Indirect handle on the down-quark Yukawa coupling

Area of Science:

• High Energy Physics
• Particle Physics
• Standard Model Physics

Background:

• Direct measurement of up and down quark Yukawa couplings (Yu,d) is experimentally infeasible with current technology.
• The Standard Model describes fundamental particles and forces, but precise measurements of couplings are crucial for testing its validity.
• Flavor-changing neutral currents (FCNCs) are rare processes that can provide sensitive probes of new physics beyond the Standard Model.

Purpose of the Study:

• To investigate the potential for indirect measurement of up and down quark Yukawa couplings.
• To explore the relationship between quark mass misalignment and the generation of flavor-changing neutral currents (FCNCs).
• To establish constraints on Yukawa couplings using existing experimental data on FCNCs.

Main Methods:

• Performing a general analysis of potential misalignments between quark masses and their Yukawa couplings.
• Deriving predictions for the magnitude of induced FCNCs based on shifts in first-generation quark Yukawa couplings.
• Utilizing kaon physics and other well-constrained FCNC processes to set limits.

Main Results:

• A change exceeding 50% in the down quark Yukawa coupling (Yd) would likely lead to ds transitions that conflict with experimental kaon physics data.
• This conflict suggests a non-zero direct coupling of the down quark to the Higgs boson.
• Non-observation of specific FCNC processes provides powerful indirect constraints on otherwise inaccessible Yukawa couplings.

Conclusions:

• The non-observation of FCNCs serves as a powerful indirect probe for measuring fundamental parameters like quark Yukawa couplings.
• Improvements in FCNC limits, particularly in the up-type quark sector, can yield valuable information about Yu.
• This approach transforms the limitations of current experiments into a tool for discovering new physics and refining the Standard Model.