A. Lindner

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)% .

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.

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.

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.

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 + .

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 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)%.

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)%.

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.