U. Matthiesen

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.

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)%.

Observation of charmless B meson decays

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II, we have observed charmless decays of B mesons into the final states ppπ ± and ppπ + π − . The significance of the signal corresponds to more than five standard deviations. The branching ratios are (5.2±1.4±1.9)×10 −4 for the three-body and (6.0±2.0±2.2)×10 −4 for the four-body final state. These decays cannot proceed via the dominant b→c transitions, and we show that they are not the result of penguin-type processes. Thus, the observed decays must represent b→u quark transitions. Consequently, the Kobayashi-Maskawa matrix element V ub is non-zero.

An improved upper limit on the ντ-mass from the decay π−→π−π−π−π+π+ντ

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II, we have observed the decay τ − → π − π − π − π + π + ν τ in tau-pair events produced at center-or-mass energies between 9.4 and 10.6 GeV. From the 5π invariant mass distribution we derive an upper limit of m ( ν τ ) c 2 at the 95% confidence level. The branching ratio for this decay channel is found to be (0.064±0.023±0.01)%.

B meson decays into charmonium states

Abstract Using the ARGUS detector at the e +e− storage ring DORIS II, we have studied the colour-suppressed decays B → J ψ X and B→ψ′ X. We find the inclusive branching ratios for these two channels to be (1.07±0.16±0.19) % and (0.46±0.17±0.11) %, respectively. From a sample of reconstructed exclusive events the masses of the B0 and B+ mesons are determined to be (5279.5±1.6±3.0) MeV c 2 and (5278.5±1.8±3.0) MeV c 2 , respectively. Branching ratios are determined from five events of the type B 0 → J ψ K ∗0 and three of B+→J/ψ K+. In the same data sample a search for B0→e+e−, μ+μ− and μ±e∓ leads to upper limits for such decays.

An Improved Upper Limit on the tau-neutrino Mass from the Decay tau- ---> pi- pi- pi- pi+ pi+ tau-neutrino

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II, we have observed the decay τ − → π − π − π − π + π + ν τ in tau-pair events produced at center-or-mass energies between 9.4 and 10.6 GeV. From the 5π invariant mass distribution we derive an upper limit of m ( ν τ ) c 2 at the 95% confidence level. The branching ratio for this decay channel is found to be (0.064±0.023±0.01)%.

Evidence for the decay τ−→ντωπ−

Abstract Using the ARGUS detector at DORIS II, we have analyzed tau decays into π − π − π + π 0 ν τ . The branching ratio was determined to be (4.2±0.5±0.9)%. From a study of the three pion sub-system, we find evidence for the decay τ − → ωπ − ν τ with a branching ratio of (1.5±0.3±0.3)%. The ωπ − system is found to be predominantly in a J P =1 − state. There is no evidence for second-class axial-vector currents. The spectral function for the ωπ − final state is in agreement with a prediction based on CVC and σ e + e −→ ωπ 0 .

Direct evidence for W exchange in charmed meson decay

Abstract Using the ARGUS detector at DORIS II, we have observed a signal of 36.7±8.0 events in the decay channel D 0 →K s 0 φ . In the same data sample, we have observed the well established decay D 0 →K s 0 π + π − , and find the ratio, Br(D) 0 → K s 0 φ) Br(D) 0 → K s 0 π + π − ) , to be 0.186±0.052. The substantial value of (0.99±0.32±0.17)% then derived for the branching ratio for D 0 → K 0 φ gives direct evidence that W exchange contributes D 0 decay.

THE ARGUS ELECTRON-PHOTON CALORIMETER III. Electron-hadron separation

Abstract The high granularity of the ARGUS shower counter system allows one to determine the lateral energy deposition. An algorithm based on the number of shower counters set by an incoming particle, the amount of energy deposited in the shower counters and the lateral distribution of the energy allows to separate electrons from muons and hadrons. For electrons with momenta larger than 0.7 GeV/c the hadron rejection rate is of the order of 50 at an electron detection efficiency of 80%.