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Using the OKA setup, which is working at the secondary 18 GeV beam of the U-70 synchrotron of the Institute for high energy physics of the NRC "Kurchatov institute" , the following results are obtained in 2022-2024:
Study of the decay K+ → e+ ν π0 γ (Ke3γ). As a result of the analysis, ~ 105 events of this decay were selected, with an expected background of ~ 18%. To search for T-odd effects, the distribution of events over the T-odd variable ξπeγ – the mixed product of the momenta of the pion, positron and photon in the kaon rest frame was studied. As an estimate of the asymmetry of the distribution, the value Aξ=(N+−N−)/(N++N−) is used, where N+(−) is the number of events with ξ>(<) zero. For the A_ξ asymmetry, the following values were obtained for the photon energy and lepton-photon angle in the Kaon rest system E_γ > 10 MeV, θ_{eγ} > 10° : A_ξ = (0.1 ± 3.9 (stat.)± 1.7 (syst.))× 10^{-3} or |Aξ| < 5.44× 10-3 at 90% CL. This result is in agreement with CM predictions and allows us to obtain constraints on vector and axial constants for SM extensions.
A search for the light axion-like particle in the decay K+ → π+ π0 a is performed. A peak search method in the missing mass spectrum is used in the analysis. No signal is observed and the upper limits on branching in the mass range 0–200 MeV are set. The 90% CL upper limit is changing from 2.5 · 10−6 to 2 · 10−7 for the ALP mass from 0 to 200 MeV, except for the region of π0 mass, where the upper limit is 4.4 · 10−6. Our analysis uses the realistic matrix element and improves the existing limit by a factor of 3.5–10 depending on the axion mass. Using this result we calculated the lower limits for |FAsd|, which is an effective coupling constant of the axion to the axial vector flavor changing -sd- current.The limit is about 6.4 · 107 GeV for axion mass below 70 MeV, which is the best limit for |FA sd |, among the HEP experiments.
A rare radiative decay K+ → π+ π0 π0 γ is discovered . The major background source, the decay K+ → π+ π0 π0, is ≈ 5000 times more intense than the radiative decay in question. The RBFN neural network is employed to suppress the background down to Signal:Noise 1 : 1 level; about 60 events of the decay observed. The branching ratio, BR(K+ → π+ π0 π0 ) =(4.1 ± 0.9(stat) ± 0.4(syst)) × 10−6 for Eγ > 10MeV agrees with ChPT prediction of 3.76 × 10−6. The γ energy spectrum is also in agreement with ChPT although the errors are large.
The K+→π0μ+νγ (Kμ3γ) decay is studied with the highest statistics in the world. More than 900 signal events were found in the region 30< E_γ <60 MeV. E_γ being the energy of the emitted photon in the rest frame of the kaon. Using 4.48 × 106 events sample of the decay K+→π0 μ+ ν (Kμ3), the branching ratio Br(Kμ3γ)/Br(Kμ3) was found to be (4.45 ± 0.25(stat)) × 10−4. This value can be transformed (Br(Kμ3) = 3.352% from PDG) to Br(Kμ3γ) = (1.492 ± 0.085(stat)) × 10−5. Our results are preliminary, with systematic errors being estimated.
A new precise measurement of the vector and axial-vector form factor difference FV − FA in
the K+ → μ+ ν μ γ decay is performed. About 144 K events of K+ → μ+νμγ are selected in the OKA
experiment. The result is FV − FA = 0.135 ± 0.017(stat) ± 0.024(syst). The number of events is about 1.5
times higher and both errors are smaller than in the published earlier OKA result. The FV − FA difference has been measured : FV − FA = 0.135 ± 0.017(stat) ± 0.024(syst). The result is 2.9σ above χPT O(p4) theory prediction and is in a better agreement (2σ) with the recent Lattice calcullations.
A search for a super-rare decay K+ → e+ ν π0 π0 π0 is performed. This decay has not been observed so far. Due to the limited phase space the expected SM branching Br~10-11. This gives a possibility to search for the contributions from exotic processes. The obtained upper limit is Br(K+ → e+ ν π0 π0 π0) < 5.4 x 10-8 90% C.L. improves the old result published in PDG by a factor of 65. The analysis methods developed during this study allowed to completely suppress the background which opens a possibility of successful continuation of the search by increase of the statistics.
| Тематическая секция | Физика фундаментальных взаимодействий |
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