[Paper Review] Recent results from SND detector at VEPP-2M

М. Н. Ачасов, V.M. Aulchenko+28 more|ArXiv.org|Oct 31, 2000

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents high-precision measurements from the SND detector at VEPP-2M, reporting the first observation of the rare radiative decay ρ→π⁰π⁰γ and a new measurement of ω→π⁰π⁰γ, both based on 20 million φ meson decays. It confirms significant branching fractions for φ→f₀(980)γ and φ→a₀(980)γ, suggesting non-conventional four-quark structure for these scalar mesons.

ABSTRACT

The current status of experiments with SND detector at VEPP-2M $e^+e^-$ collider in the energy range $2E_0=0.4-1.4$ GeV is given. The new results of analysis of $ϕ$ decay into $π^0π^0γ$, $ηπ^0γ$ are based on the full SND statistics corresponding 20 million of $ϕ$ decay. New measurement of $ω oπ^0π^0γ$ decay and a first observation of $ρ oπ^0π^0γ$ are presented. The accuracy of many other rare decays of light vector mesons was improved. In the energy range $2E_0=1.0\div1.4$ GeV the cross sections of the processes $e^+e^- oωπ^0$ and $e^+e^- oπ^+π^-π^0$ were measured. The results of the fitting of data are discussed.

Motivation & Objective

• To measure rare radiative decays of light vector mesons (φ, ω, ρ) in the 0.4–1.4 GeV energy range.
• To investigate the nature of scalar mesons f₀(980) and a₀(980) via their decays to π⁰π⁰γ and ηπ⁰γ final states.
• To improve the accuracy of branching ratios for rare φ decays and measure e⁺e⁻ cross sections for ωπ⁰ and π⁺π⁻π⁰ processes.
• To test models of resonance structure, particularly the role of energy-dependent widths in ρ′ and ω′ resonances.
• To provide input for future high-luminosity studies at VEPP-2000, aiming for definitive resonance parameters.

Proposed method

• Analysis of 32 pb⁻¹ of integrated luminosity collected by the SND detector at VEPP-2M, covering φ, ω, and ρ/ω energy regions.
• Use of a 3-layer spherical NaI(Tl) electromagnetic calorimeter with 90% 4π coverage and 1620 crystals for photon detection.
• Employment of two cylindrical drift chambers for charged particle tracking with 98% 4π angular coverage.
• Application of background subtraction and detection efficiency corrections to π⁰π⁰ and ηπ⁰ invariant mass spectra.
• Fitting of resonance shapes using both constant-width (Model 1) and energy-dependent width (Model 2, 3) parametrizations.
• Combination of SND data with CLEO and DM2 data for global fits of e⁺e⁻ → ωπ⁰ and e⁺e⁻ → π⁺π⁻π⁰ cross sections.

Experimental results

Research questions

• RQ1 What are the branching fractions for φ→π⁰π⁰γ and φ→ηπ⁰γ decays, and do they indicate resonant contributions from f₀(980) and a₀(980)?
• RQ2 Can the observed mass spectra in φ→π⁰π⁰γ and φ→ηπ⁰γ decays be explained by scalar meson intermediate states with kaon loop contributions?
• RQ3 Is the first observation of ρ→π⁰π⁰γ consistent with vector dominance model (VDM) and resonance exchange mechanisms?
• RQ4 How do energy-dependent width models affect the extracted parameters of ρ′ and ω′ resonances in e⁺e⁻ → ωπ⁰ and e⁺e⁻ → π⁺π⁻π⁰ processes?
• RQ5 What is the impact of phase space energy dependence on the fitting of resonance parameters in vector meson resonances?

Key findings

• The branching fraction for φ→π⁰π⁰γ was measured as (1.22 ± 0.12) × 10⁻⁴, with 419 ± 31 events observed.
• The branching fraction for φ→ηπ⁰γ was measured as (0.88 ± 0.17) × 10⁻⁴, with 36 ± 6 events observed.
• The decay φ→f₀(980)γ was extracted with a branching fraction of (3.5 ± 0.3⁺¹.³₋₀.⁵) × 10⁻⁴, indicating a significant contribution from the f₀(980).
• The decay φ→a₀(980)γ was measured with a branching fraction of (0.88 ± 0.17) × 10⁻⁴, suggesting non-conventional quark structure.
• The first observation of ρ→π⁰π⁰γ was confirmed, with a branching fraction consistent with VDM predictions involving ρ→ωπ⁰→π⁰π⁰γ.
• Fits to e⁺e⁻ → ωπ⁰ and e⁺e⁻ → π⁺π⁻π⁰ cross sections showed that energy-dependent width models (e.g., Γ ∝ q³/(1 + (qR)²)) yield better agreement with data than constant-width models, especially for ω′ resonance parameters.