KLEVER: An experiment to measure BR($K_L\to {\pi }^0\nu \bar{\nu }$) at the CERN SPS

Abstract

Precise measurements of the branching ratios for the flavor-changing neutral current decays $K\to \pi \nu \bar{\nu }$ can provide unique constraints on CKM unitarity and, potentially, evidence for new physics. It is important to measure both decay modes, $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ and $K_L\to {\pi }^0\nu \bar{\nu }$, 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($K_L\to {\pi }^0\nu \bar{\nu }$) to $\sim$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 $K_L\to {\pi }^0{\pi }^0$ 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($K_L\to {\pi }^0\nu \bar{\nu }$).

8 pages, 3 figures. Prepared for the proceedings of the International Conference on Kaon Physics 2019 (KAON 2019), Perugia, Italy, 10-13 September 2019. arXiv admin note: substantial text overlap with arXiv:1901.03099, arXiv:1812.01896, arXiv:1901.09966

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