The High-Intensity K+ Beam for the NA62 Experiment

The NA62 experiment, approved in 2009, aims to measure the branching ratio of the very rare decay K+ → + with a precision of ±10%. This branching ratio is calculable with good precision in the Standard Model to be (8.5±0.7) 10-11. Predictions of this branching ratio are also available for various extensions of the Standard Model, including supersymmetry and a 4th generation of quarks and leptons, and show that this decay mode is very sensitive to physics beyond the Standard Model. To achieve the K+ decay rate required to collect some 100 events of this very rare process in two years, a new 75 GeV/c secondary hadron beam line has been designed, which provides a ≈ 40x higher kaon flux per incident proton than the previous K12 beam for NA48/2 and at the same time sufficiently small halo rates to allow the very high veto efficency necessary to perform this delicate measurement. The K+ content in the beam is ≈ 6%. In this note we describe in detail the design of this new beam line, which will be built in the underground caverns TCC8 and ECN3 in the North Area of the CERN SPS. The beam line has been carefully optimised to match the stringent requirements of the experiment, of which it constitutes an integral part. The experiment itself and the detectors are described in the NA62 proposal [1] and in the NA62 Technical Design Report [2], for which the orginal version of this note was written as section 2.1. The a pparatus is optimised for kaon intensity, signal acceptance and very strong background suppresssion at the 10-12 level, achieved by excellent resolution and timing and minimal material along the beam line. Section 1 summarizes the evolution of the beam design since the NA62 proposal. In section 2 we give the rationale for the main design criteria of the beam. In section 3 we describe in detail the layout and characteristics of the beam line.