An Indirect Handle on the Down-Quark Yukawa Coupling

Abstract

To measure the Yukawa couplings of the up and down quarks, Y_{u,d}, seems to be far beyond the capabilities of current and (near) future experiments in particle physics. By performing a general analysis of the potential misalignment between quark masses and Yukawa couplings, we derive predictions for the magnitude of induced flavor-changing neutral currents (FCNCs), depending on the shift in the physical Yukawa coupling of first-generation quarks. We find that a shift of -100% in the down-quark Yukawa Y_d would generically result in ds transitions in conflict with Kaon physics. This could already be seen as evidence for a non-vanishing direct coupling of the down quark to the newly discovered Higgs boson. The non-observation of certain, already well-constrained, processes is thus turned into a powerful indirect measure of physical parameters of the effective Standard-Model, which are so far basically unconstrained from experiment and extremely challenging to access with other methods. In particular, we can already deduce that Y_d should vary at most by ~50% from its Standard Model value, barring an alignment of new physics effects with the SM Yukawa couplings. Such an (orthogonal) alignment scenario is however in general much easier to test at the LHC. Similarly, improvements in limits on FCNCs in the up-type quark sector can lead to valuable information on the physical Yukawa coupling of the up-quark.