[Paper Review] Workshop summary -- Kaons@CERN 2023

G. Anzivino, S. Arguedas Cuendis+61 more|arXiv (Cornell University)|Nov 6, 2023

Particle physics theoretical and experimental studies232 references4 citations

TL;DR

This paper summarizes the Kaons@CERN 2023 workshop, highlighting the pivotal role of kaon physics in probing new physics beyond the Standard Model. It emphasizes that future experiments like HIKE and KOTO-II, combined with advanced theoretical tools, will enable precision tests of rare kaon decays—offering high-sensitivity probes of new physics at TeV scales and potentially revealing lepton number violation or new symmetries.

ABSTRACT

Kaon physics is at a turning point -- while the rare-kaon experiments NA62 and KOTO are in full swing, the end of their lifetime is approaching and the future experimental landscape needs to be defined. With HIKE, KOTO-II and LHCb-Phase-II on the table and under scrutiny, it is a very good moment in time to take stock and contemplate about the opportunities these experiments and theoretical developments provide for particle physics in the coming decade and beyond. This paper provides a compact summary of talks and discussions from the Kaons@CERN 2023 workshop.

Motivation & Objective

• To assess the current and future experimental and theoretical landscape of kaon physics in light of upcoming experiments such as HIKE, KOTO-II, and LHCb-Phase-II.
• To evaluate the potential of rare kaon decays—especially $K^+ \to \pi^+ \nu\bar{\nu}$ and $K_L \to \pi^0 \nu\bar{\nu}$—as clean, theoretically precise probes of new physics.
• To explore the interplay between kaon physics and other areas of particle physics, including flavor physics, the strong CP problem, and leptogenesis.
• To identify key theoretical and experimental challenges and opportunities for constraining or discovering new physics through combined analyses of $K$ and $B$ decays.
• To advocate for a next-generation kaon factory (HIKE) as a critical infrastructure for advancing precision flavor physics and testing the microscopic structure of new physics.

Proposed method

• Systematic review and synthesis of talks and discussions from the Kaons@CERN 2023 workshop, involving over 100 international experts in experiment and theory.
• Analysis of theoretical frameworks such as the Standard Model Effective Field Theory (SMEFT) and chiral perturbation theory (ChPT) to assess sensitivity to new physics in kaon decays.
• Evaluation of future experimental sensitivities from HIKE and KOTO-II, particularly for rare and radiative kaon decays, and their complementarity with $B$-meson decays.
• Investigation of model-building scenarios, including $U(1)_H$ flavor symmetries and their implications for axion dark matter and the strong CP problem.
• Use of combined fits to multiple decay modes to improve constraints on new physics models, especially when long-distance contributions are refined.
• Assessment of the role of precision measurements in $K^+$ and $K_L$ decays for testing CKM unitarity and probing non-standard currents.

Experimental results

Research questions

• RQ1 How can future experiments like HIKE and KOTO-II achieve the necessary precision to probe new physics at the TeV scale through rare kaon decays?
• RQ2 What is the potential of $K^+ \to \pi^+ \nu\bar{\nu}$ and $K_L \to \pi^0 \nu\bar{\nu}$ decays as clean, theoretically robust probes of new physics?
• RQ3 Can a $U(1)_H$ flavor symmetry simultaneously solve the flavor problem and generate a viable axion dark matter candidate that also addresses the strong CP problem?
• RQ4 How do precision measurements of $K^+$ and $K_L$ decays complement LHC searches and electroweak precision data in constraining new physics models?
• RQ5 To what extent can improved theoretical calculations of long-distance contributions enhance the sensitivity of future kaon experiments to new physics?

Key findings

• The rare decay modes $K^+ \to \pi^+ \nu\bar{\nu}$ and $K_L \to \pi^0 \nu\bar{\nu}$ are among the theoretically cleanest standard candles in the Standard Model, with minimal hadronic uncertainties.
• Theoretical precision for these rare decays already matches the projected sensitivities of future experiments like HIKE and KOTO-II, enabling robust tests of new physics.
• Generic new physics models show strong complementarity between $B$ and $K$ decays, with future kaon factories uniquely positioned to constrain the microscopic structure of new physics.
• Beyond gold-plated modes, HIKE and KOTO-II will measure a wide range of $K^+$ and $K_L$ decays with unprecedented precision, aiding the development of ChPT, dispersion theory, and lattice QCD+QED tools.
• Improved measurements of non-gold-plated decays can enhance constraints on new physics models, especially when analyzed in global fits, particularly if long-distance contributions are better understood.
• Even in the absence of new physics discovery, precision measurements of the Standard Model in kaon decays by HIKE and KOTO-II will yield strong exclusion limits, making them essential for future flavor physics.