Double $K_S^0$ photoproduction off the proton at CLAS

The $f_0\left(1500\right)$ meson resonance is one of several contenders to have significant mixing with the lightest glueball. This resonance is well established from several previous experiments. Here we present the first photoproduction data for the $f_0\left(1500\right)$ via decay into the $K_S^0{K}_S^0$ channel using the CLAS detector. The reaction $\gamma p\to f_{J}p\to K_S^0{K}_S^{0}p$, where $J=0,2$, was measured with photon energies from 2.7–5.1 GeV. A clear peak is seen at 1500 MeV in the background subtracted invariant mass spectra of the two kaons. This is enhanced if the measured four-momentum transfer to the proton target is restricted to be less than 1.0 ${\mathrm{GeV}}^2$. By comparing data with simulations, it can be concluded that the peak at 1500 MeV is produced primarily at low $t$, which is consistent with a $t$-channel production mechanism.

Figure 1

Schematic diagrams of reaction mechanisms for (a) $s$-channel and (b) $t$-channel photoproduction of a scalar meson.

Figure 2

Missing mass for the reaction $\gamma p\to 2{\pi }^{+}2{\pi }^{-}X$ was calculated from the incident photon energy, the target proton mass and the momentum of the detected charged pions. A clear peak is seen at the mass of the proton. The vertical lines enclose the selected events.

Figure 3

Invariant mass spectrum for one combination of ${\pi }^{+}{\pi }^{-}$. A clear $K_S^0$ peak is seen over a two-pion background. Other combinations show a similar mass distribution.

Figure 4

Correlation between the invariant mass of one ${\pi }^{+}{\pi }^{-}$ pair and the other ${\pi }^{+}{\pi }^{-}$ pair. The spot at the center shows the situation where both pion pairs come from decays of two $K_S^0$.

Figure 5

The left histogram with the error bars shows the signal + background, whereas the shaded (yellow online) histogram is the average sideband background. The right histogram is the sideband subtracted histogram. The peak near 1.50 GeV is the region of interest.

Figure 6

Background subtracted plots for the $4\pi$ invariant mass for $\left|t\right|<1{\mathrm{GeV}}^2$ (left) and $\left|t\right|>1{\mathrm{GeV}}^2$ (right).

Figure 7

Dalitz plots of the three decay particles, the two kaons and the proton. Structures making horizontal bands represent two-kaon resonances. The intense region at the bottom, near $M^2(K_S^0,K_S^0)=1.0{\mathrm{GeV}}^2$, is due to the $f_0\left(980\right)$ decay.

Figure 8

Invariant mass spectrum of $4\pi$ phase space plus simulated $f_0\left(1500\right)$ and $f_0\left(980\right)$ mesons decaying to two $K_S^0$. Momentum transfer cuts $\left|t\right|<1{\mathrm{GeV}}^2$ (left) and $\left|t\right|>1{\mathrm{GeV}}^2$ (right) are shown.

Figure 9

Dalitz plots of simulated events for $M^2(K_S^0,K_S^0)$ vs $M^2(K_S^0,p)$ for $\left|t\right|<1{\mathrm{GeV}}^2$ (left) and $\left|t\right|>1{\mathrm{GeV}}^2$ (right) for generated phase space plus $f_0\left(980\right)$ and $f_0\left(1500\right)$ mesons.

Figure 10

Fits to $cos{\theta }_{c.m.}$ distributions in the Gottfied-Jackson frame for simulated pure $S$ wave and pure $D$ wave (top) and data from the S+B and sideband regions (bottom) for bin 1450–1500 MeV of the $K_S^0{K}_S^0$ mass. The fit curves are: $S$ wave (red), $D$ wave (blue), and total (green). The parameters of the fits are shown to the right of the plots.

Figure 11

Same as in Fig. 10, but for bin 1500–1550 MeV of the $K_S^0{K}_S^0$ mass.

Figure 12

Same as in Fig. 10, but for bin 1550–1600 MeV of the $K_S^0{K}_S^0$ mass.