Search for π0→\pi^0 \to invisible decays with the NA62 experiment

The search for new physics beyond the Standard Model (SM) is one of the most active fields in particle physics. Complementary to direct searches for new processes at high-energy scale, rare or forbidden SM decays are investigated to look for deviations from the predictions. The NA62 experiment at the CERN SPS, designed for the measurement of the ultra-rare decay $K^+ \to \pi^+ \nu \bar\nu$, investigates new-physics contributions in the kaon and pion sector. The highly efficient, hermetic photon-veto system makes NA62 a perfect apparatus for a high-sensitivity search for $\pi^0$ decays into invisible particles. In a fraction of data collected by NA62 in 2017, about $8.4 \times 10^9 \pi^0$ mesons have been tagged from the reconstruction of the second most- abundant $K^+$ decay, $K^+ \to \pi^+ \pi^0 (\gamma)$. The background rejection power for visible $\pi^0$ decays, ranging from O($10^8$) to O($10^9$), is estimated by the combination of single-photon detection efficiencies measured in data with Monte Carlo simulations of $K^+ \to \pi^+ \pi^0 (\gamma)$ decays, a novel experimental technique with respect to that used in the most sensitive previous experimental result. The analysis is performed with the blind technique for a cut-based signal region. The $\pi^+$ momentum range is optimized to have the best sensitivity for the expected upper limit on the $\pi^0 \to$ invisible branching ratio in absence of a signal. No signal is observed in excess of the expected background fluctuations. The resulting upper limit on the branching ratio, BR($\pi^0 \to$ invisible) $\leq 4.4 \times 10^{-9}$ at 90% confidence level, improves on the previous best limit by a factor of 60. As a by-product of the analysis, the decay of $K^+ \to \pi^+ X$ is investigated, where $X$ is a neutral particle escaping detection with a mass in the range 0.110–0.155 GeV/$c^2$ and rest lifetime greater than 100 ps

Search for heavy neutrinos at the NA48/2 and NA62 experiments at CERN

The NA48/2 experiment at CERN has collected large samples of charged kaons decaying into a pion and two muons for the search of heavy nuetrinos. In addition, its successor NA62 has set new limits on the rate of charged kaon decay into a heavy neutral lepton (HNL) and a lepton, with l = e; μ, using the data collected in 2007 and 2015. New limits on heavy neutrinos from kaon decays into pions, muons and positrons are presented in this report

Search for heavy neutrinos at the NA48/2 and NA62 experiments at CERN

The NA48/2 experiment at CERN has collected large samples of charged kaons decaying into a pion and two muons for the search of heavy nuetrinos. In addition, its successor NA62 has set new limits on the rate of charged kaon decay into a heavy neutral lepton (HNL) and a lepton, with = e, µ, using the data collected in 2007 and 2015. New limits on heavy neutrinos from kaon decays into pions, muons and positrons are presented in this report

SEARCH FOR AN INVISIBLE VECTOR BOSON FROM π0\pi^{0} DECAYS AT NA62

The high-intensity setup, trigger system flexibility and detector performance make the NA62 experiment at CERN particularly suitable to perform direct searches for long-lived hidden-sector particles, such as dark photons, dark scalars, axion-like particles, and heavy neutral leptons, using kaon and pion decays as well as operating the experiment in dump mode. Results from NA62 will be presented on a search for $π^0$ decays to one photon and an invisible massive dark photon. From about 400 M $π^0$ decays, no signal is observed beyond the expected fluctuation of the background and limits are set in the plane of the dark photon coupling to ordinary photon versus the dark photon mass. The analysis has been also interpreted in terms of the branching ratio for the electroweak decay $π^0 → γνν^¯$

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

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

International audienceThe 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$/c^2$

Search for leptonic decays of the dark photon at NA62

International audienceThe 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

Search for leptonic decays of the dark photon at NA62

International audienceThe 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