Erlangen 2026 – scientific programme

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T: Fachverband Teilchenphysik

T 94: Flavour Physics VI

T 94.1: Talk

Friday, March 20, 2026, 09:00–09:15, KH 01.011

Semileptonic Kaon Decays at NA62 — •Atakan Akmete — Mainz University

Semileptonic charged kaon decays K⁺ → π⁰ ℓ⁺ ν(γ) (K_ℓ3) provide a clean test of e-µ lepton universality and a direct probe of the first row of the CKM matrix unitarity |V_ud|² + |V_us|² + |V_ub|² = 1. Current measurements indicate a near three-sigma tension in the determination of V_ud and V_us, with semileptonic kaon decays alone contributing at the 2.6σ level.

This study aims to update the branching fractions of K_ℓ3 together with the other dominant K⁺ modes using an unbiased low-intensity dataset collected by the NA62 experiment at CERN, providing a clean environment for per-mille-level statistical precision.

From this dataset, events with a single positively charged downstream track are selected, defining a topology that allows all six main decay modes to be measured simultaneously without explicit particle identification. This approach naturally separates decay modes into distinct kinematic regions, effectively acting as a PID substitute and reducing the systematic uncertainties associated with neutral-pion reconstruction and calorimetry.

In this talk, I will present the current status of the analysis, including preliminary results and ongoing systematic studies.

Keywords: Cabbibo Angle; CKM Matrix; Kaon Physics; Precision Measurement; Fixed-target