Flavor Constraints from Unitarity and Analyticity.

Area of Science:

• High Energy Physics
• Theoretical Physics
• Particle Physics

Background:

• The Standard Model Effective Field Theory (SMEFT) extends the Standard Model by including higher-dimension operators.
• Four-fermion operators at mass dimension 8 are particularly important for probing new physics beyond the Standard Model.
• Unitarity and analyticity are fundamental principles governing scattering processes in quantum field theory.

Purpose of the Study:

• To derive constraints on Wilson coefficients of dimension-8 four-fermion operators in SMEFT.
• To investigate the implications of these constraints for flavor-violating couplings.
• To assess the impact of existing experimental data on these theoretical constraints.

Main Methods:

• Utilizing unitarity and analyticity of scattering amplitudes.
• Scattering flavor superpositions in fixed Standard Model representations.
• Analyzing the positivity of the contraction between Wilson coefficients and density matrices.

Main Results:

• Wilson coefficients of dimension-8 four-fermion operators are constrained to ensure positivity.
• Flavor-violating couplings are found to be upper bounded by their flavor-conserving counterparts.
• Existing Large Electron-Positron Collider (LEP) data already restricts certain operators relevant for future experiments.

Conclusions:

• Unitarity and analyticity provide powerful, model-independent constraints on new physics.
• These constraints significantly impact the parameter space of SMEFT, particularly for flavor-changing processes.
• Future experiments searching for rare decays like muon to three electrons will benefit from these theoretical insights.