B. Schmid

The Crystal Barrel spectrometer at LEAR

The crystal Barrel spectrometer used at LEAR, CERN to study the products of pd annihilations is described. A 1380 element array of Csl crystals measures photons from the decay of π0, η, η′ and ω mesons. A segmented drift chamber in a 1.5T magnetic field is used to identify and measure charged particles. A fast on-line trigger on charged and neutral multiplicities and on the invariant mass of secondary particles is available. The performance of the detector is discussed.

Observation of a 2+ + resonance at 1515 MeV in proton-antiproton annihilations into 3π0

Abstract Antiproton-proton annihilations at rest into 3 π 0 are analyzed in terms of π 0 π 0 final state interactions. A J PC = 2 + + resonance at a mass of 1515 ± 10 MeV with a width of 120 ± 10 MeV is required to explain the data. This result is supported by previous observations made on the ππ system in p p and p n annihilations which show that the resonance has I = 0. This resonance is practically degenerate in mass with the f′ 2 (1525) but the two resonances have very different production and decay characteristics.

The pseudoscalar mixing angle ΘPS from ν and ν′ production in p annihiltion at rest

Abstract We have determined ratios of branching ratios for p p annihilations at rest into π 0 η and π 0 η ′, into ηη and ηη ′, and into ωη and ωη ′. All reactins are observed in two different all-neutral final states. The ratios permit a determination of the pseudoscalar mixing angle. We find Θ PS = −(17.3±1.8)°.

P- versus S-wave pp-annihilation at rest in LH2

Abstract The annihilation p p →π 0 π 0 was measured for antiprotons stopped in liquid hydrogen (LH2). This reaction is only allowed from odd angular momentum states of the p p - atom . The resulting branching ratio BR ( p p →π 0 π 0 ) LH 2 =(6.93±0.22 stat ±0.37 syst )×10 −4 , combined with a previous measurement of the branching ratio BR ( p p →π + π − ) 2 P in gas from the 2P-state of p p suggests a fraction of P-wave annihilation in LH2 of (28.8±3.5)%, much larger than the values obtained from other annihilation channels. A method of reconciling the contradictory results is discussed.

Antiproton-proton annihilation at rest into ωπ0π0

The annihilation channel pp → ωπ0π0, where ω decays to π0γ, was studied with antiprotons stopped in liquid hydrogen. This reaction is dominated by the production of b10(1235) with contributions from f2(1270) and the ππ S-wave. The branching ratio for pp → ωπ0π0 is (2.00±0.21)% of all annihilations. The branching ratios for b10(1235)π0 (b1 → ωπ0) and f2(1270)ω (f2 → π0π0) are (0.92 ± 0.11)% and (0.57 ± 0.07)% of all annihilations, respectively. Upper limits for the production of vector mesons decaying to ωπ are given and the ππ mass distribution around 1 GeV is discussed.

Construction and performance of a thin cylindrical multiwire proportional chamber with cathode pad readout for the H1-experiment

Abstract We discuss the construction of a thin (0.14% radiation length) cylindrical multiwire proportional chamber with cathode pad readout used in the first level vertex trigger of the H1-experiment at HERA (DESY). The chamber has an active length of 2190 mm, and anode plane radii of 157 and 166 mm with a 3 mm gap. The cathode consists of a graphite layer on a Kapton foil with a resistivity of 400 kΩ/⦜. In order to have a two-dimensional readout the Al-layer on the back of the outer cathode of each chamber is segmented in 36 mm wide pads covering 45° in azimuth. The performance of the chamber with various gas mixtures was studied. Typically a length of the high voltage plateau of 200 V and a time resolution of 75 ns base width has been achieved, sufficient to separate individual HERA bunch crossings. The behavior of the chamber at pad boundaries has been measured and the performance for particles crossing at angles down ϑ = 16° is described.

η-decays into three pions

Abstract The branching ratios Γ(η → 3π 0 ) Γ(η → π + π − π 0 ) = 1.44 ± 0.09 ± 0.10 and Γ(η → γγ) Γ(η → π + π − π 0 ) = 1.78 ± 0.10 ± 0.13 have been determined using the Crystal Barrel detector at LEAR. The latter ratio yields a partial width of Γ ( η → π + π − π 0 ) = 0.26 ± 0.03 keV using the known value for Γ ( η → γγ ). The η → π + π − π 0 ) Dalitz plot is compared to the predictions of chiral perturbation theory.

Protonium annihilation into KL0KL0π0 and KL0KS0η

The annihilation of pp at rest into KL0KS0π0 and KL0KS0η was investigated with the Crystal Barrel detector at LEAR. These final states with negative C-parity are dominated by φ and K0∗ resonances. The ratios of the branching ratios are B(pp→φπ0)B(pp→φη)=8.3±2.1 and B(pp→K0K0∗+K0K0∗→KL0KS0π0)B(pp→φπ0→KL0KS0π0)= 2.04±0.21.

A large acceptance, high-resolution detector for rare K+-decay experiments

Abstract The detector of the E865-collaboration at the Brookhaven-AGS described here combines a magnetic spectrometer for the charged decay products of 6 GeV /c K + with excellent electromagnetic calorimetry and efficient particle identification for electrons and muons. Its high-resolution, large acceptance and high rate capability made it well suited for the study of extremely rare or forbidden decays with multi-leptonic final states such as K+→π+μ+e−, K+→π+l+l−, K+→l+νle−e+ and K+→π+π−e+νe down to branching ratios below 10−11 in an intense K+ beam (≈108 per AGS spill).

Compact frontend-electronics and bidirectional 3.3 Gbps optical datalink for fast proportional chamber readout

Abstract The 9600 channels of the multi-wire proportional chamber of the H1 experiment at HERA have to be read out within 96 ns and made available to the trigger system. The tight spatial conditions at the rear end flange require a compact bidirectional readout electronics with minimal power consumption and dead material. A solution using 40 identical optical link modules, each transferring the trigger information with a physical rate of 4×832 Mbps via optical fibers, has been developed and commissioned. The analog pulses from the chamber can be monitored and the synchronization to the global HERA clock signal is ensured.