Measurement of the total cross section of the reaction $K^{-}p\to {\Sigma }^0\gamma$ between 514 and 750 MeV/$c$

We report the first measurements of the total cross section of the reaction $K^{-}p\to {\Sigma }^0\gamma$ at eight beam momenta from 514 to 750 MeV/$c$. The data were obtained at the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory using the Crystal Ball detector consisting of 672 NaI crystals. All charged particles were vetoed by a barrel of plastic scintillators, resulting in a study of only the neutral decays of the ${\Sigma }^0$. The prompt photon and the photons from the decay products of ${\Sigma }^0$ were detected for each reconstructed event; the neutron was also detected in a small fraction of the events.

Figure 1

Invariant mass of two photons from ${\pi }^{-}p$ data shown after optimizing the gains of the 672 PMTs so as to minimize (a) any offset of the mass peaks from ${\pi }^0$ and $\eta$ masses and (b) the width of the mass peaks. The ${\pi }^0$ and $\eta$ mass peaks are centered at 136.0 and 545.8 MeV, respectively. The observed low-energy cutoff is due to the tight requirements that remove background events.

Figure 2

Plot of reconstructed missing mass $({\mathrm{MM}}_{{\gamma }_1})$ off the production photon showing the excellent agreement between data (solid circles) and the Monte Carlo simulation (line). The peak of the data spectrum corresponds to the mass of ${\Sigma }^0$ with a small uncertainty (sigma).

Figure 3

Difference between the optimized primary vertex $z1$ and the actual location of the vertex obtained from Monte Carlo, for the $K^{-}p\to {\Sigma }^0\gamma$ signal and for the three background channels $K^{-}p\to {\Sigma }^0{\pi }^0$, $K^{-}p\to \Lambda {\pi }^0$, and $K^{-}p\to K_s^{0}n$. The background channels most often return values of $z1$ that are outside the target. The dashed lines show the boundaries of the target.

Figure 4

Missing mass (Monte Carlo) off ${\gamma }_1$ for the $K^{-}p\to {\Sigma }^0\gamma$ signal and for the three background channels. A peak at 1193 MeV corresponding to the mass of ${\Sigma }^0$ identifies signal events. Cuts (shown by the dashed lines) are placed on this mass to optimize the signal-to-background ratio.

Figure 5

The number of signal events (reconstructed events − background) surviving the data cuts at 750 MeV/$c$, as a function of the applied cut number described in the text, showing how the number of events changed with successively tighter requirements.

Figure 6

This plot of the ratio of the signal-to-background at 750 MeV/$c$ as a function of the applied cut number described in the text shows how successive cuts improved this ratio by approximately a factor of 5.

Figure 7

Total measured cross section for $K^{-}p\to {\Sigma }^0\gamma$ at 750 MeV/$c$ as a function of the applied cut number. The plot shows how successive cuts leave the cross section unchanged but reduce the uncertainty on the cross section.

Figure 8

Measured total cross section for $K^{-}p\to {\Sigma }^0\gamma$ as a function of the incident kaon momentum. The uncertainties shown are statistical only.