Description and performance of the Fermilab E687 spectrometer

The magnetic spectrometer and charged particle tracking system used in Fermilab experiment 687 to study the photoproduction and decay of charm particles are described in detail. The photons are produced by a wideband electron beam which can operate at energies up to 600 GeV/c. The spectrometer consists of a high resolution silicon microstrip detector, a large aperture dipole magnet, proportional chambers, a second large aperture dipole, and more proportional chambers. Three multi-cell threshold Cherenkov counters provide charged particle identification. The tracking system is capable of resolving the secondary decay vertices of charm and beauty mesons and baryons from the primary interaction vertex. It also determines the invariant mass of the multibody final states of particles containing heavy quarks with excellent resolution. The particle identification system allows one to identify kaons and protons present in these final states clearly. This collection of detectors produces very clean signals for charm particles and permits one to make many cross checks of the apparatus. The performance is illustrated for a variety of charm signals. Of particular interest is a description of the tracking through the silicon microstrip detector and its use in isolating downstream decay vertices. Two complementary approaches to the reconstruction of secondary decay vertices are presented and insight is gained by comparing their strengths and weaknesses.