L3 Experiment

% L3 \\ \\ The detector consists of a large volume low field solenoid magnet, a small central tracking system with very high spatial resolution, a high resolution electromagnetic calorimeter encapsulating the central detector, a hadron calorimeter acting also as a muon filter, and high precision muon tracking chambers. \\ \\The detector is designed to measure energy and position of leptons with the highest obtainable precision allowing a mass resolution $\Delta$m/m smaller than 2\% in dilepton final states. Hadronic energy flux is detected by a fine-grained calorimeter, which also serves as muon filter and tracking device. \\ \\The outer boundary of the detector is given by the iron return-yoke of a conventional magnet, using aluminium plates for the coil. The field is 0.5~T over a length of 12~m. This large volume allows a high precision muon momentum measurement, performed by three sets of drift chambers in the central detector region. From the multiple measurement of the coordinate in the bending plane a momentum resolution of 2.5\% for 45~GeV muons is obtained. A forward-backward muon detection system extends the polar angle coverage to 22 degrees in the forward region. This system is made out of three layers of trapezoidal multi-wire drift chambers for the reconstruction of $\mu$ trajectories, two layers of Resistive Plate Chambers~(RPC) providing a fast $\mu$ trigger, and aluminium coils surrounding the main magnet doors to create a toroidal field which provides additional bending power. \\ \\Going radially inwards, the combined hadron calorimeter and muon absorber consists of wire chambers operating in the proportional mode, sandwiched with U-plates as energy converter. A subdivision into 3000 space elements covering the full solid angle (except the beam pipe) allows localization and determination of the hadronic energy flow with around 11\% energy resolution at the Z pole energy. \\ \\The electromagnetic energy flow is determined by approximately 11000 crystals of BGO \linebreak (Be$ _{4} $Ge$ _{3} $O$ _{1} _{2} $), a transparent scintillator with 1.1~cm radiation length. Full electromagnetic shower containment over nearly 4$\pi$ solid angle coverage is achieved. An energy resolution about 1.4\% and a spatial resolution better than 2~mm is obtained for photon energies larger than 1~GeV, with a rejection against hadrons of better than 1000. \\ \\Surrounding the 10~cm diameter beam pipe, a high precision Silicon Microstrip Detector~(SMD) and a small drift chamber operating in the time expansion mode (TEC) act as charged particle vertex detectors. The drift chamber has achieved a space resolution down to 45~$\mu$m. The SMD consists of two layers, each providing readout if the R-$\phi$ and Z coordinates with an intrinsic resolution of about 7~$\mu$m and 15~$\mu$m, respectively. The polar angle of the micro vertex detector extends from 22~degrees to 158 degrees. The combination of TEC and SMD allows a good momentum and charge determination for particles up to 45~GeV, an efficient electron-photon separation and a measurement of the impact parameter of the tracks of long-lived charged particles.