RESUMO
The NA62 experiment at CERN, configured in beam-dump mode, has searched for dark photon decays in flight to electron-positron pairs using a sample of 1.4×10^{17} protons on dump collected in 2021. No evidence for a dark photon signal is observed. The combined result for dark photon searches in lepton-antilepton final states is presented and a region of the parameter space is excluded at 90% confidence level, improving on previous experimental limits for dark photon mass values between 50 and 600 MeV/c^{2} and coupling values in the range 10^{-6} to 4×10^{-5}. An interpretation of the e^{+}e^{-} search result in terms of the emission and decay of an axionlike particle is also presented.
RESUMO
Searches for the lepton number violating K^{+}âπ^{-}µ^{+}e^{+} decay and the lepton flavor violating K^{+}âπ^{+}µ^{-}e^{+} and π^{0}âµ^{-}e^{+} decays are reported using data collected by the NA62 experiment at CERN in 2017-2018. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% confidence level: B(K^{+}âπ^{-}µ^{+}e^{+})<4.2×10^{-11}, B(K^{+}âπ^{+}µ^{-}e^{+})<6.6×10^{-11} and B(π^{0}âµ^{-}e^{+})<3.2×10^{-10}. These results improve by 1 order of magnitude over previous results for these decay modes.
RESUMO
A new measurement of the branching ratio R_{e/µ}=Γ(π^{+}âe^{+}ν+π^{+}âe^{+}νγ)/Γ(π^{+}âµ^{+}ν+π^{+}âµ^{+}νγ) resulted in R_{e/µ}^{exp}=[1.2344±0.0023(stat)±0.0019(syst)]×10^{-4}. This is in agreement with the standard model prediction and improves the test of electron-muon universality to the level of 0.1%.
RESUMO
The imaging performance of a high-resolution preclinical micro-positron emission tomography (micro-PET) system employing liquid xenon (LXe) as the gamma-ray detection medium was simulated. The arrangement comprises a ring of detectors consisting of trapezoidal LXe time projection ionization chambers and two arrays of large area avalanche photodiodes for the measurement of ionization charge and scintillation light. A key feature of the LXePET system is the ability to identify individual photon interactions with high energy resolution and high spatial resolution in three dimensions and determine the correct interaction sequence using Compton reconstruction algorithms. The simulated LXePET imaging performance was evaluated by computing the noise equivalent count rate, the sensitivity and point spread function for a point source according to the NEMA-NU4 standard. The image quality was studied with a micro-Derenzo phantom. Results of these simulation studies included noise equivalent count rate peaking at 1326 kcps at 188 MBq (705 kcps at 184 MBq) for an energy window of 450-600 keV and a coincidence window of 1 ns for mouse (rat) phantoms. The absolute sensitivity at the center of the field of view was 12.6%. Radial, tangential and axial resolutions of (22)Na point sources reconstructed with a list-mode maximum likelihood expectation maximization algorithm were ≤0.8 mm (full-width at half-maximum) throughout the field of view. Hot-rod inserts of <0.8 mm diameter were resolvable in the transaxial image of a micro-Derenzo phantom. The simulations show that a LXe system would provide new capabilities for significantly enhancing PET images.
Assuntos
Tomografia por Emissão de Pósitrons/estatística & dados numéricos , Algoritmos , Animais , Fenômenos Biofísicos , Simulação por Computador , Humanos , Processamento de Imagem Assistida por Computador/estatística & dados numéricos , Camundongos , Imagens de Fantasmas , Tomografia por Emissão de Pósitrons/instrumentação , Ratos , XenônioRESUMO
Three events for the decay K+-->pi+ nunu have been observed in the pion momentum region below the K+-->pi+pi0 peak, 140 < Ppi < 199 MeV/c, with an estimated background of 0.93+/-0.17(stat.) -0.24+0.32(syst.) events. Combining this observation with previously reported results yields a branching ratio of B(K+-->pi+ nunu) = (1.73(-1.05)+1.15) x 10(-10) consistent with the standard model prediction.
RESUMO
An additional event near the upper kinematic limit for K+-->pi(+)nunu; has been observed by experiment E949 at Brookhaven National Laboratory. Combining previously reported and new data, the branching ratio is B(K+-->pi(+)nunu;)=(1.47(+1.30)(-0.89))x10(-10) based on three events observed in the pion momentum region 211
RESUMO
Additional evidence for the rare kaon decay K+-->pi+nu(nu) has been found in a new data set with comparable sensitivity to the previously reported result. One new event was observed in the pion momentum region examined, 211
pi+nu(nu)) = 1.57(+1.75)(-0.82)x10(-10).