Experimental study of the ${}^{12}\mathrm{C}\left(K_{\text{stopped}}^{-},{\pi }^0\right){}_{\Lambda }{}^{12}\mathrm{B}$ reaction

The $\left(K_{\text{stopped}}^{-},{\pi }^0\right)$ strangeness and charge changing reaction was investigated by producing a ${}_{\Lambda }{}^{12}\mathrm{B}$ hypernucleus from a carbon target. The branching ratio for $K^{-}$ capture to the ground and $p$-shell states of this hypernucleus was found to be $(0.28\pm 0.08)\times {10}^{-3}$ and $(0.35\pm 0.09)\times {10}^{-3}$, respectively, which after correction for isospin was lower than a previously measured value for the ${}^{12}\mathrm{C}(K_{\text{stopped}}^{-},{\pi }^{-}){}_{\Lambda }{}^{12}\mathrm{C}$ reaction, but still above the theoretical predictions for the ground state. The experiment obtained a missing-mass resolution comparable to in-flight reactions, however the higher background and lower selectivity of the $K_{\text{stopped}}^{-}$ reaction limits its usefulness in the study of new hypernuclear species.

Figure 1

(Color online) Schematic of E907 beam line. The distances are in centimeter.

Figure 2

Time of flight spectrum between S1 and S2 for $\left(K\text{beam}·\mathrm{S}3\right)·\text{OR}·\left(\text{prescaled}\pi \text{beam}·\mathrm{S}3\right)$ trigger.

Figure 3

Energy loss in S3 for $\left(K\text{beam}·\mathrm{S}3\right)·\text{OR}·\left(\text{prescaled}\pi \text{beam}·\mathrm{S}3\right)$ trigger.

Figure 4

(Color online) Schematic of one of the NMS arms.

Figure 5

(Color online) Schematic of NMS chamber package.

Figure 6

Data flow diagram for the experiment.

Figure 7

The distribution of measured charge for all ATC channels is shown. The plot on the left shows the distribution for the case of no gain calibration applied to the channels, and the plot on the right shows the measured charge with gain corrections applied to all channels. The shift in the peak channel number is due to an overall change in the gain settings.

Figure 8

The total energy of the ${\pi }^0$ using photon conversions in both the CsI and the BGO arrays. The plot on the left shows the energy sum with all events included. The plot on the right shows the same spectrum with the edge crystal cut.

Figure 9

The total ${\pi }^0$ energy spectrum with photon conversions on CsI only. The $\sigma$ of the distribution is $\sim 5\text{MeV}$.

Figure 10

The total energy of the ${\pi }^0$ as measured by the CsI and the BGO crystals. The $\sigma$ of the distribution is $6.5\text{MeV}$.

Figure 11

Invariant ${\pi }^0$ mass. The mean value is $134.5\text{MeV}$, and the FWHM $\sim 10\text{MeV}$.

Figure 12

The left side plot shows the opening angle between two photons without any invariant ${\pi }^0$ mass cut. The right side plot shows the same quantity with an invariant mass cut of $\text{FWHM}=10\text{MeV}$. The vertical line on both plots indicates the kinematical limit of smallest possible opening angle of $66.72°$.

Figure 13

The entire ${\pi }^0$ energy spectrum, showing the various regions as discussed in the text. The ${\Sigma }^{+}$ peak is clearly evident in the data.

Figure 14

(Color online) The ${\pi }^0$ energy spectrum near ${}_{\Lambda }{}^{12}\mathrm{B}$ ground state region. The solid line is a fit to the data in the signal region and shows the position and strength of the peaks.

Figure 15

The NMS solid angle as calculated by Monte Carlo.