Abstract
The NA62 experiment reports an investigation of the {K}^{+}to {pi}^{+}nu overline{nu} mode from a sample of K+ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of (0.389 ± 0.024) × 10−10, corresponding to 2.2 events assuming the Standard Model branching ratio of (8.4 ± 1.0) × 10−11. Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of 1.78 × 10−10 for the {K}^{+}to {pi}^{+}nu overline{nu} branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of ( {0.48}_{-0.48}^{+0.72} ) × 10−10, are currently the most precise results worldwide, and are able to constrain some New Physics models that predict large enhancements still allowed by previous measurements.
Highlights
The K+ → π+ννdecay is a flavour-changing Neutral-Current process that proceeds through electroweak box and penguin diagrams in the Standard Model (SM), allowing an exploration of its flavour structure thanks to unique theoretical cleanliness
The mechanisms giving rise to accidental upstream events are the following: a) the π+ comes from a K+ decaying in the region upstream of GTK3; the KTAG signal produced by the parent K+ is associated with a pileup beam π+ or proton track, which is reconstructed as a kaon in the GTK; additional particles produced in the decay are absorbed by material in the beam line;
The distribution of the m2miss of the upstream background is extracted from an upstream-event-enriched data sample and is normalized to the number of upstream background events expected in the signal regions
Summary
The K+ → π+ννdecay is a flavour-changing Neutral-Current process that proceeds through electroweak box and penguin diagrams in the Standard Model (SM), allowing an exploration of its flavour structure thanks to unique theoretical cleanliness. Where ∆EM = −0.003 accounts for the electromagnetic radiative corrections; xt = m2t /MW2 ; λ = |Vus| and λi = Vi∗sVid (i = c, t) are combinations of Cabibbo-Kobayashi-Maskawa λ8 0.225 (1.2) Based on these conditions, upstream events can be classified as follows: 1. The mechanisms giving rise to accidental upstream events are the following: a) the π+ comes from a K+ decaying in the region upstream of GTK3; the KTAG signal produced by the parent K+ is associated with a pileup beam π+ or proton track, which is reconstructed as a kaon in the GTK; additional particles produced in the decay are absorbed by material in the beam line; Arbitrary units.
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