We carried out a model-independent search for light scalar ($s$) and pseudoscalar axionlike ($a$) particles that couple to two photons by using the high-energy CERN SPS H4 electron beam. The new particles, if they exist, could be produced through the Primakoff effect in interactions of hard bremsstrahlung photons generated by 100 GeV electrons in the NA64 active dump with virtual photons provided by the nuclei of the dump. The $a(s)$ would penetrate the downstream HCAL module, serving as a shield, and would be observed either through their $a(s)\to \gamma \gamma$ decay in the rest of the HCAL detector, or as events with a large missing energy if the $a(s)$ decays downstream of the HCAL. This method allows for the probing of the $a(s)$ parameter space, including those from generic axion models, inaccessible to previous experiments. No evidence of such processes has been found from the analysis of the data corresponding to $2.84\times {10}^{11}$ electrons on target, allowing us to set new limits on the $a(s)\gamma \gamma$-coupling strength for $a(s)$ masses below 55 MeV.

The improved results on a direct search for a new $X(16.7\mathrm{MeV})$ boson that could explain the anomalous excess of $e^{+}{e}^{-}$ pairs observed in the decays of the excited ${}^8{\mathrm{Be}}^{*}$ nuclei (“Berillium or X17 anomaly”) are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, ${}^4\mathrm{He}$, seems to support this anomaly spurring the need for an independent measurement. If the $X$ boson exists, it could be produced in the bremsstrahlung reaction $e^{-}Z\to e^{-}ZX$ by a high energy beam of electrons incident on the active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through its subsequent decay into $e^{+}{e}^{-}$ pairs. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to $8.4\times {10}^{10}$ electrons on target collected in 2017 and 2018. This allows one to set new limits on the $X-e^{-}$ coupling in the range $1.2\times {10}^{-4}\lesssim {\epsilon }_e\lesssim 6.8\times {10}^{-4}$, excluding part of the parameter space favored by the X17 anomaly, and setting new bounds on the mixing strength of photons with dark photons ($A^{\prime }$) with a mass $\lesssim 24\mathrm{MeV}$. For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling $\epsilon$. This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics.

In this paper, we present measurements of the ortho-positronium (ortho-Ps) emission energy in vacuum from mesoporous films using the time-of-flight technique. We show evidence of quantum mechanical confinement in the mesopores that defines the minimal energy of the emitted Ps. Two samples with different effective pore sizes, measured with positron annihilation lifetime spectroscopy, are compared for the data collected in the temperature range 50\char21{}400 K. The sample with smaller pore size exhibits a higher minimal energy (73\ifmmode\pm\else\textpm\fi{}5~meV), compared to the sample with bigger pores (48\ifmmode\pm\else\textpm\fi{}5~meV), due to the stronger confinement. The dependence of the emission energy with the temperature of the target is modeled as ortho-Ps being confined in rectangular boxes in thermodynamic equilibrium with the sample. We also measured that the yield of positronium emitted in vacuum is not affected by the temperature of the target.

We report the first search for dark sectors performed at the NA64 experiment employing a high energy muon beam and a missing energy-momentum technique. Muons from the M2 beamline at the CERN Super Proton Synchrotron with a momentum of $160\mathrm{GeV}/c$ are directed to an active target. The signal signature consists of a single scattered muon with momentum $<80\mathrm{GeV}/c$ in the final state, accompanied by missing energy, i.e., no detectable activity in the downstream calorimeters. For a total dataset of $(1.98\pm 0.02)\times {10}^{10}$ muons on target, no event is observed in the expected signal region. This allows us to set new limits on the remaining $(m_{Z^{\prime }},g_{Z^{\prime }})$ parameter space of a new $Z^{\prime }$ ($L_{\mu }-L_{\tau }$) vector boson which could explain the muon $(g-2{)}_{\mu }$ anomaly. Additionally, our study excludes part of the parameter space suggested by the thermal dark matter relic abundance. Our results pave the way to explore dark sectors and light dark matter with muon beams in a unique and complementary way to other experiments.

A light $Z^{\prime }$ vector boson coupled to the second and third lepton generations through the $L_{\mu }-L_{\tau }$ current with mass below 200 MeV provides a very viable explanation in terms of new physics to the recently confirmed $(g-2{)}_{\mu }$ anomaly. This boson can be produced in the bremsstrahlung reaction $\mu N\to \mu N{Z}^{\prime }$ after a high energy muon beam collides with a target. $\mathrm{NA}64\mu$ is a fixed-target experiment using a 160 GeV muon beam from the CERN Super Proton Synchrotron accelerator looking for $Z^{\prime }$ production and its subsequent decays, $Z^{\prime }\to \mathrm{invisible}$. In this paper, we present the study of the $\mathrm{NA}64\mu$ sensitivity to search for such a boson. This includes a realistic beam simulation, a detailed description of the detectors and a discussion about the main potential background sources. A pilot run is scheduled in order to validate the simulation results. If those are confirmed, $\mathrm{NA}64\mu$ will be able to explore all the remaining parameter space which could provide an explanation for the $g-2$ muon anomaly in the $L_{\mu }-L_{\tau }$ model.