First measurements of exclusive hadronic decays of ${\rm Y}(1S)$ and ${\rm Y}(2S)$

Using data taken with the CLEO III detector, $1.09{\mathrm{fb}}^{-1}$ at ${\rm Y}(1S)$ and $1.28{\mathrm{fb}}^{-1}$ at ${\rm Y}(2S)$, branching fractions have been measured for the first time for exclusive decays of each resonance into different final states consisting of 4–10 light hadrons, pions (including up to $2{\pi }^0$), kaons, and protons. Significant strength is found in 73 decay modes of ${\rm Y}(1S)$ and 17 decay modes of ${\rm Y}(2S)$, with branching fractions ranging from $0.3\times {10}^{-5}$ to $110\times {10}^{-5}$. Upper limits at 90% confidence level are presented for the other decay modes.

Figure 1

Distributions of $E/p$ for reconstructed events for the on-${\rm Y}(1S)$ data (left column), for the on-${\rm Y}(2S)$ data (middle column), and for the sum of the off-${\rm Y}(1S)$ and off-${\rm Y}(2S)$ data (right column). A significant contribution from events containing electrons ($E/p\approx 1$) is seen in the all-charged hadrons decays. The vertical lines show the cut of $0.9<E/p<1.1$ used to reject events containing electrons in all cases.

Figure 2

Distributions of maximum penetration depth in the muon chambers for reconstructed events from the data (points) and ${\rm Y}(nS)\to \mathrm{\text{hadrons}}$ MC (shaded histogram): for the on-${\rm Y}(1S)$ data (left column), for the on-${\rm Y}(2S)$ data (middle column), and for the sum of the off-${\rm Y}(1S)$ and off-${\rm Y}(2S)$ data (right column). In all cases, there is a clear excess in the data over MC predictions at large penetration depth. The vertical line at a depth of 7 shows the cut used to reject events containing muons.

Figure 3

Distributions of mass recoiling against pairs of pions in the ${\rm Y}(2S)$ data. In the three spectra with recoil against ${\pi }^{+}{\pi }^{-}$, signals for the decay ${\rm Y}(2S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)$ are seen, and events with a recoil mass of $M({\rm Y}(1S))\pm 15\mathrm{MeV}$ are rejected, as illustrated by the dashed vertical lines. In the spectrum with recoils against ${\pi }^0{\pi }^0$ (bottom right), the dashed vertical lines denote the region in which only 15–20 ${\rm Y}(1S)$ counts are expected.

Figure 4

A selection of mass distributions $\Delta M\equiv M({\rm Y}(nS))-M(\mathrm{\text{hadrons}})$ for individual decay modes in the ${\rm Y}(1S)$ and ${\rm Y}(2S)$ data taken on-resonance (open histograms) and off-resonance (shaded histograms). The off-resonance data have been scaled by the luminosity ratios $\mathcal{R}(1S)={\mathcal{L}}_{\mathrm{on}}(1S)/{\mathcal{L}}_{\mathrm{off}}=1.09/0.63=1.73$ for ${\rm Y}(1S)$, and $\mathcal{R}(2S)={\mathcal{L}}_{\mathrm{on}}(2S)/{\mathcal{L}}_{\mathrm{off}}=1.28/0.63=2.03$ for ${\rm Y}(2S)$.

Figure 5

Sums of branching fractions for $\sum _{100}\mathcal{B}({\rm Y}(nS)\to X)$ as a function of hadron multiplicity. Only statistical errors are shown. The ${\rm Y}(2S)\times 4.1$ values are plotted slightly displaced for clarity.

Figure 6

Mass distributions $\Delta M\equiv M({\rm Y}(1S))-M(\mathrm{\text{hadrons}})$ for individual decay modes in on-resonance ${\rm Y}(1S)$ data and the sum of off-resonance ${\rm Y}(1S)$ and ${\rm Y}(2S)$ data. The off-resonance data has been scaled by the luminosity ratio ${\mathcal{L}}_{\mathrm{on}}/{\mathcal{L}}_{\mathrm{off}}=1.73$. The ordinate scale is $\mathrm{\text{counts}}/20\mathrm{MeV}$.

Figure 7

Mass distributions $\Delta M\equiv M({\rm Y}(1S))-M(\mathrm{\text{hadrons}})$ for individual decay modes in on-resonance ${\rm Y}(1S)$ data and the sum of off-resonance ${\rm Y}(1S)$ and ${\rm Y}(2S)$ data. The off-resonance data has been scaled by the luminosity ratio ${\mathcal{L}}_{\mathrm{on}}/{\mathcal{L}}_{\mathrm{off}}=1.73$. The ordinate scale is $\mathrm{\text{counts}}/20\mathrm{MeV}$.

Figure 8

Mass distributions $\Delta M\equiv M({\rm Y}(1S))-M(\mathrm{\text{hadrons}})$ for individual decay modes in on-resonance ${\rm Y}(1S)$ data and the sum of off-resonance ${\rm Y}(1S)$ and ${\rm Y}(2S)$ data. The off-resonance data has been scaled by the luminosity ratio ${\mathcal{L}}_{\mathrm{on}}/{\mathcal{L}}_{\mathrm{off}}=1.73$. The ordinate scale is $\mathrm{\text{counts}}/20\mathrm{MeV}$.

Figure 9

Mass distributions $\Delta M\equiv M({\rm Y}(1S))-M(\mathrm{\text{hadrons}})$ for individual decay modes in on-resonance ${\rm Y}(1S)$ data and the sum of off-resonance ${\rm Y}(1S)$ and ${\rm Y}(2S)$ data. The off-resonance data has been scaled by the luminosity ratio ${\mathcal{L}}_{\mathrm{on}}/{\mathcal{L}}_{\mathrm{off}}=1.73$. The ordinate scale is $\mathrm{\text{counts}}/20\mathrm{MeV}$.

Figure 10

Mass distributions $\Delta M\equiv M({\rm Y}(2S))-M(\mathrm{\text{hadrons}})$ for individual decay modes in on-resonance ${\rm Y}(2S)$ data and the sum of off-resonance ${\rm Y}(1S)$ and ${\rm Y}(2S)$ data. The off-resonance data has been scaled by the luminosity ratio ${\mathcal{L}}_{\mathrm{on}}/{\mathcal{L}}_{\mathrm{off}}=2.03$. The ordinate scale is $\mathrm{\text{counts}}/20\mathrm{MeV}$.

Figure 11

Mass distributions $\Delta M\equiv M({\rm Y}(2S))-M(\mathrm{\text{hadrons}})$ for individual decay modes in on-resonance ${\rm Y}(2S)$ data and the sum of off-resonance ${\rm Y}(1S)$ and ${\rm Y}(2S)$ data. The off-resonance data has been scaled by the luminosity ratio ${\mathcal{L}}_{\mathrm{on}}/{\mathcal{L}}_{\mathrm{off}}=2.03$. The ordinate scale is $\mathrm{\text{counts}}/20\mathrm{MeV}$.

Figure 12

Mass distributions $\Delta M\equiv M({\rm Y}(2S))-M(\mathrm{\text{hadrons}})$ for individual decay modes in on-resonance ${\rm Y}(2S)$ data and the sum of off-resonance ${\rm Y}(1S)$ and ${\rm Y}(2S)$ data. The off-resonance data has been scaled by the luminosity ratio ${\mathcal{L}}_{\mathrm{on}}/{\mathcal{L}}_{\mathrm{off}}=2.03$. The ordinate scale is $\mathrm{\text{counts}}/20\mathrm{MeV}$.

Figure 13

Mass distributions $\Delta M\equiv M({\rm Y}(2S))-M(\mathrm{\text{hadrons}})$ for individual decay modes in on-resonance ${\rm Y}(2S)$ data and the sum of off-resonance ${\rm Y}(1S)$ and ${\rm Y}(2S)$ data. The off-resonance data has been scaled by the luminosity ratio ${\mathcal{L}}_{\mathrm{on}}/{\mathcal{L}}_{\mathrm{off}}=2.03$. The ordinate scale is $\mathrm{\text{counts}}/20\mathrm{MeV}$.