Measurement of the $K^+\to\pi^+\gamma\gamma$ decay

The NA62 collaboration Cortina Gil, Eduardo ; Minucci, Elisa ; Padolski, Sergey ; et al.
Phys.Lett.B 850 (2024) 138513, 2024.
Inspire Record 2718968 DOI 10.17182/hepdata.147259

A sample of 3984 candidates of the $K^+\to\pi^+\gamma\gamma$ decay, with an estimated background of $291\pm14$ events, was collected by the NA62 experiment at CERN during 2017-2018. In order to describe the observed di-photon mass spectrum, the next-to-leading order contribution in chiral perturbation theory was found to be necessary. The decay branching ratio in the full kinematic range is measured to be $(9.61\pm0.17)\times10^{-7}$. The first search for production and prompt decay of an axion-like particle with gluon coupling in the process $K^+\to\pi^+a$, $a\to\gamma\gamma$ is also reported.

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See caption of Figure 6.

Upper limits at 90% CL of $B(K^+\to\pi^+a)\times B(a\to\gamma\gamma)$ in the prompt ALP decay assumption.

See caption of Figure 9.

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Search for $K^+$ decays into the $\pi^+e^+e^-e^+e^-$ final state

The NA62 collaboration Cortina Gil, Eduardo ; Minucci, Elisa ; Padolski, Sergey ; et al.
Phys.Lett.B 846 (2023) 138193, 2023.
Inspire Record 2676523 DOI 10.17182/hepdata.144587

The first search for ultra-rare $K^+$ decays into the $\pi^+e^+e^-e^+e^-$ final state is reported, using a dataset collected by the NA62 experiment at CERN in 2017-2018. An upper limit of $1.4\times 10^{-8}$ at 90% CL is obtained for the branching ratio of the $K^+\to\pi^+e^+e^-e^+e^-$ decay, predicted in the Standard Model to be $(7.2\pm0.7)\times 10^{-11}$. Upper limits at 90% CL are obtained at the level of $10^{-9}$ for the branching ratios of two prompt decay chains involving pair-production of hidden-sector mediators: $K^+\to\pi^+aa$, $a\to e^+e^-$ and $K^+\to\pi^+S$, $S\to A^\prime A^\prime$, $A^\prime\to e^+e^-$.

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See caption of Fig 4.


A measurement of the $K^{+} \to \pi^{+} \mu^{+} \mu^{-}$ decay

The NA62 collaboration Cortina Gil, Eduardo ; Potrebenikov, Yuri ; Kleimenova, Alina ; et al.
JHEP 11 (2022) 011, 2022.
Inspire Record 2150453 DOI 10.17182/hepdata.135498

A sample of 2.8 × 10$^{4}$K$^{+}$ → π$^{+}$μ$^{+}$μ$^{−}$ candidates with negligible background was collected by the NA62 experiment at the CERN SPS in 2017–2018. The model-independent branching fraction is measured to be (9.15 ± 0.08) × 10$^{−8}$, a factor three more precise than previous measurements. The decay form factor is presented as a function of the squared dimuon mass. A measurement of the form factor parameters and their uncertainties is performed using a description based on Chiral Perturbation Theory at $ \mathcal{O} $(p$^{6}$).

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Reconstructed $K^+ \to \pi^+ \mu^+ \mu^-$ differential decay width. Only statistical errors are provided. The 4-body differential decay width (shown in green in Figure 3-left) is, in general, required to fit these data points; fitting the squared modulus of the form factor may therefore be preferable.

Reconstructed squared modulus of the $K^+ \to \pi^+ \mu^+ \mu^-$ form factor. Only statistical errors are provided.


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

The NA62 collaboration Cortina Gil, Eduardo ; Minucci, Elisa ; Padolski, Sergey ; et al.
JHEP 06 (2021) 093, 2021.
Inspire Record 1854186 DOI 10.17182/hepdata.106641

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.

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Observed and expected upper limits on branching ratio \(K^{+}\rightarrow\pi^{+}X\) at 90% CL.

Observed upper limits on branching ratio \(K^{+}\rightarrow\pi^{+}X\) at 90% CL as functions of X mass and lifetime.

Exclusion region limits on coupling strength \(sin^{2}\theta\) at 90% CL as a function of X mass, for visible X decays.


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

The NA62 collaboration Cortina Gil, Eduardo ; Minucci, Elisa ; Padolski, Sergey ; et al.
Phys.Lett.B 816 (2021) 136259, 2021.
Inspire Record 1843945 DOI 10.17182/hepdata.103974

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.

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See caption of Fig 5.


Search for a feebly interacting particle $X$ in the decay $K^{+}\rightarrow\pi^{+}X$

The NA62 collaboration Cortina Gil, Eduardo ; Minucci, Elisa ; Padolski, Sergey ; et al.
JHEP 03 (2021) 058, 2021.
Inspire Record 1832447 DOI 10.17182/hepdata.102393

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.

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Observed and expected upper limits on branching ratio \(K^{+}\rightarrow\pi^{+}X\) at 90% CL.

Observed upper limits on branching ratio \(K^{+}\rightarrow\pi^{+}X\) at 90% CL as functions of X mass and lifetime.

Exclusion region limits on coupling strength \(sin^{2}\theta\) at 90% CL as a function of X mass, for visible X decays.

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Search for $\pi^0$ decays to invisible particles

The NA62 collaboration Cortina Gil, Eduardo ; Minucci, Elisa ; Padolski, Sergey ; et al.
JHEP 02 (2021) 201, 2021.
Inspire Record 1822910 DOI 10.17182/hepdata.102452

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.

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The expected upper limit refers to absence of signal.

See caption of Fig 6.

ALP width dominantly visible, see caption of Fig 7.

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Search for heavy neutral lepton production in $K^+$ decays to positrons

The NA62 collaboration Cortina Gil, Eduardo ; Minucci, Elisa ; Padolski, Sergey ; et al.
Phys.Lett.B 807 (2020) 135599, 2020.
Inspire Record 1797041 DOI 10.17182/hepdata.95927

A search for heavy neutral lepton ($N$) production in $K^+\to e^+N$ 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 $|U_{e4}|^2$ are established at the level of $10^{-9}$ over most of the accessible heavy neutral lepton mass range 144--462 MeV/$c^2$, with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The $|U_{e4}|^2$ range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/$c^2$.

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See caption of Fig 6.


Search for production of an invisible dark photon in $\pi^0$ decays

The NA62 collaboration Cortina Gil, Eduardo ; Minucci, Elisa ; Padolski, Sergey ; et al.
JHEP 05 (2019) 182, 2019.
Inspire Record 1726101 DOI 10.17182/hepdata.102492

The results of a search for $\pi^0$ decays to a photon and an invisible massive dark photon at the NA62 experiment at the CERN SPS are reported. From a total of $4.12\times10^8$ tagged $\pi^0$ mesons, no signal is observed. Assuming a kinetic-mixing interaction, limits are set on the dark photon coupling to the ordinary photon as a function of the dark photon mass, improving on previous searches in the mass range 60--110 MeV/$c^2$. The present results are interpreted in terms of an upper limit of the branching ratio of the electro-weak decay $\pi^0 \to \gamma \nu \overline{\nu}$, improving the current limit by more than three orders of magnitude.

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See caption of Fig 6.

See caption of Fig 6.

90% CL expected upper limit refers to absence of signal in the region of squared missing mass above 0.0054 GeV^2.


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

The NA62 collaboration Cortina Gil, Eduardo ; Minucci, Elisa ; Padolski, Sergey ; et al.
Phys.Lett.B 778 (2018) 137-145, 2018.
Inspire Record 1640276 DOI 10.17182/hepdata.81001

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.

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Upper limits on |U_e4|^2 vs assumed neutrino mass [MeV].

Upper limits on |U_mu4|^2 vs assumed neutrino mass [MeV].