[Paper Review] mu-->e+gamma Decay Rate in the MSSM with Minimal Flavour Violation

Momchil Davidkov, D. I. Kazakov|arXiv (Cornell University)|Feb 8, 2011

Particle physics theoretical and experimental studies9 references3 citations

TL;DR

This paper calculates the branching ratio for the lepton flavor-violating decay μ→eγ in the Minimal Supersymmetric Standard Model (MSSM) under the Minimal Flavour Violation (MFV) framework, emphasizing the role of the PMNS mixing matrix and the neutrino mixing angle θ13. It finds that the decay rate is highly sensitive to θ13 and tan β, with current experimental bounds strongly constraining high tan β scenarios unless θ13 is small, making this decay a powerful probe of new physics beyond the Standard Model.

ABSTRACT

The branching ratio for the mu-->e+gamma decay in the framework of the minimal flavour violation in the MSSM is calculated for various regions of the MSSM parameter space. The lepton flavour violation goes through the PMNS mixing matrix. The dependence on tanbeta is studied in comparison with experimental data. The results crucially depend on the mixing angle theta_{13}. Observation of this decay would serve as a manifestation of new physics beyond the SM.

Motivation & Objective

• To calculate the branching ratio for the rare lepton flavor-violating decay μ→eγ in the MSSM under the Minimal Flavour Violation (MFV) assumption.
• To investigate the dependence of the decay rate on the neutrino mixing angle θ13, particularly its impact through the PMNS matrix.
• To constrain the MSSM parameter space—specifically m₀, m₁/₂, A₀, and tan β—using current experimental upper bounds on Br(μ→eγ).
• To assess the viability of high tan β scenarios in the MSSM in light of the μ→eγ decay rate and neutrino oscillation data.

Proposed method

• Uses the Minimal Flavour Violation (MFV) framework to suppress flavor-changing neutral currents in the MSSM, ensuring that lepton flavor violation arises only via the PMNS mixing matrix.
• Calculates the μ→eγ decay amplitude using penguin diagrams involving charginos, charged Higgs, and sneutrinos, with the dominant contribution from the sneutrino loop.
• Applies the PMNS matrix parametrization with three mixing angles and CP phases, but assumes CP conservation and fixes θ13 to the global fit value sin²θ13 = 0.016.
• Performs numerical computations of the branching ratio as a function of A₀, tan β, and θ13 for three benchmark points: (m₀, m₁/₂) = (500, 500), (500, 900), and (1500, 250) GeV.
• Compares results with experimental upper bounds from MEG (≤2.8×10⁻¹¹) and MEGA (≤1.2×10⁻¹¹), using these to constrain the MSSM parameter space.
• Sensitively analyzes the dependence on sin θ13 by treating it as a free parameter in selected plots, showing the branching ratio's proportionality to sin²θ13.

Experimental results

Research questions

• RQ1 How does the μ→eγ decay rate in the MSSM with MFV depend on the neutrino mixing angle θ13, particularly sin²θ13?
• RQ2 What constraints do current experimental bounds on Br(μ→eγ) place on the MSSM parameters tan β, A₀, and m₀/m₁/₂?
• RQ3 Can high tan β scenarios in the MSSM remain viable given the sensitivity of μ→eγ to θ13 and current experimental limits?
• RQ4 How does the branching ratio vary across different regions of the MSSM parameter space, such as the focus-point and co-annihilation regions?
• RQ5 To what extent does the μ→eγ decay rate serve as a more stringent probe of MSSM parameter space than other rare decays, especially when combined with θ13 measurements?

Key findings

• The branching ratio for μ→eγ is proportional to sin²θ13, and thus vanishes in the limit of zero θ13, making the decay highly sensitive to this mixing angle.
• For the benchmark point (m₀, m₁/₂) = (500, 500) GeV, the branching ratio increases with A₀ and tan β, but is constrained to be below 2.8×10⁻¹¹ for tan β ≲ 35, even for small A₀.
• In the (1500, 250) GeV point, the branching ratio decreases with increasing A₀, indicating an opposite trend compared to lighter sparticle scenarios.
• The experimental upper bound of 2.8×10⁻¹¹ from MEG excludes high tan β values (≳35) unless sin²θ13 is very small, which is consistent with current global fits.
• If sin²θ13 is as large as 0.056 (99.73% C.L. upper bound), the branching ratio exceeds the experimental limit even for small tan β, implying strong constraints on large θ13 values.
• The results suggest that a large θ13 would rule out the high tan β region of the MSSM, while small θ13 values are required for consistency with both μ→eγ bounds and dark matter abundance.