D.C. Fries

Cello-A New Detector at PETRA

A description of the design characteristics of CELLO, a large 4Π magnetic detector at PETRA, is given and first results of beam operation are presented.

An Experimental study of the process gamma gamma ---> pi+ pi-

Using the CELLO detector, we have measured cross sections for the processγγ→π+π− in the mass range 0.75–1.9 GeV/c2. A partial wave fit to the data indicates the presence of a sizeableS-wave amplitude with signs of resonant behaviour. Values for the γγ width of thef2(1270) are given, showing the model-dependence of this quantity. At higher dipion masses, the model of Brodsky and Lepage is found to give an order of magnitude description of the data.

Scalar lepton search with the CELLO detector at PETRA

Abstract We report on the search for “supersymmetric” scalar leptons conducted with the CELLO detector, at the PETRA e + e − storage ring. 11.1 pb −1 of high energy data were analysed ( 33 GeV s GeV ). At a 95% c.l., the existence of a scalar e is ruled out for masses between 2 GeV and 16.8 GeV; correspondingly, a scalar μ is excluded between 3.3 GeV and 16 GeV, and a scalar τ between 6 GeV and 15.3 GeV, as well as between the τ mass and 3.8 GeV.

A measurement of the π0, η and η′ electromagnetic form factors

We present measurement of the π0γ*γ, ηγ*γ and η′γ*γ form factors. The π0-form factor is for the first time observed in the space-like region. The transition form factor of the η-meson is determined from its decay modes π+π−π0, π+π−γ and the neutral decay mode γγ. The decay of the η′ is observed in the decay channels ργ, ηπ+π− with η→γγ and in the four charged prong final state stemming from ηπ+π− with the η decaying into π+π−(π0/γ). All form factors agree well with a simple ρ-pole predicted by the vector meson dominance model and also with the QCD inspired Brodsky-Lepage model.

Photoproduction of Φ-mesons at small t-values

Abstract The reaction γ + p → Φ + p has been measured using a spark chamber spectrometer and a tagged photon beam in the energy range from 4.6 to 6.7 GeV. Approximately 3500 photoproduced elastic Φ-events have been collected in the t -range between t min and t = −0.4 (GeV/ c ) 2 . Cross sections and t -distributions are presented.

Investigation of two photon final states in e+e− annihilation at <√s⪢ = 34.2 GeV

Abstract Two photon final states in e + e − annihilation have been analyzed at CM energies around 34 GeV. Good agreement with QED is observed. Lower limits for the QED cutoff parameters of Λ + > 59 GeV and Λ - > 44 GeV are determined. A search for two photons with missing energy yields an upper limit for the production of neutral particles which decay into a photon and a non-interacting particle. Constraints on the mass and the coupling strength of supersymmetric photinos are discussed.

Measurement of e+e−→τ+τ− at high energy and properties of the τ lepton

Abstract The e+e−→τ+τ− process has been measured using the CELLO detector at a mean total centre of mass energy of 34.2 GeV using essentially all the decay channels of the τ lepton. The measured cross section yields Rτ=1.03±0.05 (stat)±0.07 (syst). Topological branching fraction are given for τ → 1, 3 or 5 charged tracks. The angular distribution shows a clear 1 + cos2θ dependance with a forward-backward asymmetry of -0.103 ± 0.052 corresponding to an axial-vector coupling aτ of the τ to the weak neutral current given by aτ=−1.12 ± 0.57.

Elastic and inelastic φ photoproduction

Abstract The differential cross section of the reaction ( γ p → p φ ) has been measured in the t range 0 ⩽ t ⩽ 0.4 GeV 2 and for photon energies from 3.0 to 6.7 GeV. In particular for the small t region the measurement accuracy was better than 10%. We obtained for the slope parameter B in an exponential parametrization of the differential cross section d σ /d t = A e − Bt values of B ⋍ 6 ± 0.5 GeV −2 which are significantly larger than the slopes obtained by most other experiments at higher t values. This indicates a t dependence of B particularly in the small t region. An energy dependence of the optical point (d σ /d t ) t =0 , observed in our measurements, has been explained as a kinematic effect due to the VDM relation. A fit of our measurements is in excellent agreement with all other published values of (d σ /d t ) t =0 ( γ p → φ p), this implies that σ tot (φp) must be essentially energy independent in this energy range. Spin density matrix elements of the φ have been evaluated and an analysis of the helicity amplitudes has been carried out. This analysis confirmed s -channel helicity conservation. Moments of spherical harmonics of the K K angular decay distribution have been computed for 10 MeV K K mass-bins from threshold to 1.3 GeV. The mass dependence of the normalized moments is generally smooth. Contributing amplitudes have essentially only even moments. The moment 〈 Y 2 0 〉/〈 Y 0 0 〉 changes sign above the φ mass. Differential cross sections for the inelastic φ production γ p → φ X have been evaluated for the first time both with respect to t − t min and M K . The integrated inelastic cross sections are comparable in size with the elastic ones. The slopes of the differential cross sections d σ /d t appear to become flatter with increasing M X .

Search for new heavy quarks in

Abstract The total e + e − annihilation onto hadron has been measured at CM energies between 33.00 and 36.72 GeV and between 38.66 and 46.78 GeV in steps of 20 and 30 MeV respectively. The average of the ratio R = σ ( e + e − → hadrons )/ σ is 〈 R 〉=3.85±0.12 and 〈 R 〉=4.04±0.10 for the two energy ranges. The systematic error on 〈 R 〉 is 0.31. Both values are consistent with the expectation for the known coloured quarks u, d, s, c and b. No evidence was found for the production of new quarks. If the largest fluctuation in R is interpreted as a narrow resonance, it corresponds to a product of the electronic width and the hadronic branching ratio Γ ee B had >2.9 keV at the 95% confidence level, well below the value expected for the toponium vector ground state with charge 2 3 e . The observed number of aplanar final states rules out the continuum production of a a new heavy flavour with pointlike cross section up to a CM energy of 45.4 GeV for a quarck charge of 1 3 e . and up to 46.6 GeV for 2 3 e at the 95% confidence level.

e^+e^-

Abstract Hadronic events obtained with the CELLO detector at PETRA are compared with second order QCD predictions using different models for the fragmentation of quarks and gluons into hadrons. We find that the model dependence in the determination of the strong coupling constant persists when going from first to second order QCD calculations.