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Register a new userInvestigation of the $e^+e^- to omega pi^0 to pi^0 pi^0 gamma$ reaction in the energy domain near the $phi$-meson
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The $e^+e^- to omega pi^0 to pi^0 pi^0 gamma$ process was investigated in the SND experiment at the VEPP-2M collider. A narrow energy interval near the $phi$-meson was scanned. The observed cross-section reveals, at the level of three standard deviation, the interference effect caused by $phi to pi^0pi^0gamma$ decay. The cross-section parameters, as well as the real and imaginary parts of the $phi$-meson related amplitude, were measured.
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Using $e^{+}e^{-}$ collision data samples with center-of-mass energies ranging from 2.000 to 2.644 GeV, collected by the BESIII detector at the BEPCII collider, and with a total integrated luminosity of 300 pb^{-1}, a partial-wave analysis is performed for the process $e^{+}e^{-} to K^{+}K^{-}pi^{0}pi^{0}$. The total Born cross sections for the process $e^{+}e^{-} to K^{+}K^{-}pi^{0}pi^{0}$, as well as the Born cross sections f or the subprocesses $e^{+}e^{-} to phi pi^{0}pi^{0}$, $K^{+}(1460)K^{-}$, $K^{+}_{1}(1400)K^{-}$, $K^{+}_{1}(1270)K^{-}$ and $K^{*+}(892)K^{*-}(892)$, are measured versus the center-of-mass energy. The corresponding results for $e^{+}e^{-} to K^{+}K^{-}pi^{0}pi^{0}$ and $phi pi^{0}pi^{0}$ are consistent with those of BaBar and have much improved precision.By analyzing the cross sections for the four subprocesses, $K^{+}(1460)K^{-}$, $K^{+}_{1}(1400)K^{-}$, $K^{+ }_{1}(1270)K^{-}$ and $K^{*+}K^{*-}$, a structure with mass $M$ = (2126.5 $pm$ 16.8 $pm$ 12.4)~MeV/c^{2} and width $Gamma$ = (106.9 $pm$ 32.1 $pm$ 28.1)~MeV is observed with an overall statistical significance of 6.3 $sigma$, although with very limited significance in the subprocesses $e^{+}e^{-} to K^{+}_{1}(1270)K^{-}$ and $K^{*+}(892)K^{*-}(892)$. The resonant parameters of the observed structure suggest it can be identified with the $phi(2170)$, thus the results provide valuable input to the internal nature of the $phi(2170)$.
The presently world largest data sample of pi0 --> gamma e+e- decays containing nearly 5E5 events was collected using the WASA detector at COSY. A search for a dark photon U produced in the pi0 --> gamma U --> gamma e+e- decay from the pp-->pppi^0 reaction was carried out. An upper limit on the square of the U-gamma mixing strength parameter epsilon^2 of 5e-6 at 90% CL was obtained for the mass range 20 MeV <M_U< 100 MeV. This result together with other recent experimental limits significantly reduces the M_U vs. epsilon^2 parameter space preferred by the measured value of the muon anomalous magnetic moment.
A measurement of the vector to pseudoscalar conversion decay $phi to pi^0 e^+e^-$ with the KLOE experiment is presented. A sample of $sim 9500$ signal events was selected from a data set of 1.7 fb$^{-1}$ of $e^+e^-$ collisions at $sqrt{s} sim m_{phi}$ collected at the DA$Phi$NE $e^+e^-$ collider. These events were used to obtain the first measurement of the transition form factor $| F_{phi pi^0}(q^2) |$ and a new measurement of the branching ratio of the decay: $rm{BR},(phi to pi^0 e^+e^-) = (,1.35 pm 0.05^{,,+0.05}_{,,-0.10},) times 10 ^{-5}$. The result improves significantly on previous measurements and is in agreement with theoretical predictions.
By studying the invariant-mass distribution of the $e^+e^-$ in conversion decays, it is possible to learn more about mesons structure and underlying quark dynamics. At KLOE, the study of the $phi to pi^0 e^+e^-$ process is currently going to be finalized. At present, about 9000 events have been selected from a data sample of 1.7 fb$^{-1}$ from 2004/2005 data taking campaign. A very good agreement between data and Monte Carlo distributions has been achieved for all interesting kinematical variables. A preliminary invariant-mass spectrum of $e^{+}e^{-}$ is presented.
The dynamics of the process $ e^+e^- to pi^+pi^-pi^0 $ is studied in the energy region from 1.15 to 2.00 GeV using data accumulated with the SND detector at the VEPP-2000 $e^+e^-$ collider. The Dalitz plot distribution and $pi^+pi^-$ mass spectrum are analyzed in a model including the intermediate states $rho(770)pi$, $rho(1450)pi$, and $omegapi^0$. As a result, the energy dependences of the $rho(770)pi$ and $rho(1450)pi$ cross sections and the relative phases between the $rho(770)pi$ amplitude and the $rho(1450)pi $ and $omegapi^0$ amplitudes are obtained. The $rho(1450)pi$ cross section has a peak in the energy region of the $omega(1650)$ resonance (1.55-1.75 GeV). In this energy range the contributions of the $rho(770)pi$ and $rho(1450)pi$ states are of the same order of magnitude. No resonance structure near 1.65 GeV is observed in the $rho(770)pi$ cross section. We conclude that the intermediate state $rho(1450)pi$ gives a significant contribution to the decay of $omega (1650)topi^+pi^-pi^0$, whereas the $rho(770)pi$ mechanism dominates in the decay $omega(1420)topi^+pi^-pi^0$.
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