2 Meson Spectroscopy at LEAR with the

The CRYSTAL BARREL Collaboration has reported the discovery of the scalar mesons a 0 (1450) 1, 2] and f 0 (1370), together with a relatively narrow f 0 (1500), decaying into 0 0 3], 4], 0 5], K L K L 6] and 4 0 7]. The f 0 (1500) is interpreted as a glueball 8], but its precise KK decay rate has still not been measured precisely 6]. Also, a scalar ss meson decaying mainly into KK is needed to complete the scalar nonet. This has motivated the recent activities of the Zurich group. The detector, a barrel of CsI(Tl) crystals, a jet drift chamber and a pair of cylindrical proportional chambers surrounding a hydrogen target in a solenoidal magnetic eld is described in details in ref. 9]. The wire chambers have recently been replaced by a silicon microstrip vertex detector to trigger on secondary vertices, In the last year of LEAR operation we collected annihilations in gaseous hydrogen and in ight up to 1.9 GeV/c. The various target modes and trigger conditions are summarized in table 2.2. Three diierent targets were used: 1. Liquid hydrogen (LH 2) was used for pp annihilation at rest and in ight. The aim in 1996 was to get a reasonable statistics on 4-prong channels. 2. In liquid deuterium (LD 2) annihilation occurs with the proton or the neutron. The pn-system has the well deened isospin I=1, so that the number of contributing amplitudes is smaller than for pp (which has both I = 0 and 1). 3. In gaseous hydrogen (GH 2) annihilation also proceeds through atomic P-states while annihilation from atomic S-states dominates in liquid. Since the production of intermediate resonances is driven by the quantum numbers of the initial state, resonances with diierent spins can be discriminated by changing the gas 9