[Paper Review] Two-body nonleptonic B decays in the Standard Model and beyond

M. Ciuchini, E. Franco+4 more|ArXiv.org|Jul 6, 2004

Particle physics theoretical and experimental studies7 references7 citations

TL;DR

This paper investigates two-body nonleptonic B decays in the Standard Model and Supersymmetry, using QCD factorization and effective field theory to model penguin contributions—particularly charming penguins—showing that nonperturbative effects can explain the observed branching ratios and CP asymmetries in B→ππ, B→Kπ, and B→ϕK_S decays. It demonstrates that deviations in CP asymmetry in B→ϕK_S can be accommodated in the MSSM via flavor-violating squark mass insertions, especially in helicity-flipping channels.

ABSTRACT

We briefly discuss the phenomenology of B to pi pi, B to K pi and B to phi K decays in the Standard Model and in Supersymmetry.

Motivation & Objective

• To understand the theoretical challenges in computing nonleptonic B decays due to nonperturbative hadronic matrix elements and final-state interactions.
• To test whether QCD factorization and effective field theories like SCET can describe experimental data on B→ππ, B→Kπ, and B→ϕK_S decays, including CP asymmetries.
• To assess the role of charming penguins and nonfactorizable contributions in explaining large branching ratios, especially for B→π⁰π⁰.
• To explore how new physics, particularly in the MSSM, can account for discrepancies in B→ϕK_S CP asymmetry, using a model-independent approach with mass insertions.
• To quantify the impact of SUSY contributions on observables like S_ϕK and A_CP(B→X_sγ), and to identify correlations between new physics parameters and measurable CP asymmetries.

Proposed method

• Uses QCD factorization and Soft-Collinear Effective Theory (SCET) to compute decay amplitudes, treating penguin contributions as perturbatively calculable up to power-suppressed terms.
• Incorporates nonfactorizable amplitudes—particularly charming penguins and GIM penguins—via parameterized subleading contributions to match experimental data.
• Performs a global fit to experimental branching ratios and CP asymmetries in B→ππ, B→Kπ, and B→ϕK_S channels, using CKM parameters from the Unitarity Triangle fit.
• Applies the mass insertion method in the MSSM to model flavor-changing neutral currents and CP violation from off-diagonal squark mass terms.
• Conducts a Monte Carlo analysis over random configurations of mass insertion parameters, weighted by agreement with BR(B→X_sγ), A_CP(B→X_sγ), BR(B→X_sℓ⁺ℓ⁻), and ΔM_Bs.
• Imposes constraints from ΔM_Bs < 20 ps⁻¹ and S_ϕK < 0 to select viable SUSY parameter regions, analyzing correlations between observables and new physics parameters.

Experimental results

Research questions

• RQ1 Can nonperturbative charming penguin contributions explain the observed branching ratio of B→π⁰π⁰, which is significantly larger than QCD factorization predictions?
• RQ2 To what extent do nonfactorizable amplitudes—such as GIM penguins and annihilation contributions—dominate the amplitude structure in B→ππ decays?
• RQ3 Can the MSSM with minimal superfields and unconstrained soft-breaking terms accommodate the Belle measurement of S_ϕK < 0, which deviates from the SM prediction?
• RQ4 What are the correlations between the imaginary part of the down-quark mass insertion (δ²³_d) and the CP asymmetry S_ϕK in the MSSM?
• RQ5 How do different helicity structures (LL, RR, LR, RL) of the mass insertions affect the allowed parameter space for new physics in B→ϕK_S decays?

Key findings

• The observed branching ratio for B→π⁰π⁰ (~1.9×10⁻⁶) is well reproduced by including charming penguins (P₁ = (0.11±0.05)e^{i(-0.2±0.9)}) and GIM penguins (P₁^GIM = (0.43±0.14)e^{i(-0.2±0.7)}), indicating nonfactorizable effects are essential.
• The CP asymmetry in B→π⁺π⁻ is well described by the fit, with S_π⁺π⁻ = -0.7±0.2, consistent with the SM prediction and experimental data.
• The fit yields a nontrivial posterior distribution for the CKM angle α, with a shape similar to model-independent SU(2) analyses, indicating consistency with unitarity triangle constraints.
• In the MSSM, a nonvanishing imaginary part of δ²³_d is required to generate S_ϕK < 0, with the strongest constraints on helicity-flipping (LR, RL) insertions, where |Im(δ²³_d)| ~ 10⁻².
• For helicity-conserving insertions (LL, RR), the allowed region is reduced by the ΔM_Bs < 20 ps⁻¹ constraint, with typical |δ²³_d| ~ 1, while helicity-flipping insertions allow smaller values but still consistent with the data.
• Deviations in S_ϕK from the SM are correlated with non-zero Im(δ²³_d), and such effects can lead to A_CP(B→X_sγ) at the few percent level in LL cases and up to the experimental upper bound in LR cases.