PoS - Proceedings of Science
Volume 380 - Particles and Nuclei International Conference 2021 (PANIC2021) - Flavour physics - CKM and beyond
Measurement of structure dependent radiative $K^{+} \rightarrow e^{+} \nu \gamma$ decays using stopped positive kaons at t J-PARC
A. Kobayashi* and  On behalf of the E36 Collaboration
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Pre-published on: March 16, 2022
Published on: May 24, 2022
The J-PARC E36 experiment aims at searching for a lepton universality violation by precisely measuring the ratio of the branching ratio of the $K^+ \rightarrow e^+ \nu (\gamma)$ ($K_{e2(\gamma)}$) to $K^+ \rightarrow \mu^+ \nu$ ($K_{\mu 2}$) decays.
The E36 experiment was performed in 2015 at J-PARC using a stopped $K^{+}$ method in conjunction with a 12-sector iron-core superconducting toroidal spectrometer. Charged particle momenta were calculated by reconstructing the tracks in the spectrometer. Particle discrimination between $e^{+}$ and $\mu^{+}$ was carried out using an aerogel Cherenkov counter and a lead-glass Cherenkov counter, as well as by measuring the time-of-flight between TOF counters.
The peak structure due to the $K_{e2(\gamma)}$ decays was successfully observed in the $e^{+}$ momentum spectrum. The structure-dependent radiative $K^+ \rightarrow e^+ \nu \gamma$ ($K_{e2 \gamma}^{\rm SD}$) events were selected by requiring photon hits in the CsI(Tl) calorimeter or the GSC counter. The experimental spectra were reproduced by the Monte Carlo simulation, which indicates a correct understanding of the detector acceptance. The $Br(K_{e2\gamma}^{\rm SD})$ value relative to $Br(K_{e2(\gamma)})$ was obtained by calculating the ratio of the $K_{e2 \gamma}^{\rm SD}$ and $K_{e2(\gamma)}$ yields corrected for their detector acceptances. The $Br(K_{e2\gamma}^{\rm SD})/Br(K_{e2(\gamma)})$ value was obtained to be $1.22 \pm 0.07_{stat} \pm 0.04_{syst}$ in the CsI(Tl) analysis and $1.22 \pm 0.13_{stat} \pm 0.08_{syst}$ in the GSC analysis.
DOI: https://doi.org/10.22323/1.380.0161
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