Using the ARGUS detector at the DORIS II storage ring we have investigated the Lorentz structure of the electroweak interaction in semi-hadronic r decays. Spin correlations in the process e+e --+ r+r --~ F~rr+¢r ° vr~r-rr ° are exploited for a measurement of the normalized product of the vector (gv) and axial vector (ga) couplings of the r lepton, YAV = 2 Re{gag~, } / ([gv {2+ Iga [2). The correlations are sensitive to the product of the couplings in both r decays: y2av = TAr ( r v , W ) yav(r + --+ ~,W +) , which can be interpreted as the product of the neutrino helicities: T2AV = h ( v , ) h ( ~ , ) . The measured value, T2AV = 1.044 ± 0.057 d: 0.060, determines the relative sign of the neutrino helicities. Assuming CP invariance, the absolute value lYav[ = 1.022 + 0.028 :tz 0.030 was found to be in excellent agreement with the Standard Model. The contribution of scalar (gs) or pseudoscalar (gp) couplings was also investigated. No evidence for a scalar-like coupling was found. The investigation of the third lepton family, the r lepton and the r neutrino, remains a subject of major importance in particle physics [ 1 ]. Since the measured leptonic r branching ratios are in good agreement with the predictions based on new measurements of the r mass and lifetime [2] , the tau sector is consistent with a universal electroweak interaction. Nevertheless, the expected incompleteness of the Standard Model due to the lack of an explanation for particle masses and coupling strengths motivates further searches for deviations from theory. One of the most promising ways to look for physics beyond the Standard Model is through a detailed investigation o f the Lorentz Structure o f r decays. The decay of this heavy lepton should be much more sensitive to new couplings having non V A structure than those of the lighter leptons. Therefore several experimental tests of the Lorentz structure have previously been performed, e.g. the determination of the Michel l DESY, IfH Zeuthen 2 Supported by the German Bundesministerium ftir Forschung und Technologic, under contract number 054DO5 IP. 3 Supported by the German Bundesministerium far Forschung und Technologic, under contract number 056DDI 1P. 4 Supported by the German Bundesministerium ftir Forschung und Technologic, under contract number 054ER12P. 5 Supported by the German Bundesministerium ftir Forschung und Technologic, under contract number 055HD21P. 6 University of Toronto, Toronto, Ontario, Canada. 7 McGill University, Montreal, Quebec, Canada. 8 Supported by the Natural Sciences and Engineering Research Council, Canada. 9 Supported by the German Bundesministerium fiJr Forschung und Technologic, under contract number 055KAI 1P. l0 Supported by the Ministry of Science and Technology of the Republic of Slovenia and the Internationales Biiro KfA, Jiilich. parameters in purely leptonic r decays [3] and the measurement of the p polarisation in the process r --+ 7r ¢r ° v r [ 4]. The observation of parity violation in the three-pion final state allowed the first measurement of the helicity of the r neutrino [5] . A combined measurement of the r polarisation and the r neutrino helicity has been performed on Z --+ r + r events [6] . In this paper a precise measurement of the value of y2av = Y A v ( r ~ v ~ W ) Y A v ( T + ~ ~rW +) is presented, where T a v is the normalized product o f the vector ( g v ) and axial v e c t o r ( g a ) decay constants of the r lepton: 2 Re{gag~} ( 1 ) T a r igvl 2 + [gAI 2 • Since the parameter Y2AV can be interpreted as the product of the neutrino helicities, y2av = h ( v ~ ) h ( ~ ) , the measurement determines the relative sign of the neutrino helicities, as well as the absolute value of Y A V . The contribution from scalar ( g s ) and pseudoscalar ( g t , ) couplings in r decays has also been investigated. The applied method has been developed and extensively tested with simulated data, as described in re [8] . Therefore, only the basic ideas of the analysis method are described here. Angular correlations between the final state hadrons of the reaction e + e ~ r + r ~ ~ r r + T r ° v ~ c r c r ° (2) are analysed to determine the electroweak interaction in semi-hadronic r decays. The electromagnetic production of the r leptons of reaction (2) leads to correlated lepton spins because opposite spins are suppressed by a factor 2 m ~ c 4 / s ~ 1 / 16 for center-ofARGUS Collaboration/Physics Letters B 337 (1994) 383-392 385 mass energies ~ around 10 GeV. These correlations should be distinguished from polarized r leptons generated via Z ° exchange at much higher energies. Using the final state hadrons as spin analysers makes the structure of the coupling in 7decays observable. The analysis of the Lorentz structure of semihadronic 7decays presented here comprises four parts: the selection of events of reaction (2), the investigation of signal and background contributions to the data, the measurement of the parameter Yav and the determination of scalar-like couplings. The analysed data sample was collected with the ARGUS detector at the e+e storage ring DORIS II at center-of-mass energies between 9.4 GeV and 10.6 GeV. The ARGUS detector is a 47r magnetic spectrometer described in [ 7]. The event sample corresponds to an integrated luminosity of 387 p b 1, with about 373 000 produced r pairs. The ~pairs of reaction (2) were selected from a special two-prong data sample containing more than five million events preselected with moderate cuts, mainly to reject Bhabha events. Two charged tracks from a main vertex with minimum transverse momenta o f p r > 60 MeV/c and with zero net charge were required. Neutral energy deposits in the calorimeter with energies larger than 150 MeV and a minimum polar angle of I cos0[ 0 . 9 to reduce the background from the more collinear Bhabha and/x pair events. To take into account the possibility that a shower cluster produced by a charged track can fake a photon, we required photon candidates to satisfy a minimum opening angle c o s ( r l , p r ) < 0.990, where ri is defined as the direction between the main vertex and the impact point of a charged track at the calorimeter. A large suppression of two-photon reactions, hadronic processes and other QED processes was achieved with the following parabola cut on the relationship between the transverse momentum balance and the total visible momentum of charged and neutral particles: