Y. Gershtein

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)%.

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.

Observation of a new charmed baryon

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II at DESY, we have observed a new charmed baryon state in the channel Λ c + π + π − . (All references to a specific charged state also imply the charge conjugate state.) The mass of this state was measured to be (2626.6 ± 0.5 ± 1.5) MeV/ c 2 . The product of the production cross section and branching ratio for this channel was determined to be (11.5 ± 2.5 ± 3.0) pb, and the natural width estimated to be smaller than 3.2 MeV/ c 2 at 90% CL.

A Determination of two Michel parameters in purely leptonic tau decays

Abstract Using the ARGUS detector at the e+e− storage ring DORIS II, we have determined the Michel parameters ϱ and ξ of τ → μν ν and τ → eν ν decays. From a data sample with 333 events/pb around s = 10 GeV , we select 3230 events e + e − → τ + τ − → (μ ± μ μ )( e ǂv v ) and determine ϱ from the e and μ momentum spectra and ξ from the correlations between e and μ momenta. For ϱ we obtain ϱτ→e = 0.79 ± 0.08 ± 0.06 and ϱτ→μ = 0.76 ± 0.08 in accordance with the V - A structure of the decays. The momentum correlations are only sensitive to the product ξτ→eξτ→μ. Setting ξτ→e = ξτ→μ, we obtain |ξτ| = 0.90 ± 0.15 ± 0.10 also in accordance with V - A. The updated decay fractions are B(τ → e v v ) = (17.5 ± 0.3 ± 0.5)% and B(τ → μv v ) = (17.4 ± 0.3 ± 0.5)% .

Determination of the Michel parameters ξ and δ in leptonic τ decays

Abstract Using the ARGUS detector at the e + e − storage ring DORIS II, we have determined the Michel parameters ξ and δ of τ ∓ → l ∓ ν ν - decays. With an integrated luminosity of 445 pb −1 around s = 10 GeV, we have produced approximately 415000 τ-pairs. From this data sample, 3262 events were selected with e + e − → τ + τ − → (l ± ν ν - ) (π ∓ π + π − ν) . The semihadronic decay was used as analyser of the τ-spin and made possible - owing to the spin correlations - the determination of the parity-violating Michel parameters ξ and δ in the decay τ ∓ → l ∓ ν ν - . Simultaneously to the determination of the Michel parameters, a measurement of the τ-neutrino helicity h ν τ in the decay τ ∓ → π ∓ π + π − ν was obtained. We observed h ν τ = −0.85 −0.17 +0.15 ± 0.05, ϱ = 0.721 ± 0.040 ± 0.021, ξ = 1.26 −0.26 +0.30 ± 0.09, and ξδ = 0.77 −0.16 +0.18 ± 0.05. In addition, the combined ARGUS result on h ν τ , ϱ, ξ, and ξδ are reported using this work and previous measurements.

Measurement of the polarization in the decay B ---> J / psi K*

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 search for the electric dipole moment of the τ-lepton

Abstract Using the ARGUS detector at the e+e− storage ring DORIS II, we have searched for the real and imaginary part of the electric dipole formfactor dτ of the τ lepton in the production of τ pairs at q 2 =100 GeV 2 . This is the first direct measurement of this CP violating formfactor. We applied the method of optimised observables which takes into account all available information on the observed τ decay products. No evidence for CP violation was found, and we derive the following results: Re (d τ )=(1.6±1.9)·10 −16 e cm and Im (d τ )=(−0.2±0.8)·10 −16 e cm , where statistical and systematic errors have been combined.

A search for the lepton-flavour violating decays τ → eα, τ → μα

Lepton spectra in τ decays have been studied with the ARGUS detector at thee+e− storage ring DORIS II at DESY using the “pseudo-rest-frame” technique. We have improved upper limits for two-body τ-decays into a lepton and an unobservable particle.

Determination of the structure of τ decays in the reaction e+e− → τ+τ− → ϱ+vτϱ−vτ and a precision measurement of the τ-neutrino helicity

Abstract Using the ARGUS detector at the DORIS II storage ring we have investigated the Lorentz structure of the electroweak interaction in semi-hadronic τ decays. Spin correlations in the process e + e − → τ + τ − → v τ π + π 0 v τ π − π 0 are exploited for a measurement of the normalized product of the vector ( g V ) and axial vector ( g A ) couplings of the τ lepton, γin AV = 2 Re {g A g V ∗ } (|g V | 2 ) . The correlations are sensitive to the product of the couplings in both τ decays: γ AV 2 = γ AV (τ − → ν τ W − ) AV = −h(v τ )h( v τ ) . The measured value, γ AV 2 = 1.044±0.057±0.060, determines the relative sign of the neutrino helicities. Assuming CP invariance, the absolute value | γ AV | = 1.022±0.028±0.030 was found to be incellent agreement with the Standard Model. The contribution of scalar ( g s ) or pseudoscalar ( g P ) couplings was also investigated. No evidence for a scalar-like coupling was found.