A. Hölscher

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.

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)% .

Exclusive hadronic decays ofB mesons

Using the ARGUS detector at thee+e− storage ring DORIS II, we have measuredB decays into exclusive final states containing aD orD* meson plus several pions, or containing aJ/ψ or ψ′ meson plus a strange particle. Some of these channels have not been seen before, while others represent updated measurements of previous results. The branching ratios are compared with the predictions of the model of Bauer, Stech and Wirbel. Using the cleanest decay channels, the mass of the charged and neutralB meson are found to bemB−=(5280.5±1.0±2.0) MeV/c2 andmB0=(5279.6±0.7±2.0) MeV/c2 respectively, yielding a mass differencemB0−mB−=(−0.9±1.2±0.5) MeV/c2.

Study of inclusive semileptonic B meson decays

Abstract Using the ARGUS detector at the e + e − storage ring DORIS-II, we have measured the inclusive semileptonic decays of B-mesons into electrons and muons. The data originate from 220.5 events/pb on the ϒ(4S) resonance. We find BR (B→e + v e X = (10.3 ± 0.6 ± 0.2)% and BR (B→ μ + v μ X) = (10.0 ± 0.6 ± 0.2)% using the model of Altarelli et al. for extrapolating over all lepton momenta, and BR (B→e + v e X) = (9.9±0.6)%, BR(B→ μ + v μ X) = (9.7±0.6)% using the model of Grinstein et al. For semileptonic decays into baryons, we obtain an upper limit of BR ( B → p e + v X ) CL ) .

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.

Reconstruction of semileptonic b ---> u decays

Abstract Using the ARGUS detector at the e+ e− storage ring DORIS II at DESY we have updated our result on charmless semileptonic B decays and searched for direct evidence for b→u transitions through reconstruction of complete events. This approach has successfully identified two ϒ (4S) events which contain B meson decays to a non-charm final state. In one case, a B0 meson is found in the channel π + μ − ν , while in the other a decay B+→ωμ+ν is seen. These events show that the previously observed signal of charmless semileptonic B decays is due to b→u transitions from which we obtain | V ub V cb | = 0.11±0.012 using the model of Altarelli et al.

Observation of spin-parity 2+ dominance in the reactionγγ→ρ0ρ0 near threshold

The reactionγγ→π+π−π+π− has been studied with the ARGUS detector. The rate in the invariant mass region below 1.8 GeV/c2 is found to be largely due toρ0ρ0 production. A spin-parity analysis shows a dominance of the partial wave (JP,Jz)=(2+, 2) with a small admixture fromJP=0+. The contribution of negative parity states is consistent with zero. The large ratio of cross sectionsσ(γγ→ρ0ρ0)/σ(γγ→ρ+ρ−)≃4, and the dominance of theJP=2+ wave in the reactionγγ→ρ0ρ0 is a signature consistent with the production of an exotic (I=2) resonance.

Measurement of the decay B0→D∗−l+ν

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II, we have investigated the decay B 0 →D − l + ν, where l + is e + or μ + . The B 0 mesons were produced in 150 000 ϒ(4S) decays. Assuming electron-muon universality we obtain a branching ratio BR(B 0 →D − e + ν )=BR(B 0 →D − μ + ν )=(1.8±0.6±0.5)%.

Inclusive production ofD0,D+ andD*(2010)+ mesons inB decays and nonresonante+e− annihilation at 10.6 GeV

Using the ARGUS detector at thee+e− storage ring DORIS II at DESY, we have measured the inclusive production ofD0,D+ andD*(2010)+ mesons inB decays and in nonresonante+e− annihilation around 10.6 GeV. The inclusive branching ratios forB decays toD0,D+ andD*+ mesons are found to be (52.2±8.2±3.5)%, (27.2±6.3±3.5)% and (34.8±6.0±3.5)% respectively. Thus,D0 andD+ production account for about 70% of the charm produced inB decays, neglectingb→u contributions to the total width. The production cross sections and momentum spectra for continuume+e− annihilation are also presented.