Observation of a ${\chi }_{c2}^{\prime }$ Candidate in $\gamma \gamma \to D\overline{D}$ Production at Belle

We report on a search for new resonant states in the process $\gamma \gamma \to D\overline{D}$. A candidate $C$-even charmonium state is observed in the vicinity of $3.93\mathrm{GeV}/c^2$. The production rate and the angular distribution in the $\gamma \gamma$ center-of-mass frame suggest that this state is the previously unobserved ${\chi }_{c2}^{\prime }$, the $2{}^{3}P_2$ charmonium state.

Figure 1

Invariant-mass distributions of (a) $K^{\mp }{\pi }^{\pm }$ in N4 candidate events, (b) $K^{\pm }{\pi }^{\mp }{\pi }^0$ in N5 candidate events, (c) $K^{\mp }{\pi }^{\pm }$ in N6 candidate events, (d) $K^{\pm }{\pi }^{\mp }{\pi }^{\pm }{\pi }^{\mp }$ in N6 candidate events, and (e) $K^{\pm }{\pi }^{\mp }{\pi }^{\mp }$ in C6 candidate events. An accompanying $D$ meson candidate is required in each event sample.

Figure 2

Invariant-mass distributions for the (a) $D^0{\overline{D}}^0$ channels and (b) the $D^{+}{D}^{-}$ mode. (c) The combined $M(D\overline{D})$ distribution. The curves show the fits with (solid line) and without (dashed line) a resonance component. The histogram shows the distribution of the events from the $D$-mass sidebands (see the text).

Figure 3

Experimental $P_t(D\overline{D})$ distribution (points with error bars) for events in the $3.91<M(D\overline{D})<3.95\mathrm{GeV}/c^2$ region and the fit (histogram) based on the exclusive $\gamma \gamma \to D\overline{D}$ process MC plus a linear background (dotted line). The dot-dashed line shows the location of the $P_t$ selection requirement. The dashed histogram shows the expected distribution of the $\gamma \gamma \to D{\overline{D}}^{*}$ process followed by ${\overline{D}}^{*}\to \overline{D}\pi$ with an arbitrary normalization.

Figure 4

$M(D\overline{D})$ distributions for (a) $|\cos {\theta }^{*}|<0.5$ and (b) $|\cos {\theta }^{*}|>0.5$. (c) The $|\cos {\theta }^{*}|$ distributions in the $3.91<M(D\overline{D})<3.95\mathrm{GeV}/c^2$ region (points with error bars) and background scaled from the $M(D\overline{D})$ sideband (solid histogram). The solid and dashed curves are expected distributions for the spin-2 (helicity-2) and spin zero hypotheses, respectively, and contain the nonpeak background also shown separately by the dotted curve.