Using the ARGUS detector at the e+e storage ring DORIS II at DESY, we have studied lepton energy spectra in ~decays. We present a "pseudo-rest-frame" technique in which the second r in the event, decaying into a heavy hadronic system, is used as reference. This method allows for the first measurement of the Michel Parameter 7/ in ~decays. We also determine the Michel Parameter p in 7" --~ e~u decays with a precision comparable to the present world average. The measured values of the parameters p = 0.735 40.036 40.020 and r 1 = 0.03 40.18 40.12 are in good agreement with standard V-A coupling at the ~--~,-W vertex. 0370-2693/95/$09.50 Q 1995 Elsevier Science B.V. All rights reserved S S D I 0 3 7 0 2 6 9 3 ( 9 4 ) 0 1 3 8 6 1 442 ARGUS Collaboration/Physics Letters B 341 (1995) 441-447 Michel parameters [ 1-3 ] describing the space-time structure of the weak leptonic decays have been determined with high accuracy [4]. All values are in perfect agreement with the Standard Model predictions. The 7lepton discovered in 1975 [5], with its two leptonic decays 7" ~ e~ , and 7" --~/x~u, offers a unique opportunity to study the universality of the charged weak interaction. Describing the leptonic 7" decay by the most general four-fermion Hamiltonian we get the following matrix element [6]:

An analysis of the reaction yy ~ r/c was l~erformed in five different decay channels of the r/c: /~sK+~r ~, K+K-Tr+~ ", 27r+21r , ~b~b and 2K+2K . A value Fry(r/c) = ( 11.3 + 4.2) keV was obtained for the radiative decay width by combining the results from the first four channels. Using our result on the two-photon width we also determined the branching ratio for the decay r/c --~ 2K+2K . 0370-2693/94/$07.00 (~) 1994 Elsevier Science B.V. All rights reserved SSDI 0 3 7 0 2 6 9 3 ( 9 4 ) 0 1 1 1 6-8 ARGUS Collaboration / Physics Letters B 338 (1994) 390-396 391 The measurements of the radiative decay widths of hadrons have proved to be one of the most important tools to study the composition and the properties of the bound states of strong interactions. While for light quarks the understanding of the binding forces is still on a more phenomenological level, in heavy quark systems, such as charmonium, fundamental tests of QCD dynamics can be made. One of the important quantities in the charmonium system is the relation between the leptonic decay width of the J/qt and the two-photon width of the r/c which is to the lowest order given by (see for example [ 1 ] ) Frr (r&) = 3e~( Ms/¢ )2. r,+e_ ( J l Cs ) " --~< QCD corrections have been calculated to order as by Barbieri et al. [2] resulting in the relation Frr(~7~) = Fete ( J / O ) " (1 ~a+O.OT~ which, together . . . . _0.05.1, with Fe+ e(J/~b) = (5.36:t:0.29) keV [31, yields the prediction Frr(r/c) = (8.6 + 0.6) keV. Furthermore, with the assumption that the hadronic r/c decays can be described by the lowest order two-gluon diagram one obtains the ratio of the hadronic to two-photon partial width Fhadrons(~'lc)/Fyr(rlc) proportional to ( a s / a ) 2. This relation could in principle be used to determine as. However, in this case the QCD radiative corrections seem to be too large to provide a reliable prediction. The radiative decay width Frt(r/~) has been measured by two different methods: in photon-photon collision experiments [4-7] and by the measurement of the process p/~ --* ~ [ 8 ]. The most significant results I DESY, IfH Zeuthen 2 Supported by the German Bundesministerium for Forschung und Technologic, under contract number 05413051P. 3 Supported by the German Bundesministerium for Forschung und Technologie, under contract number 056DDI 1P. 4 Supported by the German Bundesministerium ?dr Forschung und Technologie, under contract number 054ERI2R 5 Supported by the German Bundesministerium ?dr Forschung und Technologie, under contract number 055HD21P. 6 University of Toronto, Toronto, Ontario, Canada. 7 McGill University, Montreal, Quebec, Canada. 8 Supported by the Natural Sciences and Eagineering Research Council, Canada. 9 Supported by the German Bundesministerium ?dr Forschung und Technologie, under contract number 055KA l 1R l0 Supported by the Ministry of Science and Technology of the Republic of SIovenia and the Intemationales Bilro KfA, Jiilich. were obtained from the analysis of the decay channel r/¢ --* K~sK+cr :F, where a good identification of the/~s was achieved by determining its decay vertex. The results range from 4 to 27 keV and have large errors because of the small number of collected r& events and uncertainties in the knowledge of r/c branching ratios. The present analysis aims at improving the precision of the Fr~ (r/c) measurement by using several decay channels of the r/c meson. The data used in this analysis were collected using the ARGUS detector at the e+e storage ring DORIS II at DESY, and correspond to an integrated luminosity of 473 pb t . The beam energies varied between 4.7 and 5.3 GeV. The ARGUS detector and details about its trigger and its particle identification capabilities were described elsewhere [9]. In what follows, we describe the main features of the analysis, while details can be found in Refs. [10,11]. The two-photon production of the r/c was studied in e+e interactions via the reaction

Using the ARGUS detector at the e+e- storage ring DORIS II, we have searched for the real and imaginary part of the electric dipole formfactor d_tau of the tau lepton in the production of tau pairs at q^2=100 GeV^2. This is the first direct measurement of this CP violating formfactor. We applied the method of optimised observables which takes into account all available information on the observed tau decay products. No evidence for CP violation was found, and we derive the following results: Re(d_tau)=(1.6+-.9)*10^(-16) ecm and Im(d_tau)=(-0.2+-0.8)*10^(-16) ecm, where statistical and systematic errors have been combined.

Using the ARGUS detector at the e(+)e(-) storage ring DORIS II, we have measured the Michel parameters rho,xi, and xi delta for tau(+/-) --> l (+/-) v (v) under bar decays in tau-pair events produced at center of mass energies in the region of the tau resonances. Using tau(-/+) --> rho(-/+) v as spin analyzing tags, we find rho(e) = 0.68 +/- 0.04 +/- 0.08, xi(e) = 1.12 +/- 0.20 +/- 0.09, xi delta(e) = 0.57 +/- 0.14 +/- 0.07, rho(mu)= 0.69 +/- 0.06 +/- 0.08, xi(mu) = 1.25 +/- 0.27 +/- 0.14 and xi delta(mu) = 0.72 +/- 0.18 +/- 0.10. In addition, we report the combined ARGUS results on rho, xi, and xi delta using this work and previous measurements. (C) 1998 Elsevier Science B.V. All rights reserved.

Using the ARGUS detector at the e+e− storage ring DORIS II at DESY, we have found evidence for the production of the excited charmed baryon state Λc(2593)+ in the channel Λc+π+π−. Its mass was determined to be (2594.6±0.9±0.4) MeV/c2, and the natural width measured to be Γ = (2.9−2.1−1.4+2.9+1.8) MeV. The production cross section times the branching ratios of σ(e+e− → Λc(2593)+X) × Br(Λc(2593)+ → Λc+π+π−) × Br(Λc+ → pK−π+) was measured to be (0.25−0.13+0.24 ±0.13) pb. The fractions of Λc(2593)+ decays proceeding through the Σc0π+ and Σc++π− channels were determined to be 0.29±0.10±0.11 and 0.37±0.12±0.13, respectively.

A maximum likelihood analysis of ARGUS data on two-photon production of π+π+π0π0π−π− is presented. A small fraction of events is due to the production of omega pairs. The γγ → ωω cross section has its maximum value close to threshold.

The impact of the ARGUS experiment to elementary particle physics is reviewed. More than ten years of data taking has allowed ARGUS to contribute significantly to our understanding of beauty and charmed hadrons, τ Leptons, ϒ mesons, ϒϒ interactions and fragmentation processes. In particular the ARGUS measurements of CKM matrix elements opened up a new window on the Standard Model.

Using the ARGUS detector at the storage ring DORIS II we have measured tau decays into three charged mesons containing K* mesons. Exploiting the good particle identification capabilities of the detector we have determined the following branching ratios: Br(tau(-) --> <(K*(0))over bar>pi(-)nu(tau)) = (0.25 +/- 0.10 +/- 0.05) %, Br(tau(-) --> K*(0)K(-)nu(tau)) = (0.20 +/- 0.05 +/- 0.04) %, and Br(tau(-) --> K*(-)(X(0))nu(tau)) = (1.19 +/- 0.15(-0.18)(+0.13)) %.

Using the ARGUS detector at the e+e− storage ring DORIS II at DESY we have measured the parameters of a charmed strange meson D* s2 + (2573) decaying into D0K+. The mass of this state was found to be (2574.5±3.3±1.6) MeV/c 2. The measured value of its width is (10.4±8.3±3.0) MeV and the product of the production cross section and branching ratio for this channel was determined to be (10.6±4.2 −1.2 +2.3 ) pb.

Lepton spectra in τ decays have been studied with the ARGUS detector at the e+e− storage ring DORIS II at DESY using the “pseudo-rest-frame” technique. We have improved upper limits for two-body τ-decays into a lepton and an unobservable particle.

Using the ARGUS detector at the e(+)e(-) storage ring DORIS II at DESY, we have studied lepton energy spectra in tau decays. We present a ''pseudo-rest-frame'' technique in which the second tau in the event, decaying into a heavy hadronic system, is used as reference. This method allows for the first measurement of the Michel Parameter eta in tau decays. We also determine the Michel Parameter rho in tau --> e $($) over bar$$ vv decays with a precision comparable to the present world average. The measured values of the parameters rho = 0.735 +/- 0.036 +/- 0.020 and eta = 0.03 +/- 0.18 +/- 0.12 are in good agreement with standard V-A coupling at the tau-v-W vertex.

Using the ARGUS detector at the DORIS II electron-positron storage ring at DESY, we have measured BR(D*+→D0π+)=(68.8±2.4±1.3)%, BR(D*+→D+π0)=(31.2±1.1±0.8)%, BR(D*+→D+γ)<5.2% (90%CL), BR(D*0→D0π0)=(59.6±3.5±2.8)%, and BR(D*0→D0γ)=(40.4±3.5±2.8)%, and the mass difference Δm0=m(D*0)−m(D0)=(142.2±0.3±0.2)MeV/c2.

Using the detector ARGUS at the e+e− storage ring DORIS II at DESY, we have observed the decays Λc+ → Ξ−K+π+ and Λc+ → Ξ∗0K+ and obtained evidence for the decay Ξc0 → ΛKs0. All three of these decays are of interest as they provide information about W-exchange processes. The products of cross sections and branching ratios for Λc+ → Ξ−K+π+ and Λc+ → Ξ∗0K+ are (1.7±0.3±0.3) pb and (0.6±0.2±0.1) pb, respectively, which implies that (35±17) % of the Ξ−K+π+ final state is generated through a two-body decay via intermediate W-exchange. The product of production cross section and branching ratio for Ξc0 → Λ Ks0 is (2.1 ± 1.0 ± 0.6) pb.

Lepton spectra in tau decays have been studied with the ARGUS detector at the e(+)e(-) storage ring DORIS II at DESY using the ''pseudo-rest-frame'' technique. We have improved upper limits for two-body tau -decays into a lepton and an unobservable particle.

Using the ARGUS detector at the storage ring DORIS II we have measured τ decays into three charged mesons containingK* mesons. Exploiting the good particle identification capabilities of the detector we have determined the following branching ratios:\(Br\left( {\tau ^ - \to \overline {K^{*0} } \pi ^ - v_\tau } \right) = \left( {0.25 \pm 0.10 \pm 0.05} \right)\% \),B r (τ−→K*0K−vτ)= (0.20±0.05±0.04)%, andB r (τ−→K*−X0vτ) =(1.15±0.15−0.18+0.13)%.

Using the ARGUS detector at the e+e− storage ring DORIS II at DESY, we have studied lepton energy spectra in τ decays. We present a “pseudo-rest-frame” technique in which the second τ in the event, decaying into a heavy hadronic system, is used as reference. This method allows for the first measurement of the Michel Parameter η in τ decays. We also determine the Michel Parameter ρ in τ → ev̄v decays with a precision comparable to the present world average. The measured values of the parameters ρ=0.735±0.036±0.020 and η=0.03±0.18±0.12 are in good agreement with standard V-A coupling at the τ-ν-W vertex.