A Hadron Shower Library for FOCUS Experiment

FOCUS is a charm photoproduction experiment at Fermilab. The experimental setup is a magnetic spectrometer capable of detecting the particles produced in the interactions of a tertiary photon beam of about 170 GeV on a BeO target. It consists of: the silicon microstrip layers for high resolution tracking and vertexing, five MWPC stations and two straw tube stations to trace the articles deflected by two large aperture magnets of opposite polarity, three Cerenkov counters for particle identification, two e.m. calorimeters to detect the photons and electrons, a hadron calorimeter to trigger on the total hadronic energy released in the interaction and two muon stations to detect the muons. The experiment has collected over 7 x 109 triggers of charm candidates. Large samples of events have to be simulated in data analysis. Tracking in each event all the particles produced during the process of the shower development in the calorimeters is a very time consuming operation and Hadron Shower Library (HSL) is a solution to increase the computational speed. With HSL in the FOCUS simulation program, the response of the calorimeter is accessed randomly from a file containing a large collection of showers, rather than simulated event by event. FOCUS calorimetry has a rather complicated structure: an outer e.m. Pb/scintillator sampling calorimeter to detect the wide angle electromagnetic particles and an inner part to detect higher energy particles that pass the second analyzing magnet, which consists of an homogenous lead-glass (e.m.) with a central vertical aperture and a Fe/scintillator (hadronic) calorimeter with cells of three different granularity. HSL was created for the inner calorimeter by simulating in GEANT its response to individual hadrons with momenta over the whole range of allowed values and storing the information about kinematical parameters of parent particles and the calorimeter response in a random access file. In a previous study, pion calibration data were shown to be well described with pion showers simulated in GEANT. A special algorithm was devised in order to circumvent the complications due to nonuniform calorimeter structure and to make the HSL independent of incidence point on the front face of calorimeter, which allowed a reasonable size for the HSL file. Other problems which had to be solved were the portability of HSL file in order to be used on different computing platforms and its integration in the general Focus Monte Carlo program with the possibility to activate its use by some flags. Work is in progress on new improvements and comparison with real showers. (authors)