M. Schmidtler

Measurement of the polarization in the decay B → J/ψK∗

Abstract Polarization in the exclusive decay B → J/ψK ∗ was measured using the ARGUS detector at the e+e− storage ring DORIS II. The ratio of the transverse to the total decay width was found to be ΓT/Γ = 0.03±0.16±0.15. This measurement demonstrates that the B 0 → J/ψK ∗0 decay channel can be effectively used for CP violation studies.

Determination of the Michel parameters ξ and δ in leptonic τ decays

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II, we have determined the Michel parameters ξ and δ of τ ∓ → l ∓ ν ν - decays. With an integrated luminosity of 445 pb −1 around s = 10 GeV, we have produced approximately 415000 τ-pairs. From this data sample, 3262 events were selected with e + e − → τ + τ − → (l ± ν ν - ) (π ∓ π + π − ν) . The semihadronic decay was used as analyser of the τ-spin and made possible - owing to the spin correlations - the determination of the parity-violating Michel parameters ξ and δ in the decay τ ∓ → l ∓ ν ν - . Simultaneously to the determination of the Michel parameters, a measurement of the τ-neutrino helicity h ν τ in the decay τ ∓ → π ∓ π + π − ν was obtained. We observed h ν τ = −0.85 −0.17 +0.15 ± 0.05, ϱ = 0.721 ± 0.040 ± 0.021, ξ = 1.26 −0.26 +0.30 ± 0.09, and ξδ = 0.77 −0.16 +0.18 ± 0.05. In addition, the combined ARGUS result on h ν τ , ϱ, ξ, and ξδ are reported using this work and previous measurements.

A search for the electric dipole moment of the τ-lepton

Abstract 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 dτ of the τ lepton in the production of τ 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 τ decay products. No evidence for CP violation was found, and we derive the following results: Re (d τ )=(1.6±1.9)·10 −16 e cm and Im (d τ )=(−0.2±0.8)·10 −16 e cm , where statistical and systematic errors have been combined.

A search for the lepton-flavour violating decays τ → eα, τ → μα

Lepton spectra in τ decays have been studied with the ARGUS detector at thee+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.

Determination of the structure of τ decays in the reaction e+e− → τ+τ− → ϱ+vτϱ−vτ and a precision measurement of the τ-neutrino helicity

Abstract Using the ARGUS detector at the DORIS II storage ring we have investigated the Lorentz structure of the electroweak interaction in semi-hadronic τ decays. Spin correlations in the process e + e − → τ + τ − → v τ π + π 0 v τ π − π 0 are exploited for a measurement of the normalized product of the vector ( g V ) and axial vector ( g A ) couplings of the τ lepton, γin AV = 2 Re {g A g V ∗ } (|g V | 2 ) . The correlations are sensitive to the product of the couplings in both τ decays: γ AV 2 = γ AV (τ − → ν τ W − ) AV = −h(v τ )h( v τ ) . The measured value, γ AV 2 = 1.044±0.057±0.060, determines the relative sign of the neutrino helicities. Assuming CP invariance, the absolute value | γ AV | = 1.022±0.028±0.030 was found to be incellent agreement with the Standard Model. The contribution of scalar ( g s ) or pseudoscalar ( g P ) couplings was also investigated. No evidence for a scalar-like coupling was found.

Determination of the radiative decay width of the ηc meson

Abstract An analysis of the reaction γ → η c was performed in five different decay channels of the η c : K S 0 K ± π ∓ , K + K − π + π − , 2 π + 2 π − , φφ and 2 K + 2 k − . A value Γ γγ ( η 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 η c → 2 K + 2 K − .

Evidence for Λc(2593)+ production

Abstract 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.

Measurement of the absolute branching fractions for D0 decays into K− π+, K− π+ π+ π−, K0 π+ π−

Abstract Using the ARGUS detector at the e+ e− storage ring DORIS II we have measured the absolute branching ratios of the D0 meson, Br(D0→K− π+)=(3.41±0.12±0.28)%, Br(D0→K− π+ π+ π−)=(6.80± 0.27±0.57)%, and Br (D 0 → K 0 π + π − )=(5.03±0.39±0.49)% .

Study ofD0 andD+ decays into final states with two or three kaons

Using the ARGUS detector at thee+e− storage ring DORIS II, we have studied Cabibbo-suppressedD0 decay modes resulting in theK+K−π+π− final state and two-bodyD0 decay modes with a ϕ meson in the final state. TheBR(D0→KS0KS0π+π− was measured for the first time. We also present a measurement of theD+→KS0KS0K+ branching ratio. The values are compared with other experimental results and model predictions.

Search for rare B decays

Abstract Using the ARGUS detector at the e+e− storage ring DORIS II at DESY, we have searched for decays b → sgluon through full reconstruction of a whole event. Two B B decays were found with one of B meson decaying into a final state without charmed particles. We also obtained an upper limit of Br(B+ → τ+ντ) of 1.04% at 90% CL.