arXiv subjects
Publications and source records attributed to M. N. Achasov.
We present a measurement of the $e^+e^-\to K_SK_L$ cross section in the energy range $\sqrt{s}=1.04 ÷1.38$ GeV. For the energy $\sqrt{s}\geq 1.2$ GeV the cross section exceeds vector meson dominance model predictions with only $ρ(770)$, $ω(783)$, and $ϕ(1020)$ mesons taken into account. Measured cross section agrees well with previous measurements.
The corrected cross section of the e^+e^-\toπ^+π^- process measured in the SND experiment at the VEPP-2M e^+e^- collider is presented. The update is necessary due to a flaw in the e^+e^-\toπ^+π^- and e^+e^-\toμ^+μ^- Monte Carlo events generators used previously in data analysis.
The update of the e^+e^-\toπ^+π^- process cross section, measured in the energy region \sqrt{s}<1 GeV with SND detector at VEPP-2M collider is presented.
The review of experimental results of the light vector mesons parameters studies with CMD-2 and SND detectors at VEPP-2M e+e- collider in the energy region 360 < sqrt{s} < 1380 MeV is given.
New results of the e+e- -> pi+pi-, K+K-, K_SK_L, pi0/gamma, eta0/gamma processes cross section measurements are presented. The results are based on the 30 pb^-1 data, accumulated by SND detector at VEPP-2M e+e- collider in the energy range sqrt(s)=0.4-1.4 GeV during 1995-2000 years. The comparison with existing experimental data shows that the measurement accuracy is close to or better than the world average. For the e+e- -> pi+pi- process the accuracy is about 0.01. This is important for calculation of hadronic contribution into the muon anomalous magnetic moment.
The cross section of the process e^+e^-\to π^+π^- was measured in the SND experiment at the VEPP-2M collider in the energy region 400<\sqrt[]{s}<1000 MeV. This measurement was based on about 12.4 \times 10^6 selected collinear events, which include 7.4\times 10^6 e^+e^-\to e^+e^-, 4.5\times 10^6 e^+e^-\toπ^+π^- and 0.5\times 10^6 e^+e^-\toμ^+μ^- selected events. The systematic uncertainty of the cross section determination is 1.3 %. The ρ-meson parameters were determined: m_ρ=774.9\pm 0.4\pm 0.5 MeV, Γ_ρ=146.5\pm 0.8\pm 1.5 MeV, σ(ρ\toπ^+π^-)=1220\pm 7\pm 16 nb as well as the parameters of the G-parity suppressed decay ω\toπ^+π^-: σ(ω\toπ^+π^-)=29.9\pm 1.4\pm 1.0 nb and ϕ_{ρω} = 113.5\pm 1.3\pm 1.7 degree.
The process e^+e^-\toπ^0γin the energy range \sqrt{s}=0.60-1.06 GeV was studied at VEPP-2M collider with SND detector using 14 pb^{-1} of integrated luminosity. Data were analyzed in the framework of the vector meson dominance model. Preliminary results on obtained cross-section and parameters of ρ^0, ω,ϕ\to π^0γradiative decays are presented.
The review of the SND results of the $e^+e^-\toπ^+π^-π^0$ process study in the energy range $\sqrt[]{s}=0.42$ -- 1.38 GeV at VEPP-2M collider, based on about $2\times 10^6$ selected events, is presented. The total cross section, parameters of the $ρ$, $ω$, $ϕ$ resonances, and $ω^\prime$, $ω^{\prime\prime}$ states were obtained. It was found that $ρπ$ and $ωπ^0$ intermediate states describe the reaction dynamics. The experimental data cannot be described by a sum of only $ω$, $ϕ$, $ω^\prime$ and $ω^{\prime\prime}$ resonances contributions. This can be interpreted as a manifestation of the $ρ\to 3π$ decay, suppressed by $G$-parity, with relative probability $B(ρ\to 3π) = (1.01\pm^{0.54}_{0.36}\pm 0.034) \times 10^{-4}$.
The cross section of the process $e^+e^-\to π^+π^-π^0$ was measured in the Spherical Neutral Detector (SND) experiment at the VEPP-2M collider in the energy region $\sqrt[]{s}$ below 980 MeV. This measurement was based on about $1.2 \times 10^6$ selected events. The obtained cross section was analyzed together with the SND and DM2 data in the energy region $\sqrt[]{s}$ up to 2 GeV. The $ω$-meson parameters: $m_ω=782.79\pm 0.08\pm 0.09$ MeV, $Γ_ω=8.68\pm 0.04\pm 0.15$ MeV and $σ(ω\to 3π)=1615\pm 9\pm 57$ nb were obtained. It was found that the experimental data cannot be described by a sum of only $ω$, $ϕ$, $ω^\prime$ and $ω^{\prime\prime}$ resonances contributions. This can be interpreted as a manifestation of $ρ\to 3π$ decay, suppressed by $G$-parity, with relative probability $B(ρ\to 3π) = (1.01\pm^{0.54}_{0.36}\pm 0.034) \times 10^{-4}$.
Results of the study of the e+e-->pi0 gamma process with SND detector at VEPP-2M collider in the c.m.s. energy range sqrt(s)=0.60-0.97 GeV are presented. Using 36513 selected events corresponding to a total integrated luminosity of 3.4 pb^-1 the e+e-->pi0 gamma cross section was measured. The energy dependence of the cross section was analyzed in the framework of the vector meson dominance model. The data are well described by a sum of phi,omega,rho0->pi0 gamma decay contributions with measured decay probabilities: Br(omega->pi0 gamma)=(9.34+-0.15+-0.31)% and Br(rho0->pi0 gamma)=(5.15+-1.16+-0.73)*10^-4 . The rho-omega relative interference phase is phi(rho,omega}=(-10.2+-6.5+-2.5) degree.
The cross section of the process $e^+e^-\to π^+π^-π^0$ was measured in the Spherical Neutral Detector experiment at the VEPP-2M collider in the energy region $\sqrt[]{s} = 980 ÷1380$ MeV. The measured cross section, together with the $e^+e^-\to π^+π^-π^0$ and $ωπ^+π^-$ cross sections obtained in other experiments, was analyzed in the framework of the generalized vector meson dominance model. It was found that the experimental data can be described by a sum of $ω$, $ϕ$ mesons and two $ω^\prime$ and $ω^{\prime\prime}$ resonances contributions, with masses $m_{ω^\prime}\sim 1490$,$m_{ω^{\prime\prime}}\sim 1790$ MeV and widths $Γ_{ω^\prime}\sim 1210$, $Γ_{ω^{\prime\prime}}\sim 560$ MeV. The analysis of the $π^+π^-$ invariant mass spectra in the energy region $\sqrt[]{s}$ from 1100 to 1380 MeV has shown that for their descriptionone should take into account the $e^+e^-\toωπ^0\toπ^+π^-π^0$ mechanism also. The phase between the amplitudes corresponding to the $e^+e^-\toωπ$ and $e^+e^-\toρπ$ intermediate states was measured for the first time. The value of the phase is close to zero and depends on energy.
The e+e- -> pi0 pi0 gamma process was studied in the SND experiment at VEPP-2M e+e- collider in the energy region 0.60-0.97 GeV. From the analysis of the energy dependence of measured cross section the branching ratios B(omega -> pi0 pi0 gamma)= (6.6 +1.4-0.8(stat) +-0.6(syst))x10^-5 and B(rho -> pi0 pi0 gamma)=(4.1 +1.0-0.9(stat) +-0.3(syst))x10^-5 were obtained.
The $e^+e^-\toηγ\to 7γ$ process was studied in the energy range $2E=600÷1060$ MeV with an SND detector on a VEPP-2M $e^+e^-$ collider. The decay branching ratios $B(ϕ\toηγ)=(1.343\pm 0.012\pm 0.055)\cdot 10^{-2}$, $B(ω\toηγ)=(4.60\pm 0.72\pm 0.19)\cdot 10^{-4}$, and $B(ρ\toηγ)=(2.69\pm 0.32\pm 0.16)\cdot 10^{-4}$ were measured.
The review of experimental results obtained with SND detector at VEPP-2M $e^+e^-$ collider in the energy region $\sqrt[]{s}=0.36$ -- 1.38 GeV is given. The presented results include the following items: studies of the light vector mesons radiative decays, OZI-rule and G-parity suppressed $ϕ$-meson rare decays, $ϕ$-meson parameters measurements, studies of $e^+e^-\toπ^+π^-π^0$ process dynamics, $η$ and $K_S$ mesons rare decays, $η$ and $ϕ$ mesons conversion decays, and study of the $e^+e^-$ annihilation into hadrons.
The invariant mass spectra of the $π^+π^-$ and $π^\pmπ^0$ pairs in the process $e^+e^- \to π^+π^-π^0$ were studied in the SND experiment at the VEPP-2M collider in the energy region $\sqrt[]{s} \simeq 1020$ MeV. These studies were based on about $0.5 \times 10^6$ experimental events. The spectra were analyzed in the framework of the vector meson dominance model. It was found that the experimental data can be described with $e^+e^- \to ρπ\to π^+π^-π^0$ transition only. Upper limit on the branching ratio of the $ϕ(1020)\toπ^+π^-π^0$ decay through intermediate states different from $ρπ$ was obtained at the 90 % confidence level: $B(ϕ\toπ^+π^-π^0)<6 \cdot 10^{-4}$. The $ρ$-meson mass and width which follow from the spectra analysis are $m_ρ=775.0\pm 1.3$ MeV, $Γ_ρ=150.4 \pm 3.0$ MeV. Neutral and charged $ρ$-mesons mass difference was found to equal $m_{ρ^\pm}-m_{ρ^0}=-1.3\pm2.3$ MeV. In the $π^+π^-$ mass spectrum the $ρ-ω$ interference was seen at two standard deviations level.
The program of upgrade of the Spherical Neutral Detector for future experiments at a new VEPP-2000 $e^+e^-$ collider is presented. Modernization includes upgrades of electromagnetic calorimeter, tracking system, detector electronics, data acquisition system, and offline software. It is also planned to equip the detector with two new subsystems: particle identification system based on aerogel Cherenkov counters and external electron tagging system for $γγ$ physics.
The results of tests of 1'' vacuum phototriodes in a magnetic field up to 4.5 T are presented. It was found that output amplitude decreases by about 6 % per tesla in the magnetic field range from 2.0 to 4.0 T. For devices with an anode mesh pitch of 16$μ$m, the output amplitude at 4.0 T is 30 % lower than that at zero field.
The cross sections of the processes $e^+e^- \to K^+K^-$, $e^+e^- \to K_SK_L$ and $e^+e^- \to π^+π^-π^0$ were measured in the SND experiment at the VEPP-2M collider in the energy region near the $ϕ(1020)$ meson. These measurements were based on about $10^6$ $K^+K^-$, $0.5 \times 10^6$ $K_SK_L$ and $0.4 \times 10^6$ $π^+π^-π^0$ selected events. Themeasured cross sections have been analyzed in the framework of the vector meson dominance model and the main parameters of the $ϕ$-resonance were obtained, such as its mass, width, the production cross section and branching ratios of the main decay modes. The measured value of the $ϕ$ meson total width, $Γ_ϕ = 4.21 \pm 0.04$ is lower than the present world average of $4.458 \pm 0.032$ MeV. Contributions in addition to the conventional vector meson dominance model were found in the $e^+e^- \to π^+π^-π^0$ reaction cross section.