent analysis of scalar leptoquarks' role in charm meson decays, including implications within an SU(5) GUT framework.

Findings

Scalar leptoquarks cannot simultaneously explain D_s --> mu nu and D_s --> tau nu decays.

Triplet leptoquarks are excluded by current data.

Singlet leptoquarks could significantly affect D_s --> tau nu decay.

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 --> mu nu and D_s --> tau nu decay widths simultaneously. While any contributions of the triplet leptoquarks are already excluded, the singlets could only contribute significantly to the D_s --> tau nu width. Finally, a moderate improvement of the experimental upper bound on the D^0 --> mu^+\mu^- decay width could exclude the doublet contribution to the D_s --> mu nu, 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.