L. Sun

Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS(2), a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS(2) crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS(2) provides new opportunities for engineering the electronic structure of matter at the nanoscale.

We present a study of the decays ${B}^{0,+}\ensuremath{\rightarrow}J/\ensuremath{\psi}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}{K}^{0,+}$, using $467\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $B\overline{B}$ pairs recorded with the BABAR detector. We present evidence for the decay mode $X(3872)\ensuremath{\rightarrow}J/\ensuremath{\psi}\ensuremath{\omega}$, with product branching fractions $\mathcal{B}({B}^{+}\ensuremath{\rightarrow}X(3872){K}^{+})\ifmmode\times\else\texttimes\fi{}\mathcal{B}(X(3872)\ensuremath{\rightarrow}J/\ensuremath{\psi}\ensuremath{\omega})=[0.6\ifmmode\pm\else\textpm\fi{}0.2(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.1(\mathrm{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$, and $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}X(3872){K}^{0})\ifmmode\times\else\texttimes\fi{}\mathcal{B}(X(3872)\ensuremath{\rightarrow}J/\ensuremath{\psi}\ensuremath{\omega})=[0.6\ifmmode\pm\else\textpm\fi{}0.3(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.1(\mathrm{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. A detailed study of the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}$ mass distribution from $X(3872)$ decay favors a negative-parity assignment.

We present a study of resonances in exclusive decays of $B$ mesons to ${\overline{D}}^{(*)}{D}^{(*)}K$. We report the observation of the decays $B\ensuremath{\rightarrow}{\overline{D}}^{(*)}{D}_{s1}^{+}(2536)$ where the ${D}_{s1}^{+}(2536)$ is reconstructed in the ${D}^{*0}{K}^{+}$ and ${D}^{*+}{K}_{S}^{0}$ decay channels. We report also the observation of the decays $B\ensuremath{\rightarrow}\ensuremath{\psi}(3770)K$ where the $\ensuremath{\psi}(3770)$ decays to ${\overline{D}}^{0}{D}^{0}$ and ${D}^{\ensuremath{-}}{D}^{+}$. In addition, we present the observation of an enhancement for the ${\overline{D}}^{*0}{D}^{0}$ invariant mass in the decays $B\ensuremath{\rightarrow}{\overline{D}}^{*0}{D}^{0}K$, at a mass of $({3875.1}_{\ensuremath{-}0.5}^{+0.7}\ifmmode\pm\else\textpm\fi{}0.5)\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ with a width of $({3.0}_{\ensuremath{-}1.4}^{+1.9}\ifmmode\pm\else\textpm\fi{}0.9)\text{ }\text{ }\mathrm{MeV}$ (the first errors are statistical and the second are systematic). Branching fractions and spin studies are shown for the three resonances. The results are based on $347\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data collected with the BABAR detector at the PEP-II $B$ factory.

A search for charmonium and other new states is performed in a study of exclusive initial-state-radiation production of $D\overline{D}$ events from electron-positron annihilations at a center-of-mass energy of 10.58 GeV. The data sample corresponds to an integrated luminosity of $384\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ and was recorded by the BABAR experiment at the PEP-II storage ring. The $D\overline{D}$ mass spectrum shows clear evidence of the $\ensuremath{\psi}(3770)$ plus other structures near 3.9, 4.1, and $4.4\text{ }\text{ }\mathrm{GeV}/{c}^{2}$. No evidence for $Y(4260)\ensuremath{\rightarrow}D\overline{D}$ is observed, leading to an upper limit of $\mathcal{B}(Y(4260)\ensuremath{\rightarrow}D\overline{D})/\mathcal{B}(Y(4260)\ensuremath{\rightarrow}J/\ensuremath{\psi}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})<1.0$ at 90% confidence level.