[Paper Review] Measurements of Branching Fractions and Direct CP Asymmetries in pi^+ pi^0, K^+ pi^0 and K^0 pi^0 B Decays

B. Aubert|ArXiv.org|Jul 24, 2002

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents preliminary measurements of branching fractions and direct CP asymmetries in B⁺→π⁺π⁰, B⁺→K⁺π⁰, and B⁰→K⁰π⁰ decays using 88 million B B̄ pairs collected by the BaBar experiment at PEP-II. The key result is no significant direct CP violation observed, with asymmetries consistent with zero within uncertainties: A_π⁺π⁰ = −0.03⁺⁰.¹⁸₋₀.¹⁷ ± 0.02, A_K⁺π⁰ = −0.09 ± 0.09 ± 0.01, and A_K⁰π⁰ = 0.03 ± 0.36 ± 0.09.

ABSTRACT

We present preliminary results of the analyses of B^+ --> h^+ pi^0 (with h^+ = pi^+, K^+) and B^0 --> K^0 pi^0 decays from a sample of approximately 88 million B bar{B} pairs collected by the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. We measure the pi^+ pi^0 branching fraction and we obtain B(B^+ --> pi^+ pi^0) = (5.5^{+1.0}_{-0.9} +/- 0.6) times 10^{-6} with a significance of 7.7 sigma including systematic uncertainties. We measure the K^+ pi^0 and K^0 pi^0 branching fractions to be B(B^+ --> K^+ pi^0) = (12.8^{+1.2}_{-1.1} +/- 1.0) times 10^{-6} and B(B^0 --> K^0 pi^0) = (10.4 +/- 1.5 +/- 0.8) times 10^{-6}. At the same time, the direct CP-violating asymmetries are investigated and we find A_{pi^+ pi^0} = -0.03_{-0.17}^{+0.18} +/- 0.02, A_{K^+ pi^0} = -0.09 +/- 0.09 +/- 0.01 and A_{K^0 pi^0} = 0.03 +/- 0.36 +/- 0.09, where the errors are statistical and systematic, respectively.

Motivation & Objective

• To measure the branching fractions of B⁺→π⁺π⁰, B⁺→K⁺π⁰, and B⁰→K⁰π⁰ decays with high precision using a large sample of B B̄ pairs.
• To investigate direct CP violation in these decay modes, which are sensitive to the CKM angle α and new physics contributions.
• To improve upon previous measurements by reducing systematic uncertainties through refined analysis techniques and larger statistics.
• To test the Standard Model prediction of CP violation in B meson decays, particularly in penguin-dominated and tree-dominated modes.
• To provide updated inputs for the determination of the CKM unitarity triangle and the extraction of the CKM angle α.

Proposed method

• The analysis uses data from the BaBar detector at the PEP-II asymmetric-energy B factory, corresponding to approximately 88 million B B̄ pairs.
• Signal decays are reconstructed using kinematic and topological variables, including invariant mass, energy difference (ΔE), and Fisher discriminant to separate signal from background.
• Branching fractions are extracted via an extended unbinned maximum-likelihood fit to the signal and background components, with corrections for detector efficiency and reconstruction effects.
• For B⁰→K⁰π⁰, the correction factor (1 + x_d²) is applied to account for B⁰–B̄⁰ mixing, translating the measured asymmetry into the direct CP asymmetry A_CP.
• Systematic uncertainties are evaluated through PDF shape variations, tagging efficiency and mistag fraction variations, and toy Monte Carlo simulations to assess residual charmless backgrounds.
• The CP asymmetry is defined as A_i = A_m^i · (1 + x_d²) for B⁰→K⁰π⁰ and A_i = A_m^i for charged modes, with x_d = Δm_d/Γ = 0.755 ± 0.015.

Experimental results

Research questions

• RQ1 What is the branching fraction of the B⁺→π⁺π⁰ decay, and how precisely can it be measured?
• RQ2 Is there evidence of direct CP violation in the B⁺→K⁺π⁰ and B⁰→K⁰π⁰ decay modes?
• RQ3 How do the measured branching fractions and CP asymmetries compare with Standard Model predictions?
• RQ4 What are the dominant sources of systematic uncertainty in the branching fraction and CP asymmetry measurements?
• RQ5 Do the results constrain the CKM angle α or indicate new physics beyond the Standard Model?

Key findings

• The branching fraction for B⁺→π⁺π⁰ is measured as (5.5⁺¹.⁰₋₀.⁹ ± 0.6) × 10⁻⁶ with a significance of 7.7σ, including systematic uncertainties.
• The branching fraction for B⁺→K⁺π⁰ is (12.8⁺¹.²₋₁.¹ ± 1.0) × 10⁻⁶, and for B⁰→K⁰π⁰ it is (10.4 ± 1.5 ± 0.8) × 10⁻⁶.
• The direct CP asymmetry in the π⁺π⁰ mode is A_π⁺π⁰ = −0.03⁺⁰.¹⁸₋₀.¹⁷ ± 0.02, consistent with zero within 90% confidence level interval [−0.32, 0.27].
• For K⁺π⁰, the asymmetry is A_K⁺π⁰ = −0.09 ± 0.09 ± 0.01, with a 90% CL interval of [−0.24, 0.06], indicating no significant CP violation.
• The K⁰π⁰ mode yields A_K⁰π⁰ = 0.03 ± 0.36 ± 0.09, with a 90% CL interval of [−0.58, 0.64], also consistent with zero.
• Systematic uncertainties are dominated by PDF shape variations and tagging efficiency/mistag fraction in the K⁰π⁰ channel, with negligible contributions from residual charmless backgrounds.