We report the results of a search for a new vector boson ( A ' ) decaying into two dark matter particles χ 1 χ 2 of different mass. The heavier χ 2 particle subsequently decays to χ 1 and an off-shell Dark Photon 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 ' → χ χ ¯ and axion-like or pseudo-scalar particles 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 ' masses from 2 m e up to 390 MeV and mixing parameter ε between 3 × 10 - 5 and 2 × 10 - 2 .

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.

D. Banerjee, 5 J. Bernhard, V. E. Burtsev, A. G. Chumakov, D. Cooke, P. Crivelli∗,16 E. Depero, A. V. Dermenev, S. V. Donskov, R. R. Dusaev, T. Enik, N. Charitonidis, A. Feshchenko, V. N. Frolov, A. Gardikiotis, S. G. Gerassimov, 8 S. N. Gninenko∗,7 M. Hösgen, M. Jeckel, V. A. Kachanov, A. E. Karneyeu, G. Kekelidze, B. Ketzer, D. V. Kirpichnikov, M. M. Kirsanov, V. N. Kolosov, I. V. Konorov, 8 S. G. Kovalenko, V. A. Kramarenko, 9 L. V. Kravchuk, N. V. Krasnikov, 7 S. V. Kuleshov, V. E. Lyubovitskij, 15 V. Lysan, V. A. Matveev, Yu. V. Mikhailov, L. Molina Bueno, D. V. Peshekhonov, V. A. Polyakov, B. Radics, R. Rojas, A. Rubbia, V. D. Samoylenko, H. Sieber, D. Shchukin, V. O. Tikhomirov, I. Tlisova, D. A. Tlisov†,7 A. N. Toropin, A. Yu. Trifonov, B. I. Vasilishin, G. Vasquez Arenas, P. V. Volkov, 9 V. Yu. Volkov, and P. Ulloa

The improved results on a direct search for a new X(16.7 MeV) boson which could explain the anomalous excess of $e^+e^-$ pairs observed in the excited 8Be nucleus decays ("Berillium anomaly") are reported. Due to its coupling to electrons, the X boson could be produced in the bremsstrahlung reaction e-Z -> e-ZX by a high-energy beam of electrons incident on active target in the NA64 experiment at the CERN SPS and observed through its subsequent decay into $e^+e^-$ pair. 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 to set the new limits on the $X$--$e^-$ coupling in the range 1.2 \times 10^{-4} < \epsilon_e < 6.8 \times 10^{-4}, excluding part of the parameter space favored by the Berillium anomaly. We also set new bounds on the mixing strength of photons with dark photons (A') from non-observation of the decay $A' \to e^+e^-$ of the bremsstrahlung A' with a mass below 24 MeV.

Recently, the ATOMKI experiment has reported new evidence for the excess of e+e- events with a mass ∼ 17 MeV in the nuclear transitions of 4 He, that they previously observed in measurements with 8 Be. These observations could be explained by the existence of a new vector X17 boson. So far, the search for the decay X17→e+e- with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining X17 parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the X17 decay with the proposed method. To reach this goal an optimization of the X17 production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results [1], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate show that the goal of the proposed search is feasible.

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 2.84×10^{11} 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.

The NA64 collaboration proposes to carry out further searches for dark sector and other rare processes in missing energy events from high energy muon interactions in the hermetic detector at the CERN SPS starting from the run in 2021. The dark sector of particles predominantly weakly-coupled to the second and possibly third generations of the Standard Model is motivated by several theoretically interesting models. Additional to gravity this new very weak interaction between the visible and dark sector could be mediated either by a scalar (Sμ) or U ′(1) gauge bosons (Zμ) interacting with ordinary muons. In a class of Lμ − Lτ models the corresponding Zμ could be light and have the coupling strength laying in the experimentally accessible region. If such Zμ mediator exists it could also explain the muon (g−2)μ anomaly the discrepancy between the predicted and measured values of the muon anomalous magnetic moment. We propose an extension of the experiment NA64 called NA64μ to search for invisible decays of the Zμ either to neutrinos or light DM aiming to probe the still unexplored area of the coupling strengths and masses MZμ . 200 MeV with the ' 100 − 160 GeV M2 beam. The NA64μ has the capability of a sensitive search for Sμs decaying invisibly to dark-sector particles, such as dark matter and millicharged particles and will cover a part of the parameter space complementary to what is intended to be probed by other searches Morevoer, the experiment will probe completely the Zμ parameter space (coupling vs mass) that could explain the muon (g− 2)μ anomaly and could be used to search lepton flavour violation process of the μ− τ conversion.

The experiment NA64 is aimed at a direct search for sub-GeV vector mediator A′ of Dark Matter production in invisible A′ decay mode. Another goal is to search for of a new light X boson, which could explain a recently observed excess of e+e− events from excited Be transitions. The NA64 Collaboration requests to carry out further both searches with the H4 electron beam in the year 2021 and beyond.

In the framework of model with Lorentz violation (LV) we discuss a physical observables for q\(\bar q\) pair production at lepton–lepton colliders and describe the experimental signal to be detected. We obtain a conservative limits on Lorentz-violating dimensionless coupling for quark sector from LEP data. We also make a phenomenological prediction for LV model at the future lepton collider.

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 Be-8 nucleus decays. Due to its coupling to electrons, the X could be produced in the bremsstrahlung reaction e- Z -u003e e- Z X by a 100 GeV e- beam incident on an active target in the NA64 experiment at the CERN SPS and observed through the subsequent decay into a e+e- pair. With 5.4times 10^{10} electrons on target, no evidence for such decays was found, allowing to set first limits on the X-e^- coupling in the range 1.3times 10^{-4}lesssim epsilon_e lesssim 4.2times 10^{-4} excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (Au0027) from non-observation of the decay Au0027-u003ee+e- of the bremsstrahlung Au0027 with a mass lesssim 23 MeV.

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.

We present the performance of multiplexed XY resistive Micromegas detectors tested in the CERN SPS 100 GeV/c electron beam at intensities up to 3.3×105e−∕(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.

A search is performed for a new sub-GeV vector boson (A') mediated production of dark matter (chi) 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' -> chi(chi) over bar. 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 x 10(10) electrons on target no evidence of such a process has been found. New stringent constraints on the A' mixing strength with photons, 10(-5) less than or similar to epsilon less than or similar to 10(-2), for the A' mass range m(A') less than or similar to 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) less than or similar to y less than or similar to 10(-6) on the dark-matter parameter y = epsilon(2)alpha(D)(m(chi)/m(A'))(4) are obtained for the dark coupling constant alpha(D) = 0.5 and dark-matter masses 0.001 less than or similar to m(chi) less than or similar to 0.5 GeV. The lower limits alpha(D) greater than or similar to 10(-3) for pseudo-Dirac dark matter in the mass region m(chi) less than or similar to 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 Weizsacker-Williams approximation for the mass region m(A') greater than or similar to 0.1 GeV.

In high energy experiments such as active beam dump searches for rare decays and missing energy events, the beam purity is a crucial parameter. In this paper we present a technique to reject heavy charged particle contamination in the 100 GeV electron beam of the H4 beam line at CERN SPS. The method is based on the detection with BGO scintillators of the synchrotron radiation emitted by the electrons passing through a bending dipole magnet. A 100 GeV pi(-) beam is used to test the method in the NA64 experiment resulting in a suppression factor of 10(-5) while the efficiency for electron detection is similar to 95%. The spectra and the rejection factors are in very good agreement with the Monte Carlo simulation. The reported suppression factors are significantly better than previously achieved. (C) 2017 Elsevier B.V. All rights reserved.

The experiment NA64 is aimed at a direct search for invisible decays of sub-GeV dark photons (A′). The main goal in 2016 was to probe a region of the A′ parameter space, particularly interesting for the explanation of the muon gμ − 2 anomaly. The status and results from two NA64 runs in July and October 2016, obtained, respectively, with 2.75×10 and 4×10 accumulated electron on target are reported. A feasibility study of the search for the X → e+e− decay of a new light X boson, which could explain a recently observed excess of e+e− events from excited Be transitions is also presented.

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.

The time-dependent and time-independent CP asymmetries \(A_{CP}^{B_q^0 \to f} \left( \tau \right)\) and \(A_{CP}^{B_q^0 \to f} \left( {\hat s} \right)\) for rare semileptonic and radiative leptonic decays of B mesons are calculated by the method of helicity amplitudes. The sensitivity of CP asymmetries to various extensions of the Standard Model that have an operator basis that is identical to the operator basis of the Standard Model is investigated. It is shown that, by combining information about the form of the charge lepton asymmetry A FB at small values of the square of the invariant dilepton mass and information about the average value of the time-dependent CP asymmetry, one can in principle determine the relative phases of the Wilson coefficients C 7γ , C 9V , and C 10A in the effective Hamiltonian for b → {d, s}ℓ+ℓ− transitions.

The possibility of recording the rare muonic decays B d,s 0 → μ + μ − at the ATLAS detector (LHC, CERN) is studied. The question of what results can be expected within the first three years of LHC operation in the initial-luminosity mode is examined. The most important noncombinatorial background processes are listed for rare muonic decays of B mesons.