Dengfeng Zhang

Measurement of energetic single-photon production at LEP

Abstract We describe the sample of energetic single-photon events (Eγ > 15 GeV) collected by L3 in the 1991–1993 LEP runs. The event distributions agree with expectations from the Standard Model. The data are used to constrain the ZZγ coupling and to set an upper limit of 4.1 × 10−6, μB (90% C.L.) on the the magnetic moment of the τ neutrino.

B∗ production in Z decays at LEP

Abstract The production of B∗ mesons in Z decays has been measured at LEP with the L3 detector. A sample of Z → b b events was obtained by tagging muons in 1.6 million hadronic Z decays collected in 1991, 1992 and 1993. A signal with a peak value of Eγ = 46.3 ± 1.9 (stat) MeV in the B rest frame energy spectrum was interpreted to come from the decay B∗ → γB. The inclusive production ratio of B∗ mesons relative to B mesons was determined from a fit to the spectrum to be N B ∗ (N B ∗ + N B ) = 0.76 ± 0.08 ± 0.06 , where the first error is statistical and the second is systematic.

Measurement of exclusive branching fractions of hadronic one-prong tau decays

Abstract We have measured the branching fractions for the hadronic τ decays, τ → π K nπ° ν (0≤ n ≤3), with the L3 detector at LEP. Multiphoton final states are analyzed using the fine-grained, high-resolution electromagnetic calorimeter. The decay channels are identified using a neural network method. The results are: BR (τ → π K ν ) = (11.82 ± 0.26 ± 0.43) %, BR (τ → π K π° ν) = (25.05 ± 0.35 ± 0.50) %, BR (τ → π K 2π° ν) = (8.88 ± 0.37 ± 0.42) %, BR (τ → π K 3πδ ν) = (1.70 ± 0.24 ± 0.38) %, where the first error quoted is statistical, the second systematic.

Measurement of inclusive b --> .tau..nu.X branching ratio

Using the L3 detector, the branching ratio BR(b → πνX) has been measured using a sample of Z → bb events tagged by high momentum and high transverse momentum leptons in one hemisphere and with missing energy in the opposite hemisphere. From a sample of 948 000 hadronic events we find BR(b → πνX) = (2.4 ± 0.7 (stat.) ± 0.8 (syst.))%.

Measurement of the weak charged current structure in semileptonic b-hadron decays at the Z peak

The neutrino energy spectrum in semileptonic b-hadron decays with identified energetic electrons and muons has been measured. The observed relative energy sharing between the neutrino and the cahrged lepton is found to be well described with a W± polarization obtained from a free b-quark decay model with a (Vf-A)×(Vf-A) decay structure. The alternative of a (V+A)×(Vf-A) decay structure is excluded with a significance of more than 6 standard deviations. The possibility that hadronic corrections to the b-hadron decay destroy any W± polarization is disfavored by more than 3 standard deviations.

Energy and particle flow in three-jet and radiative two-jet events from hadronic Z decays

Abstract We report on a detailed study of the energy and particle flow in the event plane of three-jet events (q q g) and radiative two-jet events (q q γ) in hadronic Z decays recorded with the L3 detector. We find a significant decrease in particle and energy density in the angular region between quark and antiquark jets for q q g events as compared with q q γ events. Several QCD model predictions are compared with the observed effect.

Search for anomalous

Abstract We have searched for anomalous Z → γγγ events with the L3 detector at LEP. No significant deviations from the expected QED e + e − → γγγ events are observed. The branching ratio upper limit for a compoite Z decaying directly into three photons is found to be 1.0 × 10 −5 at 95% C.L. The branching ratio upper limits for the process Z → γX, X → γγ are in the range of 0.4 to 1.3 × 10 −5 , depending on the mass and width of the scalar particle X. In the context of a model with magnetic monopoles coupling to the Z, we find BR(Z → γγγ) −5 at 95% C.L.; this results in a lower mass limit of 510 GeV for a magnetic monopole.We have searched for anomalous Z -+ yyy events with the L3 detector at LEI? No significant deviations from the expected QED e+e--+ yyy events are observed. The branching ratio upper limit for a composite Z decaying directly into three photons is found to be 1.0 x lo-’ at 95% C.L. The branching ratio tqper limits for the process Z+ yX. X -+ yy are in the range of 0.4 to 1.3 x IO-s, depending on the mass and width of the scalar particle X. In the context of a model with magnetic monopoles coupling to the Z, we find BR(Z-1 yyy) < 0.8 x 10e5 at 95% C.L; this results in a lower mass limit of 5 IO GeV for a magnetic monopole.