R. Mankel

Determination of the tau-neutrino helicity

Abstract Using the ARGUS detector at the DORIS II e + e − storage ring we measured for the first time the τ neutrino helicity from a parity violating asymmetry in the τ decay into three charged pions. From the measured asymmetry we derived the normalized product of the axial and vector coupling constant 2 g A g v / ( g A 2 + g V 2 ) = 1.14±0.34 −0.17 0.34 , which is in good agreement with the standard model prediction.

A concurrent track evolution algorithm for pattern recognition in the HERA-B main tracking system

Abstract A strategy for pattern recognition in the main tracking system of a forward B spectrometer like HERA-B or LHC-B is presented. Intrinsically a local method, it combines the virtues of track following procedures with the necessary ability to optimize between many available paths in a high occupancy environment. A hit-locating procedure suitable for a multiplanar detector geometry has been developed. The performance of the method is tested on HERA-B Monte Carlo events with full detector simulation and a realistic spectrometer geometry.

Observation of the D∗0(2459) IN e+e− annihilation

Abstract Using the ARGUS detector at the DORIS II storage ring at DESY, we have observed a charmed meson of mass (2455±3±5) MeV/c2, decaying to D + π − . The natural width of this state is determined to be (15 +13+5 −10−10 ) MeV c 2 . The fragmentation function is hard, as expected for a leading charmed particle from nonresonant e + e − annihilation. Analysis of the decay angular distribution supports the hypothesis that the observed state is an L =1 excited charmed meson with spin-parity 2 + .

HERA-B, an experiment to study CP violation at the HERA proton ring using an internal target

Abstract This paper discusses a dedicated fixed target experiment at the HERA proton ring with the goal to observe CP violation in the B 0 → J/ψK s 0 decay channel, using a detector with a highly selective trigger for lepton pairs from the J/ψ decay. The precision of the asymmetry, sin 2β, is expected to reach 0.13 within one year of running.

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.

The Outer Tracker detector of the HERA-B experiment. Part II: Front-end electronics

The HERA-B Outer Tracker is a large detector with 112674 drift chamber channels. It is exposed to a particle flux of up to 2x10^5/cm^2/s thus coping with conditions similar to those expected for the LHC experiments. The front-end readout system, based on the ASD-8 chip and a customized TDC chip, is designed to fulfil the requirements on low noise, high sensitivity, rate tolerance, and high integration density. The TDC system is based on an ASIC which digitizes the time in bins of about 0.5 ns within a total of 256 bins. The chip also comprises a pipeline to store data from 128 events which is required for a deadtime-free trigger and data acquisition system. We report on the development, installation, and commissioning of the front-end electronics, including the grounding and noise suppression schemes, and discuss its performance in the HERA-B experiment.

Measurement of the lifetime ratio τ(B=)τ(B0)

Abstract Using the ARGUS detector at the e + e - storage ring DORIS II the ratio of semileptonic branching ratios of B + and B 0 mesons has been determined by comparing the yields of D 0 , D - , and D * (2010) - mesons in semileptonic B decays. We derive the lifetime ratio τ( B + ) τ( B 0 ) =1.00∓0.23±0.14 .

Measurement of the polarization in the decay B ---> J / psi K*

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.

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.