Implementation of Trigger Tiles for ALFA Simulation

The Absolute Luminosity For ATLAS (ALFA) experiment was designed to accurately measure the luminosity of the intersecting proton beams at the ATLAS interaction point [1]. However, the ALFA experiment has shifted its primary purpose from luminosity measurement to elastic and inelastic proton collisions. This change was the result of difficulty in fitting parameters in the region governed by Coulomb scattering. The operational principle for luminosity measurement with ALFA relied on detecting elastic proton collisions, so the detector is suited to its role in proton collision measurements. The ALFA detector consists of several sensitive components, including the main detector (MD), overlap detectors (ODs) and trigger tiles. A diagram of the ALFA detector is shown in Figure 1. The main detector is composed of layers of 0.5 × 0.5 mm2 cross section scintillating fibres with an active area of 0.48 × 0.48 mm2, which are directed diagonally across the detector with 64 fibres in each layer. The 20 total layers are stacked behind each other with alternating U and V layers, which are oriented in opposite directions, crossing each other at right angles. This design is intended to allow the exact position of the incoming proton to be tracked as it passes through the layers. This is achieved by monitoring the scintillated light from the fibres, and the point where hits are recorded in both U and V fibres determine the precise location of the particle. The fibres are pitched in each successive layer, which provides greater precision in track reconstruction.