We report on a direct search for sub-GeV dark photons (A^{'}), which might be produced in the reaction e^{-}Z→e^{-}ZA^{'} via kinetic mixing with photons by 100 GeV electrons incident on an active ...target in the NA64 experiment at the CERN SPS. The dark photons would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75×10^{9} electrons on target. We set new limits on the γ-A^{'} mixing strength and exclude the invisible A^{'} with a mass ≲100 MeV as an explanation of the muon g_{μ}-2 anomaly.
We report the first results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of e^{+}e^{-} 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 e^{+}e^{-} pair. With 5.4×10^{10} 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}≲ε_{e}≲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 (A^{'}) from nonobservation of the decay A^{'}→e^{+}e^{-} of the bremsstrahlung A^{'} with a mass ≲23 MeV.
A search for sub-GeV dark matter production mediated by a new vector boson A′, called a 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.
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) → γγ 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 × 10 11 electrons on target, allowing us to set new limits on the a(s)γγ -coupling strength for a (s) masses below 55 MeV.
A search is performed for a new sub-GeV vector boson (A′) mediated production of dark matter (χ) in the fixed-target experiment, NA64, at the CERN SPS. The A′, called dark photon, can be generated in ...the reaction e−Z→e−ZA′ of 100 GeV electrons dumped against an active target followed by its prompt invisible decay A′→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 A′ mixing strength with photons, 10−5≲ε≲10−2, for the A′ mass range mA′≲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=ε2αD(mχmA′)4 are obtained for the dark coupling constant αD=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 A′ production cross sections, which turn out to be significantly smaller compared to the one obtained in the Weizsäcker-Williams approximation for the mass region mA′≳0.1 GeV.
The Baksan Experiment on Sterile Transitions (BEST) was designed to investigate the deficit of electron neutrinos νe observed in previous gallium-based radiochemical measurements with high-intensity ...neutrino sources, commonly referred to as the "gallium anomaly," which could be interpreted as evidence for oscillations between νe and sterile neutrino (νs) states. A 3.414-MCi 51Cr νe source was placed at the center of two nested Ga volumes and measurements were made of the production of 71Ge through the charged current reaction, 71Ga(νe,e-)71Ge, at two average distances. The measured production rates for the inner and the outer targets, respectively, are 54.9$_{-2.4}^{+2.5}$(stat)±1.4(syst) and 55.6$_{-2.6}^{+2.7}$(stat)±1.4(syst) atoms of 71Ge/d. The ratio (R) of the measured rate of 71Ge production at each distance to the expected rate from the known cross section and experimental efficiencies are Rin=0.79±0.05 and Rout=0.77±0.05. The ratio of the outer to the inner result is 0.97±0.07, which is consistent with unity within uncertainty. The rates at each distance were found to be similar, but 20%-24% lower than expected, thus reaffirming the anomaly. Therefore these results are consistent with νe→νs oscillations with a relatively large Δm2 (> 0.5 eV2) and mixing sin22θ (≈0.4).
Reprocessing of spent nuclear fuel (SNF) is an important task in a frame of ecology and rational use of natural resources. Uranium, as the main component of SNF (>95%), can be recovered for further ...use as fresh nuclear fuel. To minimize an amount of solid radioactive waste generated during SNF reprocessing, new extractants are under investigation. Diamides of 1,10-phenanthroline-2,9-dicarboxylic acid are perspective tetradentate N-donor ligands that form strong complexes with f-elements, which are soluble in polar organic solvents. As an example of three ligands of this class, we conducted a comparative study and showed how the substituent in the amide functional group affects the extraction ability toward uranyl nitrate from nitric acid media. We have performed a careful study (NMR, FT-IR, XRD, RMC-EXAFS) of the structures of synthesized complexes of new ligands with uranyl nitrate and used quantum mechanical calculations to explain the discovered regularities through.
•BaFe12−xTixO19 (x ≤ 1) hexaferrites polycrystalline ceramics was investigated.•Substitution model by titanium cations was defined.•Mossbauer and Raman spectrometry was established.•The saturation ...magnetization decreasing with substitution level increasing was shown.•Dielectric constant and dielectric loss tangent values and temperature dependence were measured.
Titanium substituted BaFe12−xTixO19 (x ≤ 1) barium hexaferrites have been synthesized using the solid phase method. The phase purity and the crystal structure of the obtained solid solutions have been studied by X-ray diffraction. It was found that the unit cell parameters change a non-monotonically. The a parameter has a maximum of 5.896 Å at x = 0.25 and then it decreases almost linearly down to 5.886 Å at x = 1. The c parameter almost linearly increases from 23.216 Å up to 23.282 Å with x increasing. It has been discovered that titanium cations are located in the following positions: 4fIV tetrahedral and octahedral 4fVI and 12k ones. Results of the Mossbauer and Raman spectrometry confirm the established substitution mechanism. The homogeneous ferrimagnetic ordering is detected by magnetic measurements down to ~35 K. Below this temperature the noncollinear magnetic structure is formed as a result of Fe2+ cations change in the spin state from high to low configuration. Magnetization for all the samples is saturated at room temperature in fields of ~1 T. The saturation magnetization decreases from ~70.6 emu/g for x = 0.25 to ~56.7 emu/g for x = 1. The real part of the dielectric constant has value of ~0.2 for all the samples and gradually decreases at heating from room temperature up to ~500 K, after which it begins to increase sharply. The dielectric loss tangent has a maximum in the region of ~500 K and reaches a value of ~0.25 at a frequency of 104 Hz. With increasing the substitution level, the dielectric constant and the dielectric loss of all the samples increase.
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 a process was found in the full NA64 dataset 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 1 MeV ≲ mX ≲ 1 GeV. These results are an order of magnitude more sensitive compared to the current accuracy on a e from the electron g − 2 experiments and recent high-precision determination of the fine structure constant.