Observation of the destructive interference in the radiative kaon decay K ! µ ¯

AbstractUsing data collected with the “ISTRA+” spectrometer at U70 protonsynchrotron of IHEP, we report the first measurement of the destructiveinterference in the radiative kaon decay K − → µ − νγ. We find the difference ofthe vector and axial form factors F V −F A = 0.126±0.027(stat)±0.043(syst).ThemeasuredvalueistwostandarddeviationsabovetheO(p 4 )ChPTpredictionequal to 0.055. Inclusion of exotic tensor interaction gives F V −F A = 0.144±0.044(stat)±0.035(syst) and F T = −0.0079±0.0113(stat)±0.0073(syst), i.e.−0.03 < F T < 0.01 at 90% CL, consistent both with zero and with recenttheoretical prediction equal to |F T | = 0.022. 1 Introduction The decay K − → µ − νγ proceeds via two distinct mechanisms: the internalBremsstrahlung (IB) with the photon emitted by the kaon or the muon, andthe structure-dependent(SD) decay involving emission of the photon fromintermediate states. SD is sensitive to the electroweak structure of the kaonand allows for good test of theories describing hadron interactions and decays,like Chiral Perturbation Theory (ChPT)[1, 2]. This decay is also a good placefor searches for possible tensor interactions, see theoretical [3, 4, 5, 6, 7] andexperimental [8, 9, 10, 11] papers, devoted mainly to the π → eνγ decay.The differential probability of the decay can be written in terms of x =