NA48-3 Abstract

We propose to measure the very rare kaon decay K⁺-> pi⁺ nu nubar at the CERN SPS to extract a 10% measurement of the CKM parameter |V_td|.
The proposed experiment aims to collect about 80 K⁺-> pi⁺ nu nubar events at the SM prediction with a signal to background ratio of 10:1 in two years of data taking.

We propose to use 400 GeV/c protons from the SPS to perform the experiment.
The are several challenging aspects for this experiment. They include:

Performing tracking at 1~GHz total rate, 40 MHz/cm², within a minimal material budget, minimal detector dead-time, and excellent time resolution ( 150 ps).

Achieving positive kaon identification in a high rate environment by means of a differential Cherenkov counter insensitive to pions and protons with minimal accidental mistagging.

Constructing and operating hermetic photon vetoes to provide a pi⁰ rejection of 10⁸.

Achieving a muon rejection of at least 10⁵ using a magnetised iron/scintilla tor detector.

Achieve a two or more standard deviations pi/mu separation up to 35 GeV/c momentum by means of a RICH counter.

Performing redundant measurement of the momentum of the incoming K⁺ and outgoing pi⁺ for suppression of the tails in the reconstruction of the missing mass for two body decays.

Vetoing the charged particles coming from three and four body kaon decays.

Minimising the accidental activity from non-kaon decays (e.g. muons from the proton dump and tracks coming from pion and kaon decays occurring upstream of the decay region).