C. Kiesling

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.

Beam tests and calibration of the H1 liquid argon calorimeter with electrons

Results are presented on the energy calibration of the H1 liquid argon calorimeter modules with electrons from a test beam in the energy range of 3.7 GeV to 80 GeV. The method to determine the calibration for the Hl experiment from these measurements by the use of detailed simulations is described . Various systematic checks of this calibration are given. The calorimeter response is uniform in space within ±1% and linear with energy within ±1%. An average energy resolution of about 11.5%/ E [GeV] is achieved .

On the Λp enhancement near ΣN threshold

Abstract New high statistics bubble chamber data on K−d → Λpπ− at incident K− momenta between 680 and 840 MeV/c were used to study the enhancement in the effective Λp mass near the ΣN threshold (∼2130 MeV). An analysis of the t-dependence of this effect shows that for minimum 4-momentum transfer |t| to the recoiling dibaryon system a narrow, symmetric peak with M = 2129.0 ± 0.4 MeV and Λ = 5.9 ± 1.6 MeV is observed. At larger negative t-values a broad asymmetric shoulder is seen with a sharp rise at ΣN threshold and extending to ∼2180 MeV. The broad structure is attributed to ΣN → Λp conversion. The dynamical origin of the peripherally produced narrow enhancement is discussed. The data favour strongly a 3S1 state like the deuteron. In a simple coupled channel formalism it is shown that its interpretation as a Λp resonance is acceptable, but the mass and width parameters in this formalism need not correspond to the mass and width of the observed peak. Since the enhancement is found to be very close to ∑+n threshold, an alternative explanation might be a threshold cusp in the elastic Λp scattering.

Measurement of the reaction γd → π−pp, and determination of cross sections for the reaction γn → π−p, at photon energies between 0.2 and 2.0 GeV

Abstract The reaction γ d→ π − pp has been measured in a bubble chamber experiment at DESY with a bremsstrahlung beam of E γ max = 5.5 GeV. Total and differential cross sections are presented for the energy region from 0.2 to 2.0 GeV. The extraction of cross sections on the neutron is discussed in detail. The impulse approximation and the closure relation, with corrections for the exclusion principle and the Fermi motion effects, as well as the Chew-Low extrapolation technique are applied. Total cross sections and angular distributions for the reaction γ n→ π − p, deduced by these methods, are presented in graohs and tables. The results are compared with other experiments and with various predictions. No positive evidence for an isotensor electromagnetic current is found. The results disagree with some of the measurements of radiative pion capture, if detailed balance is assumed to connect the two reactions.

Realization of a second level neural network trigger for the H1 experiment at HERA

Abstract Since 1996 the H1 experiment is fully equipped with two independent fast pattern recognition systems operating as second level triggers (L2). The decision time is 20 μs. One of the two is the neural network trigger. It runs an array of presently ten VME-boards with CNAPS 1064 chips (20 MHz, 128 Mcps) by Adaptive Solutions. The input trigger data from the detector components arrive in various formats on a 8 × 16 bit wide 10 MHz bus. Before usable as 8-bit input values to the CNAPS they are preprocessed by several bit-manipulating algorithms and arithmetic functions implemented on XILINX 4008 field programmable gate arrays (FPGA). The startup strategy for the new system is to concentrate on photoproduction channels or low multiplicity final states which so far could only be efficiently triggered with unacceptable high rates.

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.

New information about the Kaon-nucleon-hyperon coupling constants g(KNΣ(1197)), g(KNΣ(1385)) and g(KNΛ(1405))

The reactions K − d→ Σ − p, K − d→ Σ − (1385)p, K − d→ Λ (1405)n and K − d→ Λ (1520)n have been studied at K − momenta between 686 and 844 MeV/ c in an experiment with the 81 cm Saclay bubble chamber at CERN. About 630 000 pictures have been analyzed. Partial and differential cross sections are presented. A one-nucleon-exchange model is used to extract the kaon-nucleon-hyperon coupling constants from these results. For g( K N Σ(1197)), g( K N Σ(1385)) and g( K N Λ(1405)) we find values which are compatible with the SU(3) predictions. The coupling constant g( K N Λ(1520)) obtained by our method agrees with that determined from the partial decay width for Λ(1520)→ K N .

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.