T. Kirchhoff

Beam tests and calibration of the H1 liquid argon calorimeter with electrons

Results are presented on the energy calibration of the H1 liquid argon calorimeter modules with electrons from a test beam in the energy range of 3.7 GeV to 80 GeV. The method to determine the calibration for the Hl experiment from these measurements by the use of detailed simulations is described . Various systematic checks of this calibration are given. The calorimeter response is uniform in space within ±1% and linear with energy within ±1%. An average energy resolution of about 11.5%/ E [GeV] is achieved .

A measurement of the tau mass

Using the ARGUS detector at the DORIS II storage ring, a new measurement of the mass of the τ lepton has been obtained. An analysis of the tau pseudomass spectrum for decays of the type τ− → π−π−π+ντ finds mτ = 1776.3±2.4±1.4 MeV/c2. This result also leads to an improvement of the upper limit on the ντ mass to mντ < 31 MeV/c2 at the 95% confidence level.

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.

A Model independent determination of the inclusive semileptonic decay fraction of B mesons

Abstract With the ARGUS detector at the e + e - storage ring DORIS II, we have determined decay fraction and electron momentum spectrum of the inclusive decay mode B → eν X. Usinng lepton tags from the second B meson. in 209 000 γ(4 S ) → B B decays, we could determine the spectrum for all electron momenta p e > 0.6 GeV/ c . Including the small extrapolation to p e > 0, we find the model-independent decay fraction B (B → eν X) = (9.6 ± 0.5 ± 0.4)%. Adding D meson tags, our result is (9.7 ± 0.5 ± 0.4)%.

A study of B0 → D∗+ ℓ−vandB0B0 mixing using partial D∗+ reconstruction

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II at DESY, we have studied the decay B 0 → D ∗+ l − v by exploiting a partial D ∗+ reconstruction technique. The branching ratio for this mode was thereby determined to be (4.5 ± 0.3 ± 0.4)%. Using the corresponding sample of tagged B 0 mesons, we measured the B 0 B 0 mixing parameter to be r d = 0.194 ± 0.062 ± 0.054, a result only weakly dependent upon the ratio of semileptonic widths for and production rates of B 0 and B + mesons. We have also determined the branching ratio Br ( B 0 → X l − v to be (9.3 ± 1.1 ± 1.5)%. By comparing the results for full and partial D ∗+ reconstuction we found the absolute branching ratios for D 0 → K − π + and D 0 → K − π + π + π − to be (4.5 ± 0.6 ± 0.4)% and (7.9 ± 1.5 ± 0.9)%, respectively.

Observation of a new charmed baryon

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II at DESY, we have observed a new charmed baryon state in the channel Λ c + π + π − . (All references to a specific charged state also imply the charge conjugate state.) The mass of this state was measured to be (2626.6 ± 0.5 ± 1.5) MeV/ c 2 . The product of the production cross section and branching ratio for this channel was determined to be (11.5 ± 2.5 ± 3.0) pb, and the natural width estimated to be smaller than 3.2 MeV/ c 2 at 90% CL.

A partial wave analysis of the decay D0 → KS0π+π−

Abstract Using the ARGUS detector at the DORIS-II electron-positron storage ring at DESY, we have investigated the exclusive decay D ∗+ → D 0 π + , D 0 → K S 0 π + π − . From a partial wave analysis of the KS0π+π− system we find that (71.8± 4.2 ± 3.0)% are D 0 → K ∗− π + and (22.7 ± 3.2 ± 0.9)% are D 0 → K 0 ϱ 0 with a relative phase of (−137 ± 7 ± 3)° between the channels. The remaining fraction can be described by several channels involving excited resonances, but not by a three-body phase space decay, thus giving first evidence for D0 decays into K 0 ∗ (1430) − π + , K 0 f 0 (1975), K 0 f 2 (1270) , and K 0 f 0 (1400) .

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.

Evidence for the production of the charmed, doubly strange baryon Ωc in e+e− annihilation

Abstract Using the detector ARGUS at the storage ring DORIS II of DESY, we have found evidence for the production of the charmed and doubly strange baryon Ω c through its decay channel Ξ − K − π + π + . Its mass has been determined to be ((2719.0±7.0±2.5)MeV/ c 2 , and the product of production cross section and branching ratio the above channel to be (2.41±0.90±0.30) pb.

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.