Abstract The NA62 experiment reports the branching ratio measurement BR K + → π + ν ν ¯ = 10.6 − 3.4 + 4.0 stat ± 0.9 syst × 10 − 11 $$ \mathrm{BR}\left({K}^{+}\to {\pi}^{+}\nu ...\overline{\nu}\right)=\left({10.6}_{-3.4}^{+4.0}\left|{}_{\mathrm{stat}}\right.\pm {0.9}_{\mathrm{syst}}\right)\times {10}^{-11} $$ 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 + → π + ν ν ¯ $$ {\pi}^{+}\nu \overline{\nu} $$ 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.
Abstract The NA62 experiment reports an investigation of the K + → π + ν ν ¯ $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ 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 + → π + ν ν ¯ $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of ( 0.48 − 0.48 + 0.72 $$ {0.48}_{-0.48}^{+0.72} $$ ) × 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.
A search for heavy neutral lepton (N) production in K+→e+N decays using the data sample collected by the NA62 experiment at CERN in 2017–2018 is reported. Upper limits of the extended neutrino mixing ...matrix element |Ue4|2 are established at the level of 10−9 over most of the accessible heavy neutral lepton mass range 144–462 MeV/c2, with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The |Ue4|2 range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/c2.
The NA62 experiment at CERN reports searches for K+→μ+N and K+→μ+νX decays, where N and X are massive invisible particles, using the 2016–2018 data set. The N particle is assumed to be a heavy ...neutral lepton, and the results are expressed as upper limits of O(10−8) of the neutrino mixing parameter |Uμ4|2 for N masses in the range 200–384 MeV/c2 and lifetime exceeding 50 ns. The X particle is considered a scalar or vector hidden sector mediator decaying to an invisible final state, and upper limits of the decay branching fraction for X masses in the range 10–370 MeV/c2 are reported for the first time, ranging from O(10−5) to O(10−7). An improved upper limit of 1.0×10−6 is established at 90% CL on the K+→μ+ννν¯ branching fraction.
A search for heavy neutral lepton production in K+ decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the 10−7 to ...10−6 level are established on the elements of the extended neutrino mixing matrix |Ue4|2 and |Uμ4|2 for heavy neutral lepton mass in the ranges 170–448 MeV/c2 and 250–373 MeV/c2, respectively. This improves on the previous limits from HNL production searches over the whole mass range considered for |Ue4|2, and above 300 MeV/c2 for |Uμ4|2.
The NA62 experiment at CERN reports a search for the lepton number violating decays K+→π−e+e+ and K+→π−μ+μ+ using a data sample collected in 2017. No signals are observed, and upper limits on the ...branching fractions of these decays of 2.2×10−10 and 4.2×10−11 are obtained, respectively, at 90% confidence level. These upper limits improve on previously reported measurements by factors of 3 and 2, respectively.
Searches for lepton number violating K+→π−e+e+ and K+→π−π0e+e+ decays have been performed using the complete dataset collected by the NA62 experiment at CERN in 2016–2018. Upper limits of 5.3×10−11 ...and 8.5×10−10 are obtained on the decay branching fractions at 90% confidence level. The former result improves by a factor of four over the previous best limit, while the latter result represents the first limit on the K+→π−π0e+e+ decay rate.
A search for the K+→μ−νe+e+ decay, forbidden within the Standard Model by either lepton number or lepton flavour conservation depending on the flavour of the emitted neutrino, has been performed ...using the dataset collected by the NA62 experiment at CERN in 2016–2018. An upper limit of 8.1×10−11 is obtained for the decay branching fraction at 90% CL, improving by a factor of 250 over the previous search.
A
bstract
The NA62 experiment reports the branching ratio measurement
$$ \mathrm{BR}\left({K}^{+}\to {\pi}^{+}\nu \overline{\nu}\right)=\left({10.6}_{-3.4}^{+4.0}\left|{}_{\mathrm{stat}}\right.\pm ...{0.9}_{\mathrm{syst}}\right)\times {10}^{-11} $$
BR
K
+
→
π
+
ν
ν
¯
=
10.6
−
3.4
+
4.0
stat
±
0.9
syst
×
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
+
→
$$ {\pi}^{+}\nu \overline{\nu} $$
π
+
ν
ν
¯
decay, observed with a significance of 3.4
σ
. 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.
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