KLEVER: An experiment to measure BR() at the CERN SPS

M Moulson

doi:10.1088/1742-6596/1526/1/012028

Abstract

Precise measurements of the branching ratios for the flavor-changing neutral current decays can provide unique constraints on CKM unitarity and, potentially, evidence for new physics. It is important to measure both decay modes, and , since different new physics models affect the rates for each channel differently. The NA62 experiment at the CERN SPS will measure the BR for the charged channel to better than 20%. The BR for the neutral channel has never been measured. We are designing the KLEVER experiment to measure BR() to ∼20% using a high-energy neutral beam at the CERN SPS. The boost from the high-energy beam facilitates the rejection of background channels such as K L → π 0 π 0 by detection of the additional photons in the final state. On the other hand, the layout poses particular challenges for the design of the small-angle vetoes, which must reject photons from K L decays escaping through the beam exit amid an intense background from soft photons and neutrons in the beam. We present findings from our design studies, with an emphasis on the challenges faced and the potential sensitivity for the measurement of BR().

Highlights

The branching ratios (BRs) for the decays K → πννare among the observables in the quark flavor sector most sensitive to new physics
We present findings from our design studies, with an emphasis on the challenges faced and the potential sensitivity for the measurement of BR(KL → π0νν)
Because of the dominance of the diagrams with top loops, the lack of contributions from intermediate photons, and the fact that the hadronic matrix element can be obtained from Ke3 data, the Standard Model (SM) rates can be calculated very precisely: BR(K+ → π+νν) = (8.4 ± 1.0) × 10−11 and BR(KL → π0νν) = (3.4 ± 0.6) × 10−11, where the uncertainties are dominated by the external contributions from Vcb and Vub and the nonparametric theoretical uncertainties are about 3.5% and 1.5%, respectively [1]

Summary

Introduction

The branching ratios (BRs) for the decays K → πννare among the observables in the quark flavor sector most sensitive to new physics. The Standard Model (SM) rates for these flavor-changing neutral-current decays are very strongly suppressed by the GIM mechanism and the CKM hierarchy. Because of the dominance of the diagrams with top loops, the lack of contributions from intermediate photons, and the fact that the hadronic matrix element can be obtained from Ke3 data, the SM rates can be calculated very precisely: BR(K+ → π+νν) = (8.4 ± 1.0) × 10−11 and BR(KL → π0νν) = (3.4 ± 0.6) × 10−11, where the uncertainties are dominated by the external contributions from Vcb and Vub and the nonparametric theoretical uncertainties are about 3.5% and 1.5%, respectively [1].

Published under licence by IOP Publishing Ltd

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Findings

Conclusions