The ultra-rare kaon decay K+ → π+ ν anti-ν
For only few kaon decays exist a precise theoretical prediction. One of these exceptions is the very rare decay K+ → π+ ν anti-ν,which has a branching ratio of the order of 10-10. In the standard model the decay amplitude is a measure of the product |Vts * Vtd| of CKM matrix elements. The theoretical uncertainties can reliably be estimated and are of the order of a few per cent. Because of its low rate, its precise theoretical prediction inside the Standard Model, and the fact that the decay can only proceed via loop diagrams, possible contributions from physics beyond the Standard Model would easily be seen, provided a sufficient number of measured decays. Most of the existing theories of physics beyond the Standard Model predict a significant enhancement of the decay rate.
The NA62 experiment
The NA62 experiment at the CERN SPS aims to precisely measure the decay K+→π+ν anti-ν. It is a fixed-target experiment with the kaons being produced by protons from the CERN SPS hitting a beryllium target. Down-stream of the target, charged particles with 75 GeV/c momentum are selected. The particles are identified and measured by a silicon pixel detector, working at a rate of almost 1 GHz. The kaons in the beam decay in flight in a 80 m long vacuum vessel and the out-coming track is measured in a magnetic
spectrometer. Background arises from mainly two decay channels: K+ → π+ π0 and K+→ μ+ν, which together make up about 87% of all kaon decays. For suppression of the π+ π0 decay, photon veto-counters reject all events with additional photons. The rejection of K+→ μ+ν decays is partly done kinematically by the momentum measurement in the straw
chambers. To achieve the required rejection power of about 1011, we need a good muon-pion identification in addition. This is performed by a RICH detector, the LKr calorimeter and a muon veto detector, which measures the shower shape of pion and muon showers in matter.

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