[Paper Review] Three Lectures of Flavor and CP violation within and Beyond the Standard Model

Stefania Gori|arXiv (Cornell University)|Oct 9, 2016

Particle physics theoretical and experimental studies67 references3 citations

TL;DR

This paper provides a comprehensive overview of flavor and CP violation in the Standard Model and beyond, focusing on effective field theories, Minimal Flavor Violation, and New Physics models such as two-Higgs doublet models and supersymmetry. It highlights key experimental constraints from LHCb and Belle II, particularly on rare B-meson decays and top quark flavor-changing couplings, and emphasizes how low-energy flavor observables probe high-scale New Physics.

ABSTRACT

These notes are based on the lectures I gave at the 2015 European School of High-Energy Physics (ESHEP2015). I discuss 1) flavor physics within the Standard Model, 2) effective field theories and Minimal Flavor Violation, 3) flavor physics in theories beyond the Standard Model and "high energy" flavor transitions of the top quark and of the Higgs boson. As a bi-product, I present the most updated constraints from the measurements of B_s -> mu^+mu^-, as well as I discuss the most recent development in the LHC searches for top flavor changing couplings.

Motivation & Objective

• To provide a pedagogical introduction to flavor physics and CP violation within the Standard Model and beyond.
• To explore the role of effective field theories and the Minimal Flavor Violation principle in constraining New Physics.
• To analyze flavor-changing couplings of the top quark and Higgs boson as probes of New Physics at the LHC.
• To present updated experimental constraints from rare decays such as Bs →μ⁺μ⁻ and top quark flavor transitions.
• To highlight the complementarity between low-energy flavor measurements and high-energy collider searches for New Physics.

Proposed method

• Uses effective field theory techniques to integrate out heavy New Physics degrees of freedom, generating higher-dimensional operators in the SM Lagrangian.
• Applies the Minimal Flavor Violation (MFV) ansatz to constrain flavor-changing neutral currents (FCNCs) and suppress tree-level flavor violation.
• Analyzes flavor structure through the SM's global flavor symmetry and its spontaneous breaking via Yukawa couplings.
• Evaluates constraints on top quark and Higgs flavor-changing couplings using LHC Run I data and current experimental limits.
• Reviews experimental results from LHCb, Belle, Babar, and ATLAS/CMS on rare B decays, lepton universality tests, and Higgs flavor-violating decays.
• Compares theoretical predictions with experimental data, particularly on BR(Bs →μ⁺μ⁻), B →K(⁎)μμ angular observables, and B →D(⁎)τν anomalies.

Experimental results

Research questions

• RQ1 How do flavor-changing neutral currents arise in the Standard Model, and why are they so strongly suppressed?
• RQ2 To what extent do current experimental measurements of rare B decays (e.g., Bs →μ⁺μ⁻, B →K(⁎)μμ) deviate from Standard Model predictions?
• RQ3 What constraints do low-energy flavor observables place on New Physics models such as two-Higgs doublet models and supersymmetry?
• RQ4 How can top quark and Higgs boson flavor-changing couplings serve as probes of New Physics at the TeV scale?
• RQ5 What is the role of the Higgs boson in the flavor puzzle, and how can its couplings to different fermion generations reveal new physics?

Key findings

• The branching ratio for Bs →μ⁺μ⁻ is constrained to BR(Bs →μ⁺μ⁻) < 3.2 × 10⁻⁹ at 95% CL, consistent with SM predictions.
• LHCb has observed anomalies in angular observables of Bd →K⁎μμ decays, suggesting possible deviations from SM expectations.
• Measurements of BR(B →Kμμ)/BR(B →Kee) show a 2.5σ tension with lepton flavor universality, hinting at possible new physics.
• The decay B →Dτν shows a 2–3σ deviation from SM predictions, with Belle and Babar reporting enhanced branching ratios.
• Current LHC searches for top quark flavor-changing couplings (e.g., t →cH, t →cZ, t →cγ) set upper limits on BR(t →cH) < 10⁻⁴ and BR(t →cZ) < 10⁻⁴.
• Higgs flavor-violating decays such as h →τμ and h →τe remain consistent with SM expectations, with no significant signal observed so far.