DETECTING AND TRIGGERING ON NEUTRONS USING THE SPACAL Pb - SCINTILLATING FIBER CALORIMETER

We present an application of the SPACAL spaghetti calorimeter prototype 1; 2) in the CERN hyperon beam experiment WA89. First results on the performance of the upgraded SPACAL calorimeter in a high rate experimental environment are presented. In order to reconstruct invariant masses from ? hyperons in nal states, the energy of the decay neutrons and their impact point have to be determined with the best possible accuracy. As a rst step in the analysis, ? beam particles have been easily reconstructed. For future runs we intend to trigger on the neutrons from ? decays using fast calorimeter information and a scintillating tile predetector. Testbeam results on both components will be presented. 1 Motivation The performance of the almost compensating (e/h=1.15) electromagnetic and hadro-nic SPACAL spaghetti calorimeter has been extensively investigated in two testruns in 1990, and many results on its intrinsic properties as well as on shower characteristics have already been published 2; 3; 4; 5). The rst application of this calorime-ter prototype was recently made in the hyperon beam experiment WA89 6) at CERN aiming to measure the neutrons from ? particles ((? ! nn ? ; BR = 99:85%) steaming from charmed-strange baryon or "exotic" multiquark state decays. The WA89 experimental setup ((g.1)-which was up to then only sensitive to hy-perons in the nal states (via ! pp ? ; BR = 64:1%)-was extended for 2 1/2 weeks in 1991 by placing the SPACAL under an angle of 0 0 behind the Pb-glass electromagnetic calorimeter (which was moved out of the beam for 1 week). As the physical processes to be studied by the hadron calorimeter are rare processes (order of several hundreds of events per year), a trigger on nal states with high energy neutrons is essential for future runs.