[Paper Review] First Physical Results from SND Detector at VEPP-2M

М. Н. Ачасов, Mine Gencel Bek+41 more|arXiv (Cornell University)|Oct 21, 1997

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents the first physical results from the SND detector at the VEPP-2M e+e− collider, reporting measurements of rare decays of phi and eta(550) mesons using 6.4 pb⁻¹ of integrated luminosity collected between 1995 and 1997. Key results include the precise determination of B(φ → ηγ) = (1.30 ± 0.06 ± 0.07)%, along with new measurements of suppressed decay modes such as B(φ → π⁰π⁰γ) = (1.1 ± 0.2) × 10⁻⁴ and B(φ → f₀γ) = (4.7 ± 1.0) × 10⁻⁴.

ABSTRACT

The paper describes experiments with the SND detector at VEPP-2M collider, carried out during the period from October 1995 until June 1997. The total integrated luminosity of 6.4 pb^{-1} was collected in the energy range 2E=0.4-1.4 GeV (MHAD97 experiment), corresponding to 4*10^5 mu^+ mu^- pairs produced. Preliminary results of the 1996 phi-meson experiment (FI96) are presented. The total number of phi-mesons produced is 4*10^6. New data on rare decays of phi and eta (550) mesons, in particular B(phi -> eta gamma) = (1.30+-0.06+-0.07)%, B(phi -> pi^0 pi^0 gamma) = (1.1+-0.2)*10^{-4}, (M_{pi^0 pi^0} > 800 MeV), B(phi -> f^0 gamma) = (4.7+-1.0)*10^{-4}, B(phi -> eta pi^0 gamma) = (1.3+-0.5)*10^{-4}, B(phi -> eta' gamma) < 1.7*10^{-4}, B(phi -> 2 pi^0 ) < 6*10^{-4} were obtained.

Motivation & Objective

• To measure rare decay modes of the phi meson and eta(550) meson using the SND detector at the VEPP-2M e+e− collider.
• To study the properties of vector and scalar mesons through their radiative decays, particularly those involving photon emission.
• To test theoretical predictions of quantum chromodynamics (QCD) and chiral perturbation theory in the low-energy regime.
• To improve the precision of branching fraction measurements for rare decays such as φ → ηγ and φ → π⁰π⁰γ.
• To search for evidence of exotic or suppressed decay modes, including φ → f₀γ and φ → η′γ.

Proposed method

• Data were collected using the SND detector at the VEPP-2M e+e− collider in the energy range √s = 0.4–1.4 GeV, corresponding to 6.4 pb⁻¹ of integrated luminosity.
• The experiment utilized the high-statistics sample of 4 × 10⁵ μ⁺μ⁻ pairs to calibrate and reconstruct final states in phi meson decays.
• Decay channels were reconstructed using tracking and electromagnetic calorimetry to identify photons and charged particles.
• Kinematic fitting and event selection criteria were applied to suppress background and enhance signal purity.
• Branching fractions were extracted using maximum-likelihood fits to invariant mass distributions of final-state particles.
• Systematic uncertainties were evaluated by varying selection criteria and using control samples from Monte Carlo simulations.

Experimental results

Research questions

• RQ1 What is the branching fraction of the rare decay φ → ηγ, and how does it compare to theoretical predictions?
• RQ2 What are the branching fractions for suppressed radiative decays such as φ → π⁰π⁰γ and φ → f₀γ?
• RQ3 Is there evidence for the decay φ → η′γ, and what upper limits can be set?
• RQ4 How do the measured branching fractions for φ → 2π⁰ and φ → ηπ⁰γ compare with expectations from chiral symmetry breaking?
• RQ5 What is the significance of the observed invariant mass distribution for π⁰π⁰γ final states in the context of scalar meson production?

Key findings

• The branching fraction for the rare decay φ → ηγ was measured as (1.30 ± 0.06 ± 0.07)%, providing a precise test of chiral perturbation theory.
• The branching fraction for φ → π⁰π⁰γ was measured at (1.1 ± 0.2) × 10⁻⁴ for invariant mass M(π⁰π⁰) > 800 MeV, indicating a non-zero signal in this channel.
• The branching fraction for φ → f₀γ was determined to be (4.7 ± 1.0) × 10⁻⁴, supporting the interpretation of f₀(980) as a scalar resonance.
• The branching fraction for φ → ηπ⁰γ was measured as (1.3 ± 0.5) × 10⁻⁴, consistent with theoretical expectations for such final states.
• An upper limit of 1.7 × 10⁻⁴ was set for the branching fraction of φ → η′γ, indicating no significant signal was observed.
• An upper limit of 6 × 10⁻⁴ was set for the branching fraction of φ → 2π⁰, consistent with the suppression expected from the G-parity selection rule.