Observation of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup><mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo></mml:mrow></mml:msup><mml:mo stretchy="false">→</mml:mo><mml:mi>γ</mml:mi><mml:mi>X</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mn>3872</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:math>at BESIII

Abstract

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 ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\gamma}X(3872)$ is observed for the first time with a statistical significance of $6.3\ensuremath{\sigma}$. The measured mass of the $X(3872)$ is $(3871.9\ifmmode\pm\else\textpm\fi{}0.{7}_{\text{stat}}\ifmmode\pm\else\textpm\fi{}0.{2}_{\text{syst}})\text{ }\text{ }\mathrm{MeV}/{c}^{2}$, in agreement with previous measurements. Measurements of the product of the cross section $\ensuremath{\sigma}[{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\gamma}X(3872)]$ and the branching fraction $\mathcal{B}[X(3872)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}J/\ensuremath{\psi}]$ 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 $Y(4260)\ensuremath{\rightarrow}\ensuremath{\gamma}X(3872)$.