Observation of a narrow mass state decaying into ${\rm Y}(1S)+\gamma$ in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$

Using data corresponding to an integrated luminosity of $1.3{\mathrm{fb}}^{-1}$, we observe a narrow mass state decaying into ${\rm Y}(1S)+\gamma$, where the ${\rm Y}(1S)$ meson is detected by its decay into a pair of oppositely charged muons, and the photon is identified through its conversion into an electron-positron pair. The significance of this observation is 5.6 standard deviations. The mass of the state is centered at $10.551\pm 0.014(\mathrm{stat})\pm 0.017(\mathrm{syst})\mathrm{GeV}/c^2$, which is consistent with that of the state recently observed by the ATLAS Collaboration.

Figure 1

Dimuon invariant mass spectrum for opposite-charge pairs passing the muon selection criteria. The solid curve is a fit to the data assuming three ${\rm Y}$ resonances and a combinatorial background. The relative contributions from the ${\rm Y}(1S)$, ${\rm Y}(2S)$, and ${\rm Y}(3S)$ states are also shown.

Figure 2

The vertex position in the $x\mathrm{\text{−}}y$ plane for photon conversion candidates passing the photon selection requirements. The $x\mathrm{\text{−}}y$ plane is perpendicular to the beam, with $y$ pointing upwards and positive $x$ pointing to the right when viewed in the antiproton direction.

Figure 3

Mass difference $M_{\mu \mu \gamma }-M_{\mu \mu }$ for events passing all selection criteria. The curve shows the mass difference for the background model which combines ${\rm Y}(1S)$ and $\gamma$ candidates from different events, normalized to the number of data events above $1.2\mathrm{GeV}/c^2$. The hatched area shows the distribution obtained by repeating the event selection using muons with the same charge instead of those of opposite charge.

Figure 4

The distribution of $M=M_{\mu \mu \gamma }-M_{\mu \mu }+m_{{\rm Y}(1S)}$ fit with three signal functions and the mixed-event background.