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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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Search for hadronic decays of feebly-interacting particles at NA62

The NA62 collaboration Cortina Gil, Eduardo ; Jerhot, Jan ; Lurkin, Nicolas ; et al.
Eur.Phys.J.C 85 (2025) 571, 2025.
Inspire Record 2877075 DOI 10.17182/hepdata.156981

The NA62 experiment at CERN has the capability to collect data in a beam-dump mode, where 400 GeV protons are dumped on an absorber. In this configuration, New Physics particles, including dark photons, dark scalars, and axion-like particles, may be produced in the absorber and decay in the instrumented volume beginning approximately 80 m downstream of the dump. A search for these particles decaying in flight to hadronic final states is reported, based on an analysis of a sample of $1.4 \times 10^{17}$ protons on dump collected in 2021. No evidence of a New Physics signal is observed, excluding new regions of parameter spaces of multiple models.

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90% CL upper limit in dark photon coupling vs mass parameter space for combined di-lepton and hadronic final states, using bremsstrahlung production without the time-like form factor.

90% CL upper limit in dark photon coupling vs mass parameter space for combined di-lepton and hadronic final states, including mixing and bremsstrahlung production with a time-like form factor.

Number of expected $a \to K^+K^-\pi^0$ events for BR = 1 in the plane (mass, width) after full selection, for production in B meson decays assuming production coupling $C_{bs}/\Lambda=1\,\mathrm{GeV}^{-1}$. Acceptance for particles that reach the FV and decay therein and mass resolution.

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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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scalar production search, see caption of Fig 8.


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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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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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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Version 2
Search for leptonic decays of the dark photon at NA62

The NA62 collaboration Cortina Gil, Eduardo ; Jerhot, Jan ; Kleimenova, Alina ; et al.
Phys.Rev.Lett. 133 (2024) 111802, 2024.
Inspire Record 2738578 DOI 10.17182/hepdata.154438

The NA62 experiment at CERN, configured in beam-dump mode, has searched for dark photon decays in flight to electron-positron pairs using a sample of $1.4\times 10^{17}$ protons on dump collected in 2021. No evidence for a dark photon signal is observed. The combined result for dark photon searches in lepton-antilepton final states is presented and a region of the parameter space is excluded at 90% CL, improving on previous experimental limits for dark photon mass values between 50 and 600 MeV$/c^2$ and coupling values in the range $10^{-6}$ to $4\times10^{-5}$. An interpretation of the $e^+ e^-$ search result in terms of the emission and decay of an axion-like particle is also presented.

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Search for dark photon decays to $\mu^+\mu^-$ at NA62

The NA62 collaboration Cortina Gil, Eduardo ; Jerhot, Jan ; Kleimenova, Alina ; et al.
JHEP 09 (2023) 035, 2023.
Inspire Record 2642398 DOI 10.17182/hepdata.144533

The NA62 experiment at CERN, designed to study the ultra-rare decay $K^+ \to \pi^+\nu\overline{\nu}$, has also collected data in beam-dump mode. In this configuration, dark photons may be produced by protons dumped on an absorber and reach a decay volume beginning 80 m downstream. A search for dark photons decaying in flight to $\mu^+\mu^-$ pairs is reported, based on a sample of $1.4 \times 10^{17}$ protons on dump collected in 2021. No evidence for a dark photon signal is observed. A region of the parameter space is excluded at 90% CL, improving on previous experimental limits for dark photon masses between 215 and 550 MeV$/c^2$.

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Version 2
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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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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