An Indirect Handle on the Down-Quark Yukawa Coupling

Understand

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.