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Observation of $e^{+} e^{-} to gamma X(3872)$ at BESIII

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 Added by Ke Li
 Publication date 2013
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and research's language is English




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With data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies from 4.009 to 4.420 GeV, the process $e^{+} e^{-} to gamma X(3872)$ is observed for the first time with a statistical significance of $6.3sigma$. The measured mass of the $X(3872)$ is ($3871.9pm 0.7_{rm stat.}pm 0.2_{rm sys.}$) MeV/$c^2$, in agreement with previous measurements. Measurements of the product of the cross section $sigma[e^{+} e^{-} to gamma X(3872)]$ and the branching fraction $mathcal{B}[X(3872) to pi^{+} pi^{-} J/psi]$ at center-of-mass energies 4.009, 4.229, 4.260, and 4.360 GeV are reported. Our measurements are consistent with expectations for the radiative transition process $Y(4260) to gamma X(3872)$.



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We study the $e^+e^-togammaomega J/psi$ process using $11.6 ~rm fb^{-1}$ $e^+ e^-$ annihilation data taken at center-of-mass energies from $sqrt{s}=4.008~rm GeV$ to $4.600~rm GeV$ with the BESIII detector at the BEPCII storage ring. The $X(3872)$ resonance is observed for the first time in the $omega J/psi$ system with a significance of more than $5sigma$. The relative decay ratio of $X(3872)toomega J/psi$ and $pi^+pi^- J/psi$ is measured to be $mathcal{R}=1.6^{+0.4}_{-0.3}pm0.2$, where the first error is statistical and the second systematic (the same hereafter). The $sqrt{s}$-dependent cross section of $e^+e^-togamma X(3872)$ is also measured and investigated, and it can be described by a single Breit-Wigner resonance, referred to as the $Y(4200)$, with a mass of $4200.6^{+7.9}_{-13.3}pm3.0~{rm MeV}/c^2$ and a width of $115^{+38}_{-26}pm12~{rm MeV}$. In addition, to describe the $omega J/psi$ mass distribution above $3.9~rm GeV/c^2$, we need at least one additional Breit-Wigner resonance, labeled as $X(3915)$, in the fit. The mass and width of the $X(3915)$ are measured to be $3926.4pm2.2pm1.2~rm MeV/c^2$ and $3.8pm7.5pm2.6~rm MeV$, or $3932.6pm8.7pm4.7~rm MeV/c^2$ and $59.7pm15.5pm3.7~rm MeV$, respectively, depending on the fit models. The resonant parameters of the $X(3915)$ agree with those of the $Y(3940)$ in $Bto Komega J/psi$ and of the $X(3915)$ in $gammagammatoomega J/psi$ by the Belle and BABAR experiments within errors.
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