G. Folger

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 two JPC=0++ isoscalar resonances at 1365 and 1520 MeV

From a simultaneous analysis of data on pp → π0π0π0 and pp → ηηπ0 at rest, two I = 0, JPC = 0++ resonances are identified above 1 GeV. The first has mass M = 1365−55+20 MeV and width г = 268 ± 70 MeV, close to the ƒ0(1400) of the Particle Data Group. The second has M = 1520 ± 25 MeV, г = 148−25+20 MeV.

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)°.

Proton-antiproton annihilation into ηηπ-observation of a scalar resonance decaying into ηη

Abstract The results of a measurement of pp annihilation at rest into ηηπ 0 are presented. Assuming a resonance dominated model for the annihilation process an isoscalar J PC =0 ++ resonance decaying into ηη is observed with a mass of 1560±25 MeV/ c 2 and a width of 245±50 MeV/ c 2 .

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.

Observation of a scalar resonance decaying to π+π−π0π0 in pp annihilation at rest

Abstract We report an analysis of p p annihilation at rest into ϱ + ϱ − π 0 leading to the final state π + π − 3 π 0 . A strong signal is observed for a scalar resonance with mass 1374 ± 38 MeV/ c 2 and width 375 ± 61 MeV/ c 2 , decaying to ϱ + ϱ − . The resonance in ϱ + ϱ − represents 2.0 ± 0.3% of all p p annihilation at rest. It is also observed to decay into two S-wave dipions ( π + π − ) ( π 0 π 0 )). Under the assumption of isospin 0, this is probably ƒ 0 (1400) , seen for the first time in its 4 π decay mode.

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.

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.

Radiative protonium annihilation into γγ, γπ0, γη, γω and γη′

Abstract Radiative p p annihilation at rest into γγ , γπ 0 , γη , γω and γη′ has been measured with the Crystal Barrel detector at LEAR for antiprotons stopped in liquid hydrogen. The branching ratios are B γπ 0 =(44±4)×10 −6 , B γη =(9.3±1.4)×10 −6 and B γω =(68±18)×10 −6 , respectively. For B γγ and B γη ′ we find new upper limits of 0.63×10 −6 and 12×10 −6 (95% CL), respectively.