[Paper Review] New Measurements Using External Photon Conversion at a High Luminosity B Factory

H. Ishino, M. Hazumi+2 more|ArXiv.org|Mar 27, 2007

Nuclear Physics and Applications3 citations

TL;DR

This paper proposes two novel tests of the Standard Model using external photon conversion at a high-luminosity B factory: (1) measuring the mixing-induced CP-violation parameter 𝒮_{π⁰π⁰} in B⁰→π⁰π⁰ decays with a precision of 0.23 using 50 ab⁻¹ of data, reducing the discrete ambiguity in the CKM angle φ₂; and (2) measuring photon polarization in B⁰→K*⁰γ decays to constrain hypothetical right-handed current contributions beyond the Standard Model, with improved sensitivity at 500 ab⁻¹ data.

ABSTRACT

We propose two novel methods for testing the standard model using external photon conversion at a high-luminosity e^+e^- B factory proposed recently. The first method is to measure the mixing-induced CP-violation parameter S_{pi^0pi^0} in B^0 --> pi^0 pi^0 decays. The precision of S_{pi^0pi^0} is estimated to be 0.23 from a Monte Carlo study for a data sample containing 50 x 10^9 BBbar pairs. We demonstrate that this measurement is crucial for reducing the discrete ambiguity of the Cabibbo-Kobayashi-Maskawa angle phi_2 determined from the isospin analysis with B --> pi pi decays. The second method is to measure photon polarization in B^0 --> K^{*0}(--> K^+ pi^-) gamma decays using the external photon conversion, and combine it with S_{K^{*}gamma} from B^0 --> K^{*0}(--> K^0_S pi^0) gamma decays. This offers a promising way of determining the hypothetical right-handed current amplitude and phase beyond the standard model.

Motivation & Objective

• To reduce the discrete ambiguity in the CKM angle φ₂ by measuring the mixing-induced CP-violation parameter 𝒮_{π⁰π⁰} in B⁰→π⁰π⁰ decays.
• To probe new physics beyond the Standard Model by measuring photon polarization in B⁰→K*⁰γ decays using external photon conversion.
• To improve the precision of CP-violation and polarization measurements by leveraging vertex reconstruction and enhanced energy resolution from photon conversion.
• To assess the sensitivity of these measurements to hypothetical right-handed current amplitudes and phases using 50 and 500 ab⁻¹ data samples.

Proposed method

• Utilizes external photon conversion (PC) in the detector to convert photons from B decays into e⁺e⁻ pairs, enabling vertex reconstruction and improved energy resolution.
• Employs a Geant4 Monte Carlo simulation of the Belle detector to model photon conversion and reconstruction efficiency, including beampipe and SVD interactions.
• Applies time-dependent decay rate analysis to extract 𝒮_{π⁰π⁰} from the decay time difference Δt between B⁰→π⁰π⁰ and a flavor-tagged decay, using Δz ≈ cβγΔt.
• Measures photon polarization via the azimuthal angle φ of the e⁺e⁻ pair from conversion, with resolution of 23° and reconstruction efficiency of 35%.
• Combines polarization measurements with the mixing-induced CP asymmetry 𝒮_{K*γ} to constrain the right-handed current amplitude R and phase φ₋.
• Uses a frequentist statistical approach with pseudo-experiments to estimate measurement precision and confidence regions for R and φ₋ at 50 and 500 ab⁻¹ data samples.

Experimental results

Research questions

• RQ1 Can external photon conversion at a high-luminosity B factory enable the measurement of 𝒮_{π⁰π⁰} in B⁰→π⁰π⁰ decays, which is otherwise hindered by poor vertex reconstruction?
• RQ2 To what extent does measuring 𝒮_{π⁰π⁰} reduce the discrete ambiguity in the CKM angle φ₂ determined from isospin analysis of B→ππ decays?
• RQ3 Can photon polarization measured via external conversion in B⁰→K*⁰γ decays, combined with 𝒮_{K*γ}, constrain the amplitude and phase of a hypothetical right-handed current beyond the Standard Model?
• RQ4 How does the measurement precision of 𝒮_{π⁰π⁰} and polarization parameters scale with integrated luminosity, particularly at 50 and 500 ab⁻¹?

Key findings

• The measurement precision of 𝒮_{π⁰π⁰} is estimated at 0.23 for a 50 ab⁻¹ data sample, enabling a significant reduction in the discrete ambiguity of the CKM angle φ₂.
• The measurement of 𝒮_{π⁰π⁰} reduces the eightfold ambiguity in φ₂ solutions from isospin analysis to a two-fold ambiguity, providing a crucial consistency check.
• The photon polarization measurement in B⁰→K*⁰γ decays achieves a φ resolution of 23° and a reconstruction efficiency of 35%, with 7,200 events expected for 50 ab⁻¹.
• With 500 ab⁻¹ data, the polarization measurement becomes more critical than 𝒮_{K*γ} for constraining the right-handed current amplitude R and phase φ₋, due to improved statistical precision.
• The 500 ab⁻¹ data sample yields a σₓ of 0.16 for the fit parameters x and y, consistent with the expected scaling of 0.52/√10, indicating high sensitivity.
• The confidence regions in the R–φ_R plane show that the polarization measurement becomes dominant for constraining new physics parameters at high luminosity, especially for the left-right symmetric model.