[Paper Review] FCNC Processes Waiting for the Next Decade

Andrzej J. Buras|arXiv (Cornell University)|Oct 8, 2009

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper outlines 20 key goals in flavor physics for the next decade, focusing on rare processes and precision measurements to probe new physics beyond the Standard Model. It emphasizes that FCNC processes, electric dipole moments, and $(g-2)_\mu$ offer indirect access to energy scales up to 200 TeV, with supersymmetric flavor models showing potential to explain anomalies in $S_{\psi\phi}$, $\varepsilon_K$, and $\mu \to e\gamma$.

ABSTRACT

FCNC processes are expected to offer us a deep insight into the physics at very short distance scales. We present a list of 20 goals in quark and lepton flavour physics that could be reached already in the next decade. This list includes also flavour conserving observables like electric dipole moments of the neutron and leptons and (g-2)_μ. Subsequently we will present some aspects of these goals by concentrating on supersymmetric flavour models. A much more extensive presentation of this material can be found in my recent EPS09 talk.

Motivation & Objective

• To identify the most promising experimental and theoretical targets in flavor physics for the next decade.
• To assess the potential of rare decays, CP asymmetries, and electric dipole moments to reveal new physics beyond the Standard Model.
• To explore how supersymmetric flavor models can simultaneously explain anomalies in $S_{\psi\phi}$, $\varepsilon_K$, $(g-2)_\mu$, and lepton flavor violation.
• To evaluate the consistency of current data with the Standard Model, particularly the tension between $\varepsilon_K$ and $S_{\psi K_S}$.

Proposed method

• Using lattice QCD calculations to improve precision in hadronic matrix elements such as $\hat{B}_K$, $F_{B_s}$, and $F_{B_d}$, enabling accurate SM predictions for $\varepsilon_K$ and $B_{s,d} \to \mu^+\mu^-$.
• Analyzing correlations between observables like $S_{\psi\phi}$, $Br(B_{s} \to \mu^+\mu^-)$, and $Br(\tau \to \mu\gamma)$ in various new physics models, including MFV, CMFV, RVV2, AKM, and $\delta$ LL scenarios.
• Evaluating the impact of improved measurements from LHCb, Super-Belle, and future SFF facilities on determining CKM matrix elements and testing the SM.
• Applying theoretical frameworks such as the MFV and CMFV ansätze to constrain new physics contributions to FCNC processes.
• Using numerical simulations and model-dependent analyses to predict maximal enhancements in branching ratios and CP asymmetries across different NP models.
• Comparing theoretical predictions with experimental data from CDF, D0, and LHCb to test the SM prediction $S_{\psi\phi} \approx 0.04$.

Experimental results

Research questions

• RQ1 Can the observed $S_{\psi\phi} \approx 0.81$ be explained by new physics, or is it a statistical fluctuation?
• RQ2 Is the measured value of $\varepsilon_K$ consistent with the SM prediction when updated lattice QCD results for $\hat{B}_K$ are used?
• RQ3 Can supersymmetric flavor models simultaneously explain the $(g-2)_\mu$ anomaly, the $S_{\psi K_S}$ anomaly, and large $A_{\rm CP}^{bs\gamma}$?
• RQ4 What are the maximal enhancements in $Br(K^+ \to \pi^+ \nu\bar{\nu})$, $Br(K_L \to \pi^0 \nu\bar{\nu})$, $Br(B_s \to \mu^+\mu^-)$, and $S_{\psi\phi}$ in various new physics models?
• RQ5 Can precision measurements of $|V_{ub}|$, $\gamma$, and $|V_{cb}|$ from LHCb and Super-B resolve the current tensions in CKM unitarity?

Key findings

• The updated lattice QCD result for $\hat{B}_K$ suggests a potential tension between the SM prediction and the measured value of $\varepsilon_K$, indicating possible new physics contributions.
• The observed $S_{\psi\phi} \approx 0.81^{+0.12}_{-0.32}$ from CDF and D0 is significantly larger than the SM prediction of $\approx 0.04$, suggesting a possible new physics signal.
• In the $\delta$ LL supersymmetric model, a large $A_{\rm CP}^{bs\gamma}$ and a solution to the $(g-2)_\mu$ anomaly are automatically linked, while in models with right-handed currents, $A_{\rm CP}^{bs\gamma}$ remains SM-like.
• The RVV2 model predicts $Br(\mu \to e\gamma) \geq 10^{-9}$, which is within reach of Super-B machines, while the AKM model does not reach this sensitivity.
• In the MFV and CMFV models, $Br(B_s \to \mu^+\mu^-)$ can be enhanced by up to 1000%, while $S_{\psi\phi}$ remains close to the SM value of 0.04.
• The RSc model allows for a large enhancement of $Br(K_L \to \pi^0 \nu\bar{\nu})$ (up to 150%) and $S_{\psi\phi}$ (up to 0.75), making it a viable candidate for explaining anomalies.