Observation of $\psi (3770)\to \gamma {\chi }_{c0}$

From $e^{+}{e}^{-}$ collision data acquired with the CLEO-c detector at CESR, we search for the non-$D\overline{D}$ decays $\psi (3770)\to \gamma {\chi }_{cJ}$, with ${\chi }_{cJ}$ reconstructed in four exclusive decays modes containing charged pions and kaons. We report the first observation of such decays for $J=0$ with a branching ratio of $(0.73\pm 0.07\pm 0.06)\%$. The rates for different $J$ are consistent with the expectations assuming $\psi (3770)$ is predominantly a $1^3{D}_1$ state of charmonium, but only if relativistic corrections are applied.

Figure 1

Distribution of photon energy for $4\pi$ (top) and $2K2\pi$ (bottom) decay samples in CLEO-c $\psi (2S)$ (left) and $\psi (3770)$ (right) data. Solid histogram is data; smooth curve is fit to the data. Dashed line shows radiative return background contribution from $\psi (2S)$ tail and dotted line is polynomial background.

Figure 2

Distribution of photon energy for $6\pi$ (top) and $2K$ (bottom) decay samples in CLEO-c $\psi (2S)$ (left) and $\psi (3770)$ (right) data. Solid histogram is data; smooth curve is fit to the data. Dashed line shows radiative return background contribution from $\psi (2S)$ tail and dotted line is polynomial background.

Figure 3

Distribution of photon energy in CLEO-c $\psi (2S)$ (top) and $\psi (3770)$ (bottom) data summed over all analyzed modes (data points). The smooth curve shows the sum of the fits performed to the individual modes. The dashed curve shows the radiative tail from $\psi (2S)$. The dotted line shows the polynomial background.