Lyubovitskij, Valery

A search for sub-GeV dark matter production mediated by a new vector boson A′, called dark photon, is performed by the NA64 experiment in missing energy events from 100 GeV electron interactions in an active beam dump at the CERN SPS. From the analysis of the data collected in the years 2016, 2017, and 2018 with 2.84×1011 electrons on target no evidence of such a process has been found. The most stringent constraints on the A′ mixing strength with photons and the parameter space for the scalar and fermionic dark matter in the mass range ≲0.2 GeV are derived, thus demonstrating the power of the active beam dump approach for the dark matter search.

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 A0 , in collisions of 100 GeV electrons with the active target of the NA64 experiment at the CERN SPS. With 9.37 × 1011 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 A0 couplings to photons for masses mA0 ≲ 0.35 GeV, and to exclude scalar and Majorana dark matter with the χ − A0 coupling αD ≤ 0.1 for masses 0.001 ≲ mχ ≲ 0.1 GeV and 3mχ ≤ mA0 .

We found a new important tree-level contribution to muon–electron nuclear conversion from neutrino exchange between two quarks in the same nucleon and demonstrated that this process, contrary to common belief, can be observed in the near future experiments if there exists a mixed sterile-active neutrino state νh heavier than the quark confinement scale Λc ∼ 1 GeV. From the present non-observation of muon–electron conversion we derive new experimental constraints on νh − νe,µ mixing

We performed a search for a new generic X boson, which could be a scalar (S), pseudoscalar (P), vector (V) or an axial vector (A) particle produced in the 100 GeV electron scattering off nuclei, e−Z→e−ZX, followed by its invisible decay in the NA64 experiment at CERN. No evidence for such process was found in the full NA64 data set of 2.84×1011 electrons on target. We place new bounds on the S,P,V,A coupling strengths to electrons, and set constraints on their contributions to the electron anomalous magnetic moment ae, |ΔaX|≲10−15−10−13 for the X mass region mX≲1 GeV. These results are an order of magnitude more sensitive compared to the current accuracy on ae from the electron g−2 experiments and recent high-precision determination of the fine structure constant.

We study nuclear µ−–e− conversion in the general framework of an effective Lagrangian approach without referring to any specific realization of the physics beyond the Standard Model (SM) responsible for lepton flavor violation (L/f ). We show that vector meson exchange between lepton and nucleon currents plays an important role in this process. A new issue of this mechanism is the presence of the strange quark vector current contribution induced by the φ meson. This allows us to extract new limits on the L/f lepton–quark effective couplings from the existing experimental data.

We report the first results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of eþe− pairs observed in the excited 8 Be nucleus decays. Because of its coupling to electrons, the X could be produced in the bremsstrahlung reaction e−Z → e−ZX by a 100 GeV e− beam incident on an active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through the subsequent decay into a eþe− pair. With 5.4 × 1010 electrons on target, no evidence for such decays was found, allowing us to set first limits on the X − e− coupling in the range 1.3 × 10−4 ≲ ϵe ≲ 4.2 × 10−4 excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (A0 ) from nonobservation of the decay A0 → eþe− of the bremsstrahlung A0 with a mass ≲23 MeV

A search is performed for a new sub-GeV vector boson (A0 ) mediated production of dark matter (χ) in the fixed-target experiment, NA64, at the CERN SPS. The A0 , called dark photon, can be generated in the reaction e−Z → e−ZA0 of 100 GeV electrons dumped against an active target followed by its prompt invisible decay A0 → χχ¯. The experimental signature of this process would be an event with an isolated electron and large missing energy in the detector. From the analysis of the data sample collected in 2016 corresponding to 4.3 × 1010 electrons on target no evidence of such a process has been found. New stringent constraints on the A0 mixing strength with photons, 10−5 ≲ ϵ ≲ 10−2, for the A0 mass range mA0 ≲ 1 GeV are derived. For models considering scalar and fermionic thermal dark matter interacting with the visible sector through the vector portal the 90% C.L. limits 10−11 ≲ y ≲ 10−6 on the dark-matter parameter y ¼ ϵ2αDð mχ mA0 Þ4 are obtained for the dark coupling constant αD ¼ 0.5 and dark-matter masses 0.001 ≲ mχ ≲ 0.5 GeV. The lower limits αD ≳ 10−3 for pseudo-Dirac dark matter in the mass region mχ ≲ 0.05 GeV are more stringent than the corresponding bounds from beam dump experiments. The results are obtained by using exact tree level calculations of the A0 production cross sections, which turn out to be significantly smaller compared to the one obtained in the Weizsäcker-Williams approximation for the mass region mA0 ≳ 0.1 GeV.

AbstractWe report the results of a search for a new vector boson ($$ A'$$ A ′ ) decaying into two dark matter particles $$\chi _1 \chi _2$$ χ 1 χ 2 of different mass. The heavier $$\chi _2$$ χ 2 particle subsequently decays to $$\chi _1$$ χ 1 and an off-shell Dark Photon $$ A'^* \rightarrow e^+e^-$$ A ′ ∗ → e + e - . For a sufficiently large mass splitting, this model can explain in terms of new physics the recently confirmed discrepancy observed in the muon anomalous magnetic moment at Fermilab. Remarkably, it also predicts the observed yield of thermal dark matter relic abundance. A detailed Monte-Carlo simulation was used to determine the signal yield and detection efficiency for this channel in the NA64 setup. The results were obtained re-analyzing the previous NA64 searches for an invisible decay $$A'\rightarrow \chi \overline{\chi }$$ A ′ → χ χ ¯ and axion-like or pseudo-scalar particles $$a \rightarrow \gamma \gamma $$ a → γ γ . With this method, we exclude a significant portion of the parameter space justifying the muon g-2 anomaly and being compatible with the observed dark matter relic density for $$A'$$ A ′ masses from 2$$m_e$$ m e up to 390 MeV and mixing parameter $$\varepsilon $$ ε between $$3\times 10^{-5}$$ 3 × 10 - 5 and $$2\times 10^{-2}$$ 2 × 10 - 2 .

We present the performance of multiplexed XY resistive Micromegas detectors tested in the CERN SPS 100 GeV/c electron beam at intensities up to 3.3 × 105 e −∕(s ⋅ cm2 ). So far, all studies with multiplexed Micromegas have only been reported for tests with radioactive sources and cosmic rays. The use of multiplexed modules in high intensity environments was not explored due to the effect of ambiguities in the reconstruction of the hit point caused by the multiplexing feature. For the specific mapping and beam intensities analyzed in this work with a multiplexing factor of five, more than 50% level of ambiguity is introduced due to particle pile-up as well as fake clusters due to the mapping feature. Our results prove that by using the additional information of cluster size and integrated charge from the signal clusters induced on the XY strips, the ambiguities can be reduced to a level below 2%. The tested detectors are used in the CERN NA64 experiment for tracking the incoming particles bending in a magnetic field in order to reconstruct their momentum. The average hit detection efficiency of each module was found to be ∼96% at the highest beam intensities. By using four modules a tracking resolution of 1.1% was obtained with ∼85% combined tracking efficiency.

The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson A0 were set by the NA64 experiment for the mass region mA0 ≲ 250 MeV, by analyzing data from the interaction of 2.84 × 1011 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including A0 production via secondary positron annihilation with atomic electrons, we extend these limits in the 200–300 MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation p