A simple method for increasing the equal-amplitude non-uniform linear thinned array directivity

Calculating the distances between the elements of the non-uniform array is usually carried out through its parametric optimization using the computational algorithms. In this paper, a simplified method is introduced and tested for determining the distances between the neighboring elements of the equal-amplitude non-uniform linear array that provides a significant increase in the array directivity while preserving the array thinning. The developed method allows both choosing the element spacing on the non-uniform linear array aperture and ensuring an increase in the array directive gain without applying the computational algorithms

Search for heavy neutral lepton production in K+K^+ decays

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 $|U_{\ell 4}|^2$ ($\ell=e,\mu$) for heavy neutral lepton mass in the range $170-448~{\rm MeV}/c^2$. This improves on the results from previous production searches in $K^+$ decays, setting more stringent limits and extending the mass range.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

First search for K+→π+ννˉK^+\rightarrow\pi^+\nu\bar{\nu} using the decay-in-flight technique

International audienceThe NA62 experiment at the CERN SPS reports the first search for K+→π+νν¯ using the decay-in-flight technique, based on a sample of 1.21×1011 K+ decays collected in 2016. The single event sensitivity is 3.15×10−10 , corresponding to 0.267 Standard Model events. One signal candidate is observed while the expected background is 0.152 events. This leads to an upper limit of 14×10−10 on the K+→π+νν¯ branching ratio at 95% CL

A search for the K+→μ−νe+e+K^+\to\mu^-\nu e^+e^+ decay

A search for the $K^+\to\mu^-\nu e^+e^+$ 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 $8.1\times 10^{-11}$ is obtained for the decay branching fraction at 90% CL, improving by a factor of 250 over the previous search

Search for a feebly interacting particle XX in the decay K+→π+XK^+ \to \pi^+ X

A search for the $K^+ \to \pi^+ X$ decay, where $X$ is a long-lived feebly interacting particle, is performed through an interpretation of the $K^+ \to \pi^+ \nu \bar\nu$ analysis of data collected in 2017 by the NA62 experiment at CERN. Two ranges of $X$ masses, $0-110$ MeV/$c^2$ and $154-260$ MeV/$c^2$, and lifetimes above 100 ps are considered. The limits set on the branching ratio, BR($K^+ \to \pi^+ X$), are competitive with previously reported searches in the first mass range, and improve on current limits in the second mass range by more than an order of magnitude.A search for the K$^{+}$ → π$^{+}$X decay, where X is a long-lived feebly interacting particle, is performed through an interpretation of the K$^{+}$ → ${\pi}^{+}\nu \overline{\nu}$ analysis of data collected in 2017 by the NA62 experiment at CERN. Two ranges of X masses, 0–110 MeV/c$^{2}$ and 154–260 MeV/c$^{2}$, and lifetimes above 100 ps are considered. The limits set on the branching ratio, BR(K$^{+}$ → π$^{+}$X), are competitive with previously reported searches in the first mass range, and improve on current limits in the second mass range by more than an order of magnitude.A search for the $K^{+}\rightarrow\pi^{+}X$ decay, where $X$ is a long-lived feebly interacting particle, is performed through an interpretation of the $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ analysis of data collected in 2017 by the NA62 experiment at CERN. Two ranges of $X$ masses, $0$-$110\,\text{MeV}/c^{2}$ and $154$-$260\,\text{MeV}/c^{2}$, and lifetimes above $100\,\text{ps}$ are considered. The limits set on the branching ratio, $\text{BR}(K^{+}\rightarrow\pi^{+}X)$, are competitive with previously reported searches in the first mass range, and improve on current limits in the second mass range by more than an order of magnitude

Measurement of the very rare K+→π+ννˉK^+\rightarrow\pi^+\nu\bar{\nu} decay

The NA62 experiment reports the branching ratio measurement BR$(K^+ \rightarrow \pi^+ \nu\bar{\nu}) = (10.6^{+4.0}_{-3.4} |_{\rm stat} \pm 0.9_{\rm syst}) \times 10 ^{-11}$ 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 $K^+ \rightarrow \pi^+ \nu\bar{\nu}$ decay, observed with a significance of 3.4$\sigma$. The experiment achieves a single event sensitivity of $(0.839\pm 0.054)\times 10^{-11}$, corresponding to 10.0 events assuming the Standard Model branching ratio of $(8.4\pm1.0)\times10^{-11}$. This measurement is also used to set limits on BR($K^+ \to \pi^+ X$), 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

Search for π0\pi^0 decays to invisible particles

The NA62 experiment at the CERN SPS reports a study of a sample of $4 \times 10^9$ tagged $\pi^0$ mesons from $K^+ \to \pi^+ \pi^0 (\gamma)$, searching for the decay of the $\pi^0$ to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of $4.4 \times 10^{-9}$ is set on the branching ratio at 90% confidence level, improving on previous results by a factor of 60. This result can also be interpreted as a model-independent upper limit on the branching ratio for the decay $K^+ \to \pi^+ X$ where $X$ is a particle escaping detection with mass in the range $0.110–0.155$ GeV/$c^2$ and rest lifetime greater than 100 ps. Model- dependent upper limits are obtained assuming $X$ to be an axion-like particle with dominant fermion couplings or a dark scalar mixing with the Standard Model Higgs.The NA62 experiment at the CERN SPS reports a study of a sample of $4 \times10^{9}$ tagged $\pi^0$ mesons from $K^+ \to \pi^+ \pi^0 (\gamma)$, searching for the decay of the $\pi^0$ to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of $4.4 \times10^{-9}$ is set on the branching ratio at 90% confidence level, improving on previous results by a factor of 60. This result can also be interpreted as a model-independent upper limit on the branching ratio for the decay $K^+ \to \pi^+ X$, where $X$ is a particle escaping detection with mass in the range 0.110-0.155 GeV$/c^2$ and rest lifetime greater than 100 ps. Model-dependent upper limits are obtained assuming $X$ to be an axion-like particle with dominant fermion couplings or a dark scalar mixing with the Standard Model Higgs boson

A search for the K+→μ−νe+e+K^+\to\mu^-\nu e^+e^+ decay

A search for the $K^+\to\mu^-\nu e^+e^+$ 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 $8.1\times 10^{-11}$ is obtained for the decay branching fraction at 90% CL, improving by a factor of 250 over the previous search

Search for lepton number and flavour violation in K+K^+ and π0\pi^0 decays

Searches for the lepton number violating $K^+ \to \pi^− \mu^+ e^+$ decay and the lepton flavour violating $K^+ \to \pi^+ \mu^- e^+$ and $\pi^0 \to \mu^- e^+$ decays are reported using data collected by the NA62 experiment at CERN in 2017$-$2018. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% confidence level: B($K^+ \to \pi^− \mu^+ e^+$) $K+→π-μ+e+ decay and the lepton flavor violating K+→π+μ-e+ and π0→μ-e+ decays are reported using data collected by the NA62 experiment at CERN in 2017–2018. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90$K^{+} \rightarrow \pi^{-} \mu^{+} e^{+}$decay and the lepton flavour violating$K^{+} \rightarrow \pi^{+} \mu^{-} e^{+}$and$\pi^{0} \rightarrow \mu^{-} e^{+}$decays are reported using data collected by the NA62 experiment at CERN in$2017$-$2018$. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90$\mathcal{B}(K^{+}\rightarrow\pi^{-}\mu^{+}e^{+})<4.2\times 10^{-11}$,$\mathcal{B}(K^{+}\rightarrow\pi^{+}\mu^{-}e^{+})<6.6\times10^{-11}$and$\mathcal{B}(\pi^{0}\rightarrow\mu^{-}e^{+})<3.2\times 10^{-10}$. These results improve by one order of magnitude over previous results for these decay modes

Search for K+K^+ decays to a muon and invisible particles

The NA62 experiment at CERN reports searches for $K^+\to\mu^+N$ and $K^+\to\mu^+\nu X$ 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 ${\cal O}(10^{-8})$ of the neutrino mixing parameter $|U_{\mu4}|^2$ for $N$ masses in the range 200-384 MeV/$c^2$ 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/$c^2$ are reported for the first time, ranging from ${\cal O}(10^{-5})$ to ${\cal O}(10^{-7})$. An improved upper limit of $1.0\times 10^{-6}$ is established at 90% CL on the $K^+\to\mu^+\nu\nu\bar\nu$ branching fraction.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.The NA62 experiment at CERN reports searches for $K^+\to\mu^+N$ and $K^+\to\mu^+\nu X$ 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 ${\cal O}(10^{-8})$ of the neutrino mixing parameter $|U_{\mu4}|^2$ for $N$ masses in the range 200-384 MeV/$c^2$ 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/$c^2$ are reported for the first time, ranging from ${\cal O}(10^{-5})$ to ${\cal O}(10^{-7})$. An improved upper limit of $1.0\times 10^{-6}$ is established at 90% CL on the $K^+\to\mu^+\nu\nu\bar\nu$ branching fraction