R. Davis

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.

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 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.

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 + .

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.

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)%.