J. Stiewe

Observation of B0-B0 mixing

Abstract Using the ARGUS detector at the DORIS II storage ring we have searched in three different ways for B0- B 0 mixing in ϒ (4S) decays. One explicitly mixed event, a decay ϒ (4S)→B0B0, has been completely reconstructed. Furthermore, we observe a 4.0 standard deviation signal of 24.8 events with like-sign lepton pairs and a 3.0 standard deviation signal of 4.1 events containing one reconstructed B0( B 0) and an additional fast l+(l)−. This leads to the conclusion that B0- B 0 mixing is substantial. For the mixing parameter we obtain r=0.21±0.08.

Argus: A universal detector at DORIS II

Abstract The detector ARGUS has been designed as a universal tool to investigate final states from e + e − annihilation processes in the energy range of the ϒ resonances. ARGUS started operation in October 1982 and has since successfully taken data at the ϒ(1S), ϒ(2S) and ϒ(4S) energies, and in the nearby continuum. The detector combines excellent charged particle identification and good photon energy resolution over more than 90% of the full solid angle. A particle originating from the interaction vertex and leaving the beam tube traverses the following components: the vertex drift chamber, the main drift chamber which determines its momentum and specific ionization, the time-of-flight system through which its velocity is determined, and the electromagnetic calorimeter. Muons pass through the magnet coils and the flux return yoke and finally hit the muon chamber system which surrounds the detector. The momentum resolution of ARGUS is σ (p T ) P T = (0.01 2 + (0.009p T [ GeV /c]) 2 ) 1 2 , the photon energy resolution in the barrel shower counters is σ (E) E = ( 0.072 2 +0.065 2 E[ GeV ] ) 1 2 . Combining the information from all p devices, more than 80% of all charged hadrons can be recognized unambiguously. The electron-hadron and muon-hadron rejection rates are 1:200 and 1:50 respectively.

The H1 lead/scintillating-fibre calorimeter

Abstract The backward region of the H1 detector has been upgraded in order to provide improved measurement of the scattered electron in deep inelastic scattering events. The centerpiece of the upgrade is a high-resolution lead/scintillating-fibre calorimeter. The main design goals of the calorimeter are: good coverage of the region close to the beam pipe, high angular resolution and energy resolution of better than 2% for 30 GeV electrons. The calorimeter should be capable of providing coarse hadronic energy measurement and precise time information to suppress out-of-time background events at the first trigger level. It must be compact due to space restrictions. These requirements were fulfilled by constructing two separate calorimeter sections. The inner electromagnetic section is made of 0.5 mm scintillating plastic fibres embedded in a lead matrix. Its lead-to-fibre ratio is 2.3:1 by volume. The outer hadronic section consists of 1.0 mm diameter fibres with a lead-to-fibre ratio of 3.4:1. The mechanical construction of the new calorimeter and its assembly in the H1 detector are described.

Search for hadronic b→u decays

Using the ARGUS detector at the e+e− storage ring DORIS II at DESY, we searched for b→u transitions in exclusive hadronic B meson decays. A systematic analysis of B decays into pions has been performed for decay modes with 2–7 pions in the final state. In none of the decays a positive signal was observed. The upper limits obtained on various branching ratios are consistent with the current model predictions.

Observation of Semileptonic Charmless B Meson Decays

Abstract A detailed study of the lepton momentum spectrum in υ (4 S ) decays has been made using the ARGUS detector at the DORIS II e + e − storage ring at DESY. In the region from 2.3 to 2.6 GeV/ c , which is above the endpoint for contributions from B decays via b → c transitions, we observe 41±10 events in excess of known backgrounds. These events are interpreted as a signal for b → u transitions. A model dependent value of 0.10±0.01 is obtained for the ratio of Cabibbo-Kobayashi-Maskawa matrix elements | V ub |/| V cv| , using the Altarelli et al. model.

Observation of jet structure in high pT events at the ISR and the importance of parton transverse momentum

Abstract Events with a charged particle of high transverse momentum were studied in pp collisions at √s = 52.5 GeV with the Split Field magnet (SFM) at the CERN-ISR, triggering on a charged particle of pT > 2 GeV/c, in the c.m. angular region θ ≅ 20° and θ ≅ 45°. The results provide further evidence for jet-like structure in high pT reactions. One of two produced jets is associated with the triggering hadron whereas the other is produced at an azimuthal angle that differs from that of the trigger by roughly 180°. The main features of the data are found to agree with the predictions of a simple parton-parton hard scattering model with vector gluon exchange, provided that one allows for partons in the proton to have transverse momentum. The ingredients of the model are all derived from data on deep inelastic lepton-proton scattering and from hadron production in e+e− annihilation. The model retains a simple pT−4 dependence for the parton-parton scattering. However, for pT ≲ 4GeV/c, the transverse movement of partons yields an approximate pT−8 scaling for the inclusive hadron distribution in agreement with observations. For the understanding of our data, it is found to be essential to take into account the soft component arising from the fragmentation of the spectator partons into hadrons.

Measurement of the decay

Using the ARGUS detector at the e + e − storage ring DORIS II at DESY we have investigated the decays B 0 → D ∗− e + v and B 0 →D ∗− μ + ν. The B 0 mesons were produced in 39600 ϒ(4 S )→ B 0 B 0 decays. Assuming electron-muon universality we obtain a branching ratio BR ( B 0 → D ∗− e + v)= BR ( B 0 → D ∗− γm + ν)=(7.0±1.2±1.9)% .

Performance of an electromagnetic lead/scintillating-fibre calorimeter for the H1 detector

Abstract The properties of final modules of a high resolution lead/scintillating-fibre calorimeter to upgrade the backward region of the H1 detector were studied with electrons in the energy range from 2–60 GeV. The electromagnetic calorimeter consists of scintillating fibres with a diameter of 0.5 mm embedded in a lead matrix. This small fibre radius, in combination with a lead-to-fibre ratio of 2.27:1, ensures excellent energy resolution which has been measured to be δ/E=7.1%/ E/GeV ⊕ 1.0% . The spatial resolution as a function of energy for impact points at the center of a cell is given by 4.4 mm/ E/GeV + 1.0 mm . The time resolution was found to be better than 0.4 ns.

Branching ratio and mass spectrum of the decay ϒ′ → ϒπ+π−

Abstract The reaction e + e − → ϒ ′ → ϒπ + π − , with ϒ → anything, has been measured using the ARGUS detector at DORIS . We obtain a mass difference M ( ϒ ′) − M ( ϒ ) = (562 ± 3) MeV and a decay branching ratio BR( ϒ ′→ ϒπ + π − )=(17.9±0.9±2.1)%. The invariant mass spectrum of the π + π − system differs from phase space, but is well described by a matrix element of the form ( M ππ 2 − λM π 2 ) with λ =2.6±0.5. We also report preliminary results on the exclusive decay ϒ′ → ϒπ + π − , with ϒ → e + e − , and obtain BR( ϒ → e + e − ) = (2.8 ± 0.4 ± 04±0.3)%.

Measurement of the reaction γd → π−pp, and determination of cross sections for the reaction γn → π−p, at photon energies between 0.2 and 2.0 GeV

Abstract The reaction γ d→ π − pp has been measured in a bubble chamber experiment at DESY with a bremsstrahlung beam of E γ max = 5.5 GeV. Total and differential cross sections are presented for the energy region from 0.2 to 2.0 GeV. The extraction of cross sections on the neutron is discussed in detail. The impulse approximation and the closure relation, with corrections for the exclusion principle and the Fermi motion effects, as well as the Chew-Low extrapolation technique are applied. Total cross sections and angular distributions for the reaction γ n→ π − p, deduced by these methods, are presented in graohs and tables. The results are compared with other experiments and with various predictions. No positive evidence for an isotensor electromagnetic current is found. The results disagree with some of the measurements of radiative pion capture, if detailed balance is assumed to connect the two reactions.