arXiv subjects

NA62 Collaboration

Publications and source records attributed to NA62 Collaboration.

16 recordsLinked to original sources

arXiv2026

Measurement of the branching ratio of the $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ decay

The NA62 experiment has measured the branching ratio of the rare decay $K^+\to\pi^+\nu\bar{\nu}$ using data collected in 2023 and 2024 at the CERN SPS, obtaining ${\rm B}_{\rm 2023-2024}(K^+\to\pi^+\nu\bar{\nu}) = \left(7.2^{+2.3}_{-2.1}\right)\times10^{-11}$. With results from data collected between 2016 and 2022, the analysis gives ${\rm B}_{\rm 2016-2024}(K^+\to\pi^+\nu\bar{\nu}) = \left(9.6^{+1.9}_{-1.8}\right)\times10^{-11}$, corresponding to a relative precision of $20\%$. This value is consistent with Standard Model predictions.

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arXiv2025

Searches for hidden sectors using $K^+\to\pi^+X$ decays

Results from the study of the rare decays $K^+\to\pi^+\nu\bar{\nu}$, $K^{+}\rightarrow\pi^{+}\mu^{+}\mu^{-}$ and $K^{+}\rightarrow\pi^{+}\gamma\gamma$ at the NA62 experiment at CERN are interpreted in terms of improved limits for $\rm{B}(K^+\to\pi^+X)$ and coupling parameters of hidden-sector models, where $X$ is a mediator. World-leading limits are achieved for dark photon, dark scalar and axion-like particle models.

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arXiv2025

Search for heavy neutral leptons in $\pi^+$ decays to positrons

A search for heavy neutral lepton ($N$) production in $\pi^+\to e^+ N$ in-flight decays using data collected by the NA62 experiment at CERN in 2017-2024 is reported. Upper limits for the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the level of $10^{-8}$ for heavy neutral leptons with mass in the range 95-126 $MeV/c^2$ and lifetime exceeding 50 ns.

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arXiv2025

Search for hadronic decays of feebly-interacting particles at NA62

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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arXiv2024

Observation of the $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ decay and measurement of its branching ratio

A measurement of the $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. This dataset was recorded, after modifications to the beamline and detectors, at a higher instantaneous beam intensity with respect to the 2016--2018 data taking. Combining NA62 data collected in 2016--2022, a measurement of $B(K^{+}\rightarrow\pi^{+}\nu\bar{\nu}) = \left( 13.0^{+ 3.3}_{- 3.0} \right)\times10^{-11}$ is reported. With $51$ signal candidates observed and an expected background of $18^{+3}_{-2}$ events, $B(K^{+}\rightarrow\pi^{+}\nu\bar{\nu})$ becomes the smallest branching ratio measured with a signal significance above $5\,\sigma$.

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arXiv2024

First detection of a tagged neutrino in the NA62 experiment

The NA62 experiment at the CERN SPS reports the first detection of a tagged neutrino candidate based on the data collected in 2022. The candidate consists of a $K^+ \rightarrow \mu^+ \nu_\mu$ decay where the charged particles are reconstructed and the neutrino is detected through a charged-current interaction in a liquid krypton calorimeter.

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arXiv2023

Improved calorimetric particle identification in NA62 using machine learning techniques

Measurement of the ultra-rare $K^+\to\pi^+\nu\bar\nu$ decay at the NA62 experiment at CERN requires high-performance particle identification to distinguish muons from pions. Calorimetric identification currently in use, based on a boosted decision tree algorithm, achieves a muon misidentification probability of $1.2\times 10^{-5}$ for a pion identification efficiency of 75% in the momentum range of 15-40 GeV/$c$. In this work, calorimetric identification performance is improved by developing an algorithm based on a convolutional neural network classifier augmented by a filter. Muon misidentification probability is reduced by a factor of six with respect to the current value for a fixed pion-identification efficiency of 75%. Alternatively, pion identification efficiency is improved from 72% to 91% for a fixed muon misidentification probability of $10^{-5}$.

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arXiv2023

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

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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arXiv2022

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

A sample of $2.8 \times 10^4$ $K^{+} \to \pi^{+} \mu^{+} \mu^{-}$ 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 $(9.15 \pm 0.08)\times 10^{-8}$, 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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arXiv2021

Search for lepton number and flavour violation in $K^{+}$ and $\pi^{0}$ decays

Searches for the lepton number violating $K^{+} \rightarrow \pi^{-} \mu^{+} e^{+}$ 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% confidence level: $\mathcal{B}(K^{+}\rightarrow\pi^{-}\mu^{+}e^{+})<4.2\times 10^{-11}$, $\mathcal{B}(K^{+}\rightarrow\pi^{+}\mu^{-}e^{+})<6.6\times10^{-11}$ 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.

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arXiv2021

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

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arXiv2020

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

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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arXiv2020

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

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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arXiv2020

Rare decays at the CERN high-intensity kaon beam facility

Precision measurements of the branching ratios (BRs) for rare kaon decays can provide unique constraints on CKM unitarity and may reveal the existence of new physics. The BRs for two of these decays, $K^+\to\pi^+\nu\bar{\nu}$ and $K_L\to\pi^0\nu\bar{\nu}$, are strongly suppressed in the Standard Model, while their rates can be calculated with very small theoretical uncertainties.These decays are potentially sensitive to high mass scales, and several models of new physics predict large deviations from the Standard Model. Much progress has been made recently in the measurement of the $K\to\pi\nu\bar{\nu}$ BRs. Following upon NA62's successful application of the in-flight technique to measure ${\rm BR}(K^+\to\pi^+\nu\bar{\nu})$, we envision a comprehensive program for the study of the rare decay modes of both $K^+$ and $K_L$ mesons, to be carried out with high-intensity kaon beams from the CERN SPS in multiple phases, including both an experiment to measure ${\rm BR}(K^+\to\pi^+\nu\bar{\nu})$ at the 5% level and an experiment to measure ${\rm BR}(K_L\to\pi^0\nu\bar{\nu})$ at the 20% level. The detectors could also be reconfigured to allow measurements of $K_L$ decays with charged particles, such as $K_L\to\pi^0\ell^+\ell^-$.

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arXiv2020

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

The NA62 experiment reports an investigation of the $K^+\rightarrow\pi^+\nu\bar{\nu}$ mode from a sample of $K^+$ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of $(0.389\pm 0.024)\times10^{-10}$, corresponding to 2.2 events assuming the Standard Model branching ratio of $(8.4\pm1.0)\times10^{-11}$. 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 $1.78\times10^{-10}$ for the $K^+\rightarrow\pi^+\nu\bar{\nu}$ branching ratio at 90\%\,CL. This, together with the corresponding 68\%\,CL measurement of $(0.48^{+0.72 }_{-0.48})\times10^{-10}$, 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.

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arXiv2018

Search for $K^{+}\rightarrow\pi^{+}\nu\overline{\nu}$ at NA62

$K^{+}\rightarrow\pi^{+}\nu\overline{\nu}$ is one of the theoretically cleanest meson decay where to look for indirect effects of new physics complementary to LHC searches. The NA62 experiment at CERN SPS is designed to measure the branching ratio of this decay with 10\% precision. NA62 took data in pilot runs in 2014 and 2015 reaching the final designed beam intensity. The quality of 2015 data acquired, in view of the final measurement, will be presented.

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