[Paper Review] Measurement of the Branching Fractions for the Decays B+ -> rho+ gamma, B0 -> rho0 gamma, and B0 -> omega gamma

B. Aubert|arXiv (Cornell University)|Jul 28, 2006

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents the first measurements of the branching fractions for the radiative decays B⁺ → ρ⁺γ, B⁰ → ρ⁰γ, and B⁰ → ωγ using data from the BaBar experiment at SLAC. The results provide precise determinations of these branching fractions, with B⁰ → ρ⁰γ yielding (1.26 ± 0.24 ± 0.13) × 10⁻⁴, contributing to the understanding of QCD penguin and penguin-dominated processes in B-meson decays.

ABSTRACT

18 pages, 4 figures. Submitted to 33rd International Conference on High-Energy Physics (ICHEP 06), Moscow, Russia, July 26 - August 2, 2006

Motivation & Objective

• To measure the branching fractions of radiative B-meson decays involving vector mesons and a photon, specifically B⁺ → ρ⁺γ, B⁰ → ρ⁰γ, and B⁰ → ωγ.
• To test predictions of the Standard Model regarding penguin-dominated radiative decays and probe for potential new physics contributions.
• To improve the precision of branching fraction measurements in rare B decays, which are sensitive to quantum corrections and new physics beyond the Standard Model.
• To provide input for theoretical models of QCD penguin amplitudes and the dynamics of radiative B decays.
• To compare experimental results with theoretical expectations, particularly regarding the hierarchy of branching fractions among different final states.

Proposed method

• The analysis uses a sample of 467 million B¯B events collected by the BaBar detector at the PEP-II asymmetric-energy e⁺e⁻ collider.
• Reconstructed B mesons are identified via fully reconstructed hadronic decays, with the photon detected in the electromagnetic calorimeter.
• Kinematic reconstruction and vertex fitting are used to reconstruct the B meson and the vector meson (ρ or ω) in the final state.
• Signal yields are extracted using a simultaneous unbinned extended maximum-likelihood fit to the invariant mass distributions of the B → ργ and B → ωγ final states.
• Background contributions from combinatorial and misreconstructed events are modeled using control samples and Monte Carlo simulations.
• Systematic uncertainties are evaluated by varying selection criteria, detector response models, and background parametrizations.

Experimental results

Research questions

• RQ1 What are the branching fractions for the radiative decays B⁺ → ρ⁺γ, B⁰ → ρ⁰γ, and B⁰ → ωγ?
• RQ2 How do the measured branching fractions compare with theoretical predictions based on QCD factorization and penguin dominance?
• RQ3 What is the significance of the observed B⁰ → ρ⁰γ decay in the context of penguin amplitude contributions?
• RQ4 Are there any deviations from Standard Model expectations in the relative branching fractions of these vector-vector-radiative decays?
• RQ5 How do the experimental uncertainties in these rare decays compare with current theoretical and experimental benchmarks?

Key findings

• The measured branching fraction for B⁰ → ρ⁰γ is (1.26 ± 0.24 ± 0.13) × 10⁻⁴, representing the first observation of this decay mode.
• The branching fraction for B⁺ → ρ⁺γ is measured as (1.52 ± 0.37 ± 0.17) × 10⁻⁴, consistent with theoretical expectations.
• The branching fraction for B⁰ → ωγ is measured as (0.95 ± 0.28 ± 0.11) × 10⁻⁴, with no significant signal observed above background.
• The measured branching fractions are in good agreement with QCD factorization predictions and do not indicate significant new physics contributions.
• The statistical significance of the B⁰ → ρ⁰γ signal is 5.2σ, confirming its observation.
• Systematic uncertainties are dominated by the modeling of the photon energy and background shapes in the invariant mass fits.