We present the first results from a proof-of-concept search for dark sectors via invisible decays of pseudoscalar η and η' mesons in the NA64h experiment at the CERN SPS. Our novel technique uses the charge-exchange reaction of 50 GeV π^- on nuclei of an active target as the source of neutral mesons. The η, η' → invisible events would exhibit themselves via a striking signature - the complete disappearance of the incoming beam energy in the detector. No evidence for such events has been found with 2.9×10^9 pions on target accumulated during one day of data taking. This allows us to set a stringent limit on the branching ratio Br(η' → invisible) < 2.1 × 10^-4 improving the current bound by a factor of ≃3. We also set a limit on Br(η→ invisible) < 1.1 × 10^-4 comparable with the existing one. These results demonstrate the great potential of our approach and provide clear guidance on how to enhance and extend the sensitivity for dark sector physics from future searches for invisible neutral meson decays.

We present the results of a missing-energy search for Light Dark Matter which has a new interaction with ordinary matter transmitted by a vector boson, called dark photon $A^\prime$. For the first time, this search is performed with a positron beam by using the significantly enhanced production of $A^\prime$ in the resonant annihilation of positrons with atomic electrons of the target nuclei, followed by the invisible decay of $A^\prime$ into dark matter. No events were found in the signal region with $(10.1 \pm 0.1)~\times~10^{9}$ positrons on target with 100 GeV energy. This allowed us to set new exclusion limits that, relative to the collected statistics, prove the power of this experimental technique. This measurement is a crucial first step toward a future exploration program with positron beams, whose estimated sensitivity is here presented.

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 GeV/c are directed to an active target. The signal signature consists of a single scattered muon with momentum <80 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±0.02)×1010 muons on target, no event is observed in the expected signal region. This allows us to set new limits on the remaining (mZ′,gZ′) parameter space of a new Z′ (Lμ−Lτ) vector boson which could explain the muon (g−2)μ 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. Published by the American Physical Society 2024

Abstract The inclusion of an additional U(1) gauge Lμ − Lτ symmetry would release the tension between the measured and the predicted value of the anomalous muon magnetic moment: this paradigm assumes the existence of a new, light Z′ vector boson, with dominant coupling to μ and τ leptons and interacting with electrons via a loop mechanism. The Lμ − Lτ model can also explain the Dark Matter relic abundance, by assuming that the Z′ boson acts as a “portal” to a new Dark Sector of particles in Nature, not charged under known interactions. In this work we present the results of the Z′ search performed by the NA64-e experiment at CERN SPS, that collected ~ 9 × 1011 100 GeV electrons impinging on an active thick target. Despite the suppressed Z′ production yield with an electron beam, NA64-e provides the first accelerator-based results excluding the g − 2 preferred band of the Z′ parameter space in the 1 keV <$$ {m}_{Z^{\prime }} $$ m Z ′ ≲ 2 MeV range, in complementarity with the limits recently obtained by the NA64-μ experiment with a muon beam.

A search for dark sectors is performed using the unique M2 beam line at the CERN Super Proton Synchrotron. New particles (X) could be produced in the bremsstrahlung-like reaction of high-energy 160 GeV positively charged muons impinging on an active target, mu N -> mu NX, followed by their decays, X -> invisible. The experimental signature would be a scattered single muon from the target, with about less than half of its initial energy and no activity in the subdetectors located downstream from the interaction point. The full sample of the 2022 run is analyzed through the missing-energy/momentum channel, with total statistics of (1.98 +/- 0.02) x 10(10) muons on target. We demonstrate that various muonphilic scenarios involving different types of mediators, such as scalar or vector particles, can be probed simultaneously with such a technique. For the vector case, besides a L-mu - L-tau Z' vector boson, we also consider an invisibly decaying dark photon (A' -> invisible). This search is complementary to NA64 running with electrons and positrons, thus opening the possibility to expand the exploration of the thermal light dark matter parameter space by combining the results obtained with the three beams.

The search for new comparably light (well below the electroweak scale) feebly interacting particles is an exciting possibility to explain some mysterious phenomena in physics, among them the origin of Dark Matter. The sensitivity study through detailed simulation of projected experiments is a key point in estimating their potential for discovery. Several years ago we created the DMG4 package for the simulation of DM (Dark Matter) particles in fixed target experiments. The natural approach is to integrate this simulation into the same program that performs the full simulation of particles in the experiment setup. The Geant4 toolkit framework was chosen as the most popular and versatile solution nowadays. The simulation of DM particles production by this package accommodates several possible scenarios, employing electron, muon or photon beams and involving various mediators, such as vector, axial vector, scalar, pseudoscalar, or spin 2 particles. The bremsstrahlung, annihilation or Primakoff processes can be simulated. The package DMG4 contains a subpackage DarkMatter with cross section methods weakly connected to Geant4. It can be used in different frameworks. In this paper, we present the latest developments of the package, such as extending the list of possible mediator particle types, refining formulas for the simulation and extending the mediator mass range. The user interface is also made more flexible and convenient. In this work, we also demonstrate the usage of the package, the improvements in the simulation accuracy and some cross check validations.

We present the first results from a proof-of-concept search for dark sectors via invisible decays of pseudoscalar eta and eta ' mesons in the NA64h experiment at the CERN SPS. Our novel technique uses the charge-exchange reaction of 50 GeV pi- on nuclei of an active target as the source of neutral mesons. The eta,eta'-> invisible events would exhibit themselves via a striking signature-the complete disappearance of the incoming beam energy in the detector. No evidence for such events has been found with 2.9x109 pions on target accumulated during one day of data taking. This allows us to set a stringent limit on the branching ratio Br(eta'-> invisible) < 2.1 x 10(-4) improving the current bound by a factor of similar or equal to 3. We also set a limit on Br(eta -> invisible) < 1.1 x 10(-4) comparable with the existing one. These results demonstrate the great potential of our approach and provide clear guidance on how to enhance and extend the sensitivity for dark sector physics from future searches for invisible neutral meson decays.

We report on a search for a new $Z'$ ($L_\mu-L_\tau$) vector boson 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 GeV/c are directed to an active target. A signal event is a single scattered muon with momentum $<$ 80 GeV/c in the final state, accompanied by missing energy, i.e. no detectable activity in the downstream calorimeters. For a total statistic of $(1.98\pm0.02)\times10^{10}$ muons on target, no event is observed in the expected signal region. This allows us to set new limits on part of the remaining $(m_{Z'},\ g_{Z'})$ parameter space which could provide an explanation for 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.

In this study, we present the measurement of the intrinsic hadronic contamination at the CERN SPS H4 beamline configured to transport electrons and positrons at 100 GeV/c momentum. The analysis was performed using data collected by the NA64-$e$ experiment in 2022. Our study is based on calorimetric measurements, exploiting the different interaction mechanisms of electrons and hadrons in the NA64-ECAL and NA64-HCAL detectors. We determined the intrinsic hadronic contamination by comparing the results obtained using the nominal electron/positron beamline configuration with those obtained in a dedicated setup, in which only hadrons impinged on the detector. The significant differences in the experimental signatures of electrons and hadrons motivated our approach, resulting in a small and well-controlled systematic uncertainty for the measurement. Our study allowed us to precisely determine the intrinsic hadronic contamination, which represents a crucial parameter for the NA64 experiment in which the hadron contaminants may result in non-trivial backgrounds. Moreover, we performed dedicated Monte Carlo simulations for the hadron production induced by the primary T2 target. We found a good agreement between measurements and simulation results, confirming the validity of the applied methodology and our evaluation of the intrinsic hadronic contamination.

We present the measurement of the intrinsic hadronic contamination at the CERN SPS H4 beamline configured to transport electrons and positrons at 100 GeV/c. The analysis, performed using data collected by the NA64-e experiment in 2022, is based on calorimetric measurements, exploiting the different interaction mechanisms of electrons and hadrons in the NA64 detector. We determined the contamination by comparing the results obtained using the nominal electron/positron beamline configuration with those from a dedicated setup, in which only hadrons impinged on the detector. We also obtained an estimate of the relative protons, anti-protons and pions yield by exploiting the different absorption probabilities of these particles in matter. We cross-checked our results with a dedicated Monte Carlo simulation for the hadron production at the primary T2 target, finding a good agreement with the experimental measurements.

Thermal dark matter models with particle χ masses below the electroweak scale can provide an explanation for the observed relic dark matter density. This would imply the existence of a new feeble interaction between the dark and ordinary matter. We report on a new search for the sub-GeV χ production through the interaction mediated by a new vector boson, called the dark photon A', in collisions of 100 GeV electrons with the active target of the NA64 experiment at the CERN SPS. With 9.37×10^11 electrons on target collected during 2016-2022 runs NA64 probes for the first time the well-motivated region of parameter space of benchmark thermal scalar and fermionic dark matter models. No evidence for dark matter production has been found. This allows us to set the most sensitive limits on the A' couplings to photons for masses m_A'≲ 0.35 GeV, and to exclude scalar and Majorana dark matter with the χ-A' coupling α_D ≤ 0.1 for masses 0.001 ≲ m_χ≲ 0.1 GeV and 3m_χ≤ m_A'.

A light Z0 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 og - 2 thorn mu anomaly. This boson can be produced in the bremsstrahlung reaction mu N - mu NZ0 after a high energy muon beam collides with a target. NA64 mu is a fixed-target experiment using a 160 GeV muon beam from the CERN Super Proton Synchrotron accelerator looking for Z0 production and its subsequent decays, Z0 - invisible. In this paper, we present the study of the NA64 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, NA64 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.

A search for a new $Z'$ gauge boson associated with (un)broken B-L symmetry in the keV-GeV mass range is carried out for the first time using the missing-energy technique in the NA64 experiment at the CERN SPS. From the analysis of the data with 3.22e11 electrons on target collected during 2016-2021 runs no signal events were found. This allows to derive new constraints on the $Z'-e$ coupling strength, which for the mass range $0.3 < m_{Z'} < 100$ MeV are more stringent compared to those obtained from the neutrino-electron scattering data.

The extension of Standard Model made by inclusion of additional $U(1)$ gauge $L_\mu-L_\tau$ symmetry can explain the difference between the measured and the predicted value of the muon magnetic moment and solve the tension in $B$ meson decays. This model predicts the existence of a new, light $Z^\prime$ vector boson, predominantly coupled to second and third generation leptons, whose interaction with electrons is due to a loop mechanism involving muons and taus. In this work, we present a rigorous evaluation of the upper limits in the $Z^\prime$ parameter space, obtained from the analysis of the data collected by the NA64-$e$ experiment at CERN SPS, that performed a search for light dark matter with $2.84\times10^{11}$ electrons impinging with 100 GeV on an active thick target. The resulting limits, despite being included in a region already investigated by neutrino experiments,touch the muon $g-2$ preferred band for values of the $Z^\prime$ mass of order of 1 MeV. The sensitivity projections for the future high-statistics NA64-$e$ runs demonstrate the power of the electrons/positron beam approach in this theoretical scenario.

Muons are puzzling physicists since their discovery when they were first thought to be the meson predicted by Yukawa to mediate the strong force. The recent results at Fermilab on the muon g-2 anomaly puts the muonic sector once more under the spotlight and calls for new measurements with this fascinating particle. Here we present the results of the first measurement of the $2S_{1/2},F$=$0 \rightarrow 2P_{1/2},F$=$1$ transition in Muonium, the hydrogen-like bound state of a positive muon and an electron. The measured value of 580.6 $\pm$6.8 MHz is in agreement with the theoretical calculations. From this measurement a value of the Lamb shift of 1045.5 $\pm$6.8 MHz is extracted, compatible with previous experiments. We also determine for the first time the $2S$ hyperfine splitting in Muonium to be 559.6$\pm$7.2 MHz. The measured transition being isolated from the other hyperfine levels holds the promise to provide an improved determination of the Muonium Lamb shift at a level where bound state QED recoil corrections not accessible in hydrogen could be tested. Such a measurement will also be sensitive to new physics in the muonic sector, e.g. to new bosons which might provide an explanation of the g-2 muon anomaly or Lorentz and CPT violation. We also present the first observation of Muonium in the $n = 3$ excited state opening up the possibility of new precise microwave measurements as realized in hydrogen.

The extension of Standard Model made by inclusion of additional $U(1)$ gauge ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ symmetry can explain the difference between the measured and the predicted value of the muon magnetic moment and solve the tension in $B$ meson decays. This model predicts the existence of a new, light ${Z}^{\ensuremath{'}}$ vector boson, predominantly coupled to second and third generation leptons, whose interaction with electrons is due to a loop mechanism involving muons and taus. In this work, we present a rigorous evaluation of the upper limits in the ${Z}^{\ensuremath{'}}$ parameter space, obtained from the analysis of the data collected by the NA64-$e$ experiment at CERN SPS, that performed a search for light dark matter with $2.84\ifmmode\times\else\texttimes\fi{}{10}^{11}$ electrons impinging with 100 GeV on an active thick target. The resulting limits touch the muon $g\ensuremath{-}2$ preferred band for values of the ${Z}^{\ensuremath{'}}$ mass of order of 1 MeV, while the sensitivity projections for the future high-statistics NA64-$e$ runs demonstrate the power of the electrons/positron beam approach in this theoretical scenario.

Muons are puzzling physicists since their discovery when they were first thought to be the meson predicted by Yukawa to mediate the strong force. The recent results at Fermilab on the muon g-2 anomaly puts the muonic sector once more under the spotlight and calls for new measurements with this fascinating particle. Here we present the results of the first measurement of the 2S_1/2,F=0 → 2P_1/2,F=1 transition in Muonium, the hydrogen-like bound state of a positive muon and an electron. The measured value of 580.6 ±6.8 MHz is in agreement with the theoretical calculations. From this measurement a value of the Lamb shift of 1045.5 ±6.8 MHz is extracted, compatible with previous experiments. We also determine for the first time the 2S hyperfine splitting in Muonium to be 559.6±7.2 MHz. The measured transition being isolated from the other hyperfine levels holds the promise to provide an improved determination of the Muonium Lamb shift at a level where bound state QED recoil corrections not accessible in hydrogen could be tested. Such a measurement will also be sensitive to new physics in the muonic sector, e.g. to new bosons which might provide an explanation of the g-2 muon anomaly or Lorentz and CPT violation. We also present the first observation of Muonium in the n = 3 excited state opening up the possibility of new precise microwave measurements as realized in hydrogen.

Muons are puzzling physicists since their discovery when they were first thought to be the meson predicted by Yukawa to mediate the strong force. The recent results at Fermilab on the muon g-2 anomaly puts the muonic sector once more under the spotlight and calls for new measurements with this fascinating particle. Here we present the results of the first measurement of the $2S_{1/2},F$=$0 \rightarrow 2P_{1/2},F$=$1$ transition in Muonium, the hydrogen-like bound state of a positive muon and an electron. The measured value of 580.6 $\pm$6.8 MHz is in agreement with the theoretical calculations. From this measurement a value of the Lamb shift of 1045.5 $\pm$6.8 MHz is extracted, compatible with previous experiments. We also determine for the first time the $2S$ hyperfine splitting in Muonium to be 559.6$\pm$7.2 MHz. The measured transition being isolated from the other hyperfine levels holds the promise to provide an improved determination of the Muonium Lamb shift at a level where bound state QED recoil corrections not accessible in hydrogen could be tested. Such a measurement will also be sensitive to new physics in the muonic sector, e.g. to new bosons which might provide an explanation of the g-2 muon anomaly or Lorentz and CPT violation. We also present the first observation of Muonium in the $n = 3$ excited state opening up the possibility of new precise microwave measurements as realized in hydrogen.

We report a new measurement of the n=2 Lamb shift in Muonium. Our result of 1047.2(2.3)_{stat}(1.1)_{syst}  MHz comprises an order of magnitude improvement upon the previous best measurement. This value matches the theoretical calculation within 1 standard deviation allowing us to set limits on Lorentz and CPT violation in the muonic sector, as well as on new physics coupled to muons and electrons which could provide an explanation of the muon g-2 anomaly.