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A precise measurement of the vector and axial-vector form factors difference \\[F_V-F_A\\] in the \\[K^+\\rightarrow {\\mu ^+}{\\nu _{\\mu }}{\\gamma }\\] decay is presented. About 95K events of \\[K^+\\rightarrow {\\mu ^+}{\\nu _{\\mu }}{\\gamma }\\] are selected in the OKA experiment. The result is \\[F_V-F_A=0.134\\pm 0.021(stat)\\pm 0.027(syst)\\]. Both errors are smaller than in the previous \\[F_V-F_A\\] measurements.

A high statistics data sample of the decays of \\[K^+\\] mesons to three charged particles was accumulated by the OKA experiment in 2012 and 2013. This allowed to select a clean sample of about 450 events with \\[K^{+}\\rightarrow \\pi ^{+}\\pi ^{-}\\pi ^{+}\\gamma \\] decays with the energy of the photon in the kaon rest frame greater than 30 MeV. The measured branching fraction of the \\[K^{+}\\rightarrow \\pi ^{+}\\pi ^{-}\\pi ^{+}\\gamma \\], with \\[E_{\\gamma }^{*} > 30\\ \\hbox {MeV}\\] is equal to \\[(0.71 \\pm 0.05) \\times 10^{-5}\\]. The measured differential branching fraction over photon energy is compared with the prediction of the chiral perturbation theory to \\[{\\mathcal {O}}(p^{4})\\]. A search for an up-down asymmetry of the photon with respect to the hadronic system decay plane is also performed.

We report the results of a search for a new vector boson (A′) decaying into two dark matter particles χ1χ2 of different mass. The heavier χ2 particle subsequently decays to χ1 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′→χχ¯ and axion-like or pseudo-scalar particles a→γγ. 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 2me up to 390 MeV and mixing parameter ε between 3×10-5 and 2×10-2.

The [Formula omitted] decay is observed by the OKA collaboration. About 60 events of the decay observed with signal:noise [Formula omitted]. The branching ratio obtained by normalization to [Formula omitted] is measured to be [Formula omitted] for [Formula omitted]. The branching ratio, [Formula omitted] energy spectrum and angular distribution are consistent with ChPT prediction.

A high-statistics data sample of the [Formula omitted] decays is recorded by the OKA collaboration. A missing mass analysis is performed to search for a light invisible pseudoscalar axion-like particle (ALP) a in the decay [Formula omitted]. No signal is observed, and the upper limits for the branching ratio of the decay are calculated. The [Formula omitted] confidence level upper limit changes from [Formula omitted] to [Formula omitted] for the ALP mass from 0 to 200 MeV/ [Formula omitted], except for the region of [Formula omitted] mass, where the upper limit is [Formula omitted].

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 X 17 boson. So far, the search for the decay X 17 → 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 X 17 parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the X 17 decay with the proposed method. To reach this goal an optimization of the X 17 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 high-statistics data sample of the K + decays is recorded by the OKA collaboration. A missing mass analysis is performed to search for a light invisible pseudoscalar axion-like particle (ALP) a in the decay K + → π + π 0 a . No signal is observed, and the upper limits for the branching ratio of the decay are calculated. The 90 % confidence level upper limit changes from 2.5 · 10 - 6 to 2 · 10 - 7 for the ALP mass from 0 to 200 MeV/ c 2 , except for the region of π 0 mass, where the upper limit is 4.4 · 10 - 6 .

The K + → π + π 0 π 0 γ decay is observed by the OKA collaboration. About 60 events of the decay observed with signal:noise ≈ 1 . The branching ratio obtained by normalization to K + → π + π 0 π 0 is measured to be ( 3.7 ± 0.9 ( s t a t ) ± 0.3 ( s y s t ) ) × 10 - 6 for E γ ∗ > 10 MeV . The branching ratio, γ energy spectrum and angular distribution are consistent with ChPT prediction.

Results of a study of the K+→π0e+νγ decay at OKA setup are presented. More than 32,000 events of this decay are observed. The differential spectra over the photon energy and the photon–electron opening angle in kaon rest frame are presented. The branching ratios, normalized to that of Ke3 decay are calculated for different cuts on Eγ∗ and cosΘeγ∗. In particular, the branching ratio for Eγ∗>30 MeV and Θeγ∗>20∘ is measured R = Br(K+→π0e+νeγ)Br(K+→π0e+νe) = = (0.587±0.010(stat.)±0.015(syst.))×10-2, which is in a good agreement with ChPT O(p4) calculations.

A high statistics data sample of the K+→μ+νμ decay was accumulated by the OKA experiment in 2012. The missing mass analysis was performed to search for the decay channel K+→μ+νH with a hypothetic stable heavy neutrino in the final state. The obtained missing mass spectrum does not show peaks that could be attributed to existence of stable heavy neutrinos in the mass range (270

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