Abstract
The NA62 experiment reports the branching ratio measurement mathrm{BR}left({K}^{+}to {pi}^{+}nu overline{nu}right)=left({10.6}_{-3.4}^{+4.0}left|{}_{mathrm{stat}}right.pm {0.9}_{mathrm{syst}}right)times {10}^{-11} at 68% CL, based on the observation of 20 signal candidates with an expected background of 7.0 events from the total data sample collected at the CERN SPS during 2016–2018. This provides evidence for the very rare K+→ {pi}^{+}nu overline{nu} decay, observed with a significance of 3.4σ. The experiment achieves a single event sensitivity of (0.839 ± 0.054) × 10−11, corresponding to 10.0 events assuming the Standard Model branching ratio of (8.4 ± 1.0) × 10−11. This measurement is also used to set limits on BR(K+→ π+X), where X is a scalar or pseudo-scalar particle. Details are given of the analysis of the 2018 data sample, which corresponds to about 80% of the total data sample.
Highlights
Background evaluation and validationBackground contributions to the K+ → π+ννfinal state can be identified from two processes: K+ decays inside the fiducial volume (FV) to a final state different from the signal; upstream events where a π+ originates either from a K+ decay or from an interaction between a beam K+ and the material upstream of the FV
Additional constraints are imposed on the m2miss value using: the π+ momentum extracted from the ring-imaging Cherenkov counter (RICH) ring measurement in the π+ mass hypothesis instead of the STRAW measurement; the nominal beam momentum and direction instead of the K+ track measured by GTK
The sum runs over the twelve 100 ps wide bins covering the (−600, +600) ps region used to reconstruct the tracks in the GTK; Nups(|∆Ti|) is the number of events in the upstream sample in the ∆T bin i; Pmistag(|∆Ti|) is the mistagging probability; fscale = 1.15 is a scaling factor that accounts for upstream events with closest distance of approach (CDA) < 4 mm not included in Nups
Summary
The NA62 beam line and detector are sketched in figure 1 and a detailed description can be found in [30]. The L0 trigger requires a signal in the RICH to tag a charged particle The time of this signal, called trigger time, is used as a reference to define a coincidence within 6.3 ns of: a signal in one to four CHOD tiles; no signals in opposite CHOD quadrants to suppress K+ → π+π+π− decays; no signals in MUV3 to reject K+ → μ+ν decays; less than 30 GeV deposited energy and no more than one cluster in the LKr to reject K+ → π+π0 decays. The L1 trigger requires: a kaon identified in KTAG; signals within 10 ns of the trigger time in at most two blocks of each LAV station; at least one STRAW track corresponding to a particle with momentum below 50 GeV/c and forming a vertex with the nominal beam axis upstream of the first STRAW chamber. Simulated data are subject to the same reconstruction and calibration procedures as real data
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