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.

1 data table

See caption of Fig 5.


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.

3 data tables

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 $\pi^0$ 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.

5 data tables

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 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.

6 data tables

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 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/$c^2$.

1 data table

See caption of Fig 6.


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.

2 data tables

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

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


Measurement of the $\pi^0$ electromagnetic transition form factor slope

The NA62 collaboration Lazzeroni, C. ; Lurkin, N. ; Romano, A. ; et al.
Phys.Lett.B 768 (2017) 38-45, 2017.
Inspire Record 1506406 DOI 10.17182/hepdata.77001

The NA62 experiment collected a large sample of charged kaon decays in 2007 with a highly efficient trigger for decays into electrons. A measurement of the $\pi^0$ electromagnetic transition form factor slope parameter from $1.11\times10^{6}$ fully reconstructed $K^\pm \to \pi^\pm \pi^0_D, \pi^0_D \to e^+ e^-\gamma$ events is reported. The measured value $a = (3.68 \pm 0.57)\times10^{-2}$ is in good agreement with theoretical expectations and previous measurements, and represents the most precise experimental determination of the slope in the time-like momentum transfer region.

1 data table

Number of reconstructed data events and MC events as a function of the Dalitz kinematic variable $x=(M_{ee}/M_{\pi^0})^2$. The MC events are reweighted to correspond to a TFF slope a=0. The MC sample is not normalized to the size of the data. The values of $x$ quoted in the table are the barycenters of the 50 variable size bins.