The NA62 Experiment at the CERN SPS
For only few kaon decays exist a precise theoretical prediction. One of these exceptions is the very rare decay K+→π+ν¯ν,which has a branching ratio of the order of 10-10. In the framework of the standard model the decay amplitude is, apart from well-known corrections, directly proportional to the product |Vts * Vtd| of CKM matrix elements.
The theoretical uncertainties can reliably be estimated from other semileptonic decays 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. However, so far only 7 events have been reported by the E787/E949-Experiment at Brookhaven, which therefore can only set coarse limits on possible non-SM contributions.
The NA62 experiment at the CERN SPS aims to collect about 100 K+→π+ν¯ν events in two years of data taking starting and with a signal-to-background ratio of 10:1.While the NA62 Collaboration is a successor of the NA48 Collaboration, most detector parts will be newly built. The kaons are produced by protons from the CERN SPS hitting a fixed target. Down-stream of the target, an achromatic system selects charged particles with 75 GeV/c momentum. These particles (of those about 6% are kaons) are tagged and measured by a silicon pixel detector, working at a rate of 1 GHz (GigaTracker). The kaons decay then in flight in a 80 m long vacuum vessel. The pion track is measured in a spectrometer, made from 4 straw chambers, two before and two after a dipole magnet. To reduce multiple scattering, the straw tubes are operated in vacuum. Background arises from mainly two decay channels: K+ → π+ π0 and K+→ μ+ν, which together are responsible for about 87% of all kaon decays. For suppression of the π+ π0 decay, several sets of photon veto-counters reject all events with additional photons. A large angle veto (LAV), made from several rings of lead glass scintillator around the vacuum vessel, detects photons which leave the detector on the outside. The main photon rejection is performed by a liquid-krypton calorimeter (LKr), already used in the NA48 experiment, with excellent energy resolution and granularity. Finally, calorimeters at the end of the beam pipe detect photons which are emitted along the beam line. The rejection of K+→ μ+ν decays is partly done kinematically by the momentum measurement in the straw chambers. However, to achieve the required rejection power of about 1011, we need a good muon-pion identificatio in addition. This is performed by a RICH detector, the LKr calorimeter and a new muon veto detector, which measures the shower shape of pion and muon showers in matter.
In a first phase in 2007 and 2008, the NA62 Collaboration has taken data for a precise measurement of the decay rate ratio Γ(K+ → e+ ν)/Γ(K+ → μ+ ν). Due to the helicity ppression in the weak interaction, this ratio is of the order of 10-5. Within the framework of the Standard Model the ratio can precisely be calculated. However, possible contributions from a charged Higgs boson (predicted in supersymmetric models) may enhance or deplete the predicted ratio by as much as a few per cent. The data for this measurement were collected with the existing NA48 detector and minimum trigger conditions. In total, about 150000 K+ → e+ nu events were recorded and are now being analyzed. The total precision of the final result is expected to be ±0.35%, allowing to either find new physics in this channel or to set strong limits on the existence of a charged Higgs boson. A first preliminary result could be presented.