[Paper Review] Improved Measurement of CP Asymmetry in the Neutral B Meson System

T. Higuchi|arXiv (Cornell University)|May 10, 2002

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This paper presents a preliminary improved measurement of the CP violation parameter sin 2φ₁ in the neutral B meson system using 45×10⁶ B̄B pairs collected by the Belle experiment at KEKB. By analyzing time-dependent CP asymmetries in decay modes such as J/ψK_S⁰, ψ(2S)K_S⁰, and J/ψK_L⁰, the study determines sin 2φ₁ = 0.82 ± 0.12 (stat) ± 0.05 (syst), providing a precise test of the CKM matrix in the Standard Model.

ABSTRACT

We present an improved measurement of the standard model CP violation parameter sin2phi1 (also known as sin2beta) based on a sample of 45x10^6 BBbar pairs collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. One neutral B meson is reconstructed in the J/psiKS, Psi(2S)KS, chi_c1KS, eta_cKS, J/psiK^*0, or J/psiKL CP-eigenstate decay channel and the flavor of accompanying B meson is identified from its decay products. From the asymmetry in the distribution of the time intervals between the two B meson decay points, we obtain sin2phi1 = 0.82 +/- 0.12(stat) +/- 0.05(syst). The result is preliminary.

Motivation & Objective

• To improve the precision of the CP violation parameter sin 2φ₁ (sin 2β) in the neutral B meson system.
• To test the Standard Model prediction of CP violation via the CKM matrix's complex phase.
• To measure time-dependent CP asymmetries in multiple CP eigenstate decay modes of B⁰ mesons.
• To reduce systematic uncertainties in the measurement by refining vertex reconstruction and background modeling.

Proposed method

• The analysis uses time-dependent CP asymmetry in the decay of one B⁰ meson to a CP eigenstate (e.g., J/ψK_S⁰, ψ(2S)K_S⁰, η_cK_S⁰, J/ψK_L⁰, or J/ψK*⁰) and the flavor of the other B⁰ meson tagged via its decay products.
• The proper time interval Δt between the two B meson decays is reconstructed from the spatial separation (Δz) along the beam direction, using the boost parameter βγ = 0.425.
• An unbinned maximum-likelihood fit is performed on the Δt distributions, with a probability density function that includes signal, combinatorial background, and outlier components.
• The signal PDF incorporates the time-dependent CP asymmetry formula: A(Δt) = -ξ_f sin 2φ₁ sin(Δm_d Δt), where ξ_f is the CP eigenvalue of the final state.
• Systematic uncertainties are evaluated by varying vertex resolution, signal probability (w_l), resolution function parameters, and background fractions.
• The fit is validated using control samples such as non-CP eigenstate decays (e.g., D*⁻π⁺, D*⁻ρ⁺), which should show no CP asymmetry.

Experimental results

Research questions

• RQ1 What is the most precise measurement of sin 2φ₁ using the Belle experiment's 45×10⁶ B̄B pair dataset?
• RQ2 How do different CP eigenstate decay modes (e.g., J/ψK_S⁰, J/ψK_L⁰, J/ψK*⁰) contribute to the overall sin 2φ₁ determination?
• RQ3 What are the dominant systematic uncertainties affecting the time-dependent CP asymmetry measurement?
• RQ4 Is the measured sin 2φ₁ consistent with the Standard Model prediction of sin 2φ₁ ≈ 0.7–0.9?
• RQ5 Do control samples with no expected CP asymmetry show consistent results within uncertainties?

Key findings

• The preliminary measurement yields sin 2φ₁ = 0.82 ± 0.12 (statistical) ± 0.05 (systematic), representing an improved precision over previous measurements.
• The result is consistent with the Standard Model prediction of sin 2φ₁ ≈ 0.8–0.9, supporting the CKM matrix as the source of CP violation.
• The systematic uncertainty is dominated by vertex resolution tail effects, contributing ±0.030 to the error.
• The J/ψK_L⁰ mode yields a higher sin 2φ₁ value (1.14 ± 0.23) but with large uncertainty, consistent with the overall result.
• Control samples such as B⁰ → D*⁻π⁺ and D*⁻ρ⁺ show no significant CP asymmetry (S = 0.05 ± 0.04), validating the analysis method.
• The measurement of CP asymmetry in B⁰ → η′K_S⁰ yields S = 0.27⁺⁵⁴₋⁵⁵ (stat) ± 0.07 (syst), consistent with zero, as expected for this mode.