A high sensitivity investigation of $K_{\rm S}$ and neutral hyperon decays using a modified $K_{\rm S}$ beam

%NA48/1 The experiment performs a search for rare $K_{\rm S}$ and neutral hyperon decays. A neutral beam is produced by 400 GeV protons striking a beryllium target. About 1/3 of the produced $K_{\rm S}$ particles live long enough to enter the decay region 6m downstream of the target. The experiment aims to an exposure of about $3 \times 10^{10}$ neutral kaon decays. The experimental apparatus is based on the existing NA48 detector, with upgraded read-out systems. Charged particles are reconstructed by a magnetic spectrometer; photons are measured by a liquid krypton calorimeter (LKr). Among the most interesting decay decay modes there are the $\pi^0 e^+ e^-$, $ 3 \pi^0$, and $\gamma \gamma$. A measurement of the $ K_{\rm S} \rightarrow \pi^0 e^+ e^-$channel, or at least a precise upper limit, will bound the indirect CP violating term in the decay $ K_{\rm L} \rightarrow \pi^0 e^+ e^-$. The branching ratio for the $K_{\rm S}$ mode the cannot be accurately predicted and therefore a measurement for this decay is necessary. The NA48/1 Single Event Sensitivity (SES) is $\sim 3 \times 10^{-10}$. Since $K^0$ and $\overline{K^0}$ are not produced in the same amount in the target, one can search for CP violation via the $K_{\rm S,L}$ interference. NA48/1 searches for $K_{\rm S,L} \rightarrow 3 \pi$ interference at small proper time. Owing to the good energy resolution provided by the LKr calorimeter, the experiment can study rare kaon decays with photons only in the final state. These decays, as for example $K_{\rm S} \rightarrow \gamma \gamma$, provide important tests of Chiral Perturbation Theory. The neutral beam is also rich in neutral hyperons. In a year of data taking (120 days of 400 GeV protons with 50\% efficiency) the experiment can collect 25000 beta decays of the neutral cascade particle ($\Xi^0$). The data should allow a definitive measurement of the {\it g1/f1} form factor and a precise measurement of the Cabibbo angle.