| Summary: | The NA62 experiment aims at measuring the branching ratio of the ultra-rare decay K⁺ → π⁺νν ̄ with 10% precision. To achieve the desired precision, high levels of background rejection must be accomplished using techniques such as high-resolution timing, kinematic rejection, particle identification, and hermetic vetoing of photons. K⁺ → π⁺π0 (Kπ₂) decays, one of the largest background sources, are mainly rejected by kinematics reconstruction and photon vetoing in NA62. To evaluate Kπ₂ background rejection capabilities of the NA62 system, we studied the inefficiency of these two techniques. Kπ₂ and K⁺ → μ⁺ν (Kμ₂) events were selected from 2015 minimum bias data runs using Kπ₂/Kμ₂ separation cuts whose efficiency was also studied in details. The inefficiency of kinematics suppression was found to be (2.16 ± 0.21) × 10-³, and the upper limit of the photon veto inefficiency was 7 × 10-⁷ at 90% confidence level. Combined with correction factors from Monte Caro simulations, a preliminary result, S/B > 0.2, was estimated for Kπ₂ background in the measurement of K⁺ → π⁺νν ̄. Also, an upper limit on the branching ratio of the invisible decay π0 → νν ̄, 4.3×10-⁷, was obtained at 90% confidence level. === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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