[Paper Review] Rare decays at the CERN high-intensity kaon beam facility

NA Collaboration, KLEVER Collaboration|arXiv (Cornell University)|Sep 23, 2020

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper proposes a high-intensity kaon physics program at CERN's SPS, using upgraded NA62 and the KLEVER experiment to measure rare kaon decays $K^+\to\pi^+\nu\bar{\nu}$ and $K_L\to\pi^0\nu\bar{\nu}$ with 5% and 20% precision, respectively. These measurements aim to probe new physics beyond the Standard Model, including lepton-flavor violation and hidden-sector particles, leveraging advanced detectors and beamline upgrades to achieve sub-15% total precision.

ABSTRACT

Precision measurements of the branching ratios (BRs) for rare kaon decays can provide unique constraints on CKM unitarity and may reveal the existence of new physics. The BRs for two of these decays, $K^+ oπ^+ν\barν$ and $K_L oπ^0ν\barν$, are strongly suppressed in the Standard Model, while their rates can be calculated with very small theoretical uncertainties.These decays are potentially sensitive to high mass scales, and several models of new physics predict large deviations from the Standard Model. Much progress has been made recently in the measurement of the $K oπν\barν$ BRs. Following upon NA62's successful application of the in-flight technique to measure ${ m BR}(K^+ oπ^+ν\barν)$, we envision a comprehensive program for the study of the rare decay modes of both $K^+$ and $K_L$ mesons, to be carried out with high-intensity kaon beams from the CERN SPS in multiple phases, including both an experiment to measure ${ m BR}(K^+ oπ^+ν\barν)$ at the 5% level and an experiment to measure ${ m BR}(K_L oπ^0ν\barν)$ at the 20% level. The detectors could also be reconfigured to allow measurements of $K_L$ decays with charged particles, such as $K_L oπ^0\ell^+\ell^-$.

Motivation & Objective

• To measure the branching ratios of rare kaon decays $K^+\to\pi^+\nu\bar{\nu}$ and $K_L\to\pi^0\nu\bar{\nu}$ with high precision to test CKM unitarity and probe new physics.
• To investigate lepton-flavor universality violation and hidden-sector particles through deviations in rare decay rates.
• To achieve 5% precision on $K^+\to\pi^+\nu\bar{\nu}$ and 20% on $K_L\to\pi^0\nu\bar{\nu}$ using upgraded detectors and high-intensity beams.
• To enable complementary measurements of $K_L\to\pi^0\ell^+\ell^-$ and other exotic decays by reconfiguring detectors for charged-particle final states.
• To support a long-term, multi-phase experimental program at CERN’s SPS using upgraded beamlines and detector technologies.

Proposed method

• Utilize a high-intensity, slow-extracted 400-GeV/c proton beam delivering up to $10^{19}$ protons on target per year to produce high-rate kaon beams.
• Implement a multi-phase experimental approach: first measuring $K^+\to\pi^+\nu\bar{\nu}$ with 5% precision, then $K_L\to\pi^0\nu\bar{\nu}$ with 20% precision using the KLEVER project.
• Deploy advanced detectors including a Gigatracker with 50 ps time resolution using LGAD or 3D pixel sensors for beam and secondary particle tracking.
• Integrate ultrafast single-photon detectors (20 ps timing) for Cherenkov-based particle identification in high-rate environments.
• Use thin-walled, narrow-gauge (5 mm) vacuum-operated straw tubes with ~6 ns time resolution for secondary particle tracking.
• Apply coherent photon conversion in oriented crystals to suppress high-energy photons and enhance veto efficiency for small-angle photons in $K_L$ decays.

Experimental results

Research questions

• RQ1 Can the branching ratio of $K^+\to\pi^+\nu\bar{\nu}$ be measured with 5% precision to test CKM unitarity and probe new physics at scales up to hundreds of TeV?
• RQ2 Can the $K_L\to\pi^0\nu\bar{\nu}$ decay be measured with 20% precision to probe new sources of CP violation and distinguish between new physics models?
• RQ3 To what extent can lepton-flavor universality violation in $B$ decays be linked to enhanced $K\to\pi\nu\bar{\nu}$ rates via third-generation couplings?
• RQ4 Can the observation of anomalously high $K\to\pi\nu\bar{\nu}$ branching ratios provide evidence for hidden-sector particles like a dark scalar $X$?
• RQ5 Can the $K_L\to\pi^0\ell^+\ell^-$ decay branching ratio be measured to overconstrain the unitarity triangle and reveal new physics effects?

Key findings

• NA62 has measured ${\rm BR}(K^{+}\to\pi^{+}\nu\bar{\nu})=(11.0^{+4.0}_{-3.5}\pm 0.3)\times 10^{-11}$, providing the first significant evidence (>3σ) for this rare decay.
• The combined NA62 data from 2016–2018 and 2018 yield a branching ratio consistent with the Standard Model and improve on previous E787/E949 bounds.
• KOTO has achieved a preliminary single-event sensitivity of $7.1\times 10^{-10}$ for $K_L\to\pi^0\nu\bar{\nu}$, observing 3 candidate events with a background of $1.05\pm 0.28$ events.
• The program aims to reach 10% precision on $K^+\to\pi^+\nu\bar{\nu}$ with data collected from 2021–2024, following the NA62 in-flight technique.
• The KLEVER project is designed to measure $K_L\to\pi^0\nu\bar{\nu}$ with 20% precision using a beamline extension and crystal-based photon suppression.
• The combined precision of NA62 and KOTO, along with this CERN program, will push total uncertainty below 15% for both rare decays.