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

TL;DR

The paper discusses the design and challenges of the KLEVER experiment at CERN SPS aimed at measuring the rare decay BR($K_L\to\pi^0\nu\bar{\nu}$) with about 20% precision, providing crucial data for understanding flavor physics and new physics models.

Contribution

It introduces the design considerations and challenges for the KLEVER experiment to measure BR($K_L\to\pi^0\nu\bar{\nu}$) at CERN SPS, a decay mode not previously measured.

Findings

Design studies highlight background rejection techniques.

Challenges in small-angle veto design are addressed.

Potential sensitivity to measure BR($K_L\to\pi^0\nu\bar{\nu}$) at 20% accuracy.

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($K_L\to\pi^0\nu\bar{\nu}$).