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Abstract
The decay $ {K^ + } \to {\pi ^ + }v\bar v $, with a very precisely predicted branching ratio of less than 10−10 in the Standard Model framework, is one of the best candidates to reveal indirect effects of new physics at the highest mass scales. The NA62 experiment at CERN SPS is designed to measure the branching ratio of such decay with a decay-in-flight technique, novel for this channel. The main goal of NA62 is to measure such Branching Ratio of $ {K^ + } \to {\pi ^ + }v\bar v $ with an accuracy better than 20%. This will be achieved by collecting up to 100 $ {K^ + } \to {\pi ^ + }v\bar v $ events with a background contamination at the level of 10%. The NA62 detector was commissioned in 2014 and 2015 and the experiment took physics data from 2015 to 2018. NA62 has already published the result of the 2016 data analysis. The 2017 data analysis is in progress, the single event sensitivity reached and the evaluation of the main backgrounds will be shown in this contribution.
Highlights
Background evaluationA class of backgrounds that has been already estimated for the 2017 sample is the one originating from K+ decaying in the fiducial volume
The result of the data collected in 2016 [13], with one event found in the signal region (0.26 events expected for the Standard Model decay and with an expected background contribution of 0.12 events) allowed to set a 95% confidence level limit on the branching ratio: BR(K+ → π+νν) < 14 × 10−10 and demonstrated the validity of this experimental technique
The estimation of the K+ → π+π0 and K+ → μ+ν backgrounds relies on the assumption that π0 and μ+ rejections are independent from the m2miss cuts defining the signal regions
Summary
Down (flavour changing neutral current) make this process extremely rare. The Standard Model predicts [1] the branching ratio to be (8.4±1.0)×10−11, where the precision on the external inputs (measurable parameters) dominates the uncertainty. On the other hand the K+ → π+ννdecay is extremely sensitive to physics beyond the SM, probing the highest mass scales among the rare meson decays. The experiments E787 and E949 at BNL [11, 12] studied the K+ → π+ννdecay using a kaon decay–at–rest technique, their measured Branching Ratio is (17.3+−1110..55) × 10−11. The result of the data collected in 2016 [13], with one event found in the signal region (0.26 events expected for the Standard Model decay and with an expected background contribution of 0.12 events) allowed to set a 95% confidence level limit on the branching ratio: BR(K+ → π+νν) < 14 × 10−10 and demonstrated the validity of this experimental technique. The Kaon flux, the evaluation of the single event sensitivity and the contribution of most of the backgrounds will be presented
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