Recent Results from the SND Detector

Presented are recent results of experiments on the SND detector. Data were collected on the VEPP-2000 and VEPP-2M \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{e}^{ + }}{{e}^{ - }}$$\end{document} colliders (Novosibirsk) in the c.m. energy region 0.3–2.0 GeV. Integrated luminusity used is 70 and 25 pb–1 respectively. Cross section of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{e}^{ + }}{{e}^{ - }} \to {{\pi }^{0}}\gamma ,$$\end{document}\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{e}^{ + }}{{e}^{ - }} \to {{\pi }^{ + }}{{\pi }^{ - }}{{\pi }^{0}}\eta ,$$\end{document}\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{e}^{ + }}{{e}^{ - }} \to \omega \eta {{\pi }^{0}}$$\end{document} processes is measured.

The SND detector consists of several subsystems (Fig. 1). These are a spherical three-layer electromagnetic calorimeter made of NaI (Tl) crystals, a cylindrical tracking system (a drift and a proportional chambers in the same gas volume), an Aerogel threshold Cerenkov counter, a muon subsystem. The electromagnetic calorimeter covers 95% of the total solid angle, it has an equivalent thickness of , the energy resolution depends on the photon energy: = the angular resolution = 1.5°. The tracking system covers 94% of the total solid angle, the angular resolution in azimuth and polar directions, respectively = 45° and = 0.8°.
The integrated luminosity collected in experiments with the SND detector on VEPP-2M and VEPP-2000 colliders is summarized in Table 1.
During 2010-2013, luminosity was limited by the lack of positrons. To solve this problem, the acceleration complex undergone an upgrade. After the upgrade electrons and positrons are transferred from the VEPP-5 injection complex. The new data collection run started in December 2016. 1 The article was translated by the authors.

THE PROCESS
The cross section of the process [6] was measured using the full statistics collected at the VEPP-2M and VEPP-2000 colliders. This process has the third largest cross section in the c.m. energy region below 1 GeV. Its study makes it possible to obtain the parameters of the electromagnetic radiative decays of light vector mesons ( ), to extract information on the transition form factor .
We used the process for normalization. The general selection criteria include (apart from the hardware trigger conditions) the lack of charged tracks, large energy deposition in the calorimeter, small total momentum estimated by the calorimeter, muon veto.
The final selection is made after a kinematic fit with the constraints based on 4-momentum conservation is the recoil mass of the most energetic photon. The number of signal events is calculated as the number of parameter from the spectra fitting.
Our measurement of the process cross section is the most accurate at the present time (Fig. 2). The systematic uncertainty in the vicinity of the -meson peak is 1.4%, the main contributions are measuring the luminosity (1.2%) and the efficiency of the selection criteria (0.6%).  [7]), the ratio of the widths which is significantly larger (by 3.4σ) than the result of KLOE [8] 0.0897 ± 0.0016 . The measured value of the relative decay probability Γ ω → π γ) Γ ω → π π π = ±   For the ϕ-meson, we obtained the value 3. PROCESSES NOT STUDIED BEFORE The total hadronic cross section in the energy region below 2 GeV is calculated as a sum of exclusive hadronic cross sections. At present, its values obtained from inclusive and exclusive measurements is well agree at the boundary [9]. However, in the c.m. energy range 1.5-2.0 GeV, the set of exclusive measurements is incomplete. There is no experimental data on the The process [10] is studied for the first time. Its cross section is measured (Fig. 3), intermediate states are studied.
In addition, the cross section of the process with the same final state was considered (Fig. 5).
The dependence of the cross section on energy is described well with a single resonance model whose mass and width correspond to ρ(1700). The description without resonance is worse than this at the level of 1.2σ (Fig. 7). The cross sections of the processes and are measured for the first time. A notable contribution of the intermediate states and to the corresponding processes is also observed.
After the upgrade, the VEPP-2000 collider began data collection with an increased luminosity. The goal is to collect the integral luminosity 1 fb -1 .