Y(4260) and possible charmonium assignment

The newly reported Y(4260) becomes the second most massive state in the charmonium family. We argue that it displaces the $\psi (4415)$ as the (largely) $4s$ vector charmonium state, recall $s-d$ wave interference to explain the lack of a signal in $e^{-}{e}^{+}\to \mathrm{hadrons}$ and suggest some further study avenues that can exclude exotic meson assignments. The absence of a $J/\psi KK$ mode can be understood, beyond phase space suppression, to be a consequence of chiral symmetry. We also provide a model calculation in this sector showing that, although forcing the fit somewhat (which suggests a small sea quark wave function component), the state can be incorporated in a standard scheme.

Figure 1

Effect of a new charmonium state on $D^{+}{D}^{-}$, $D_s^{+}{D}_s^{-}$ production at 4260 MeV, with parameters from the BABAR observation. We employ standard Particle Data Group parameters for the known resonances (dotted line). For comparison, the total $R(e^{-}{e}^{+}\to \mathrm{hadrons})$ is given. The nonresonant $D$, $D_s$ production as well as light quark production (at the 2.4 level) is also added to the calculation for a more meaningful comparison. Observe that the spectrum looks qualitatively much better if the $d$-wave resonances $\psi (4160)$ and $\psi (4440)$ are taken to have negative coupling to the double pseudoscalar channel, as calculated by Godfrey and Isgur (dashed line). Finally, reparametrizing the resonances with $M_{\psi (3s)}=4.02\mathrm{GeV}$, ${\Gamma }_{\psi (3s)}=80\mathrm{MeV}$, ${\Gamma }_{\psi (3s)}^{e^{-}{e}^{+}}=1.5\mathrm{keV}$, $M_{\psi (2d)}=4.19\mathrm{GeV}$, and ${\Gamma }_{\psi (2d)}^{e^{-}{e}^{+}}=1.4\mathrm{keV}$ arbitrarily fits total $D$ production, although a large fraction should proceed by an intermediate $D^{*}$.

Figure 2

$R^{D_s{D}_s}=[\sigma (e^{-}{e}^{+}\to D_s^{+}{D}_s^{-})/\sigma (e^{-}{e}^{+}\to {\mu }^{+}{\mu }^{-})]$.