Introduction

The purpose of the TRT_Digitization package is to turn hits from the Geant4 simulation into digits, which is a reflection of the event format as read out from the real detector.

The hits from the Geant4 simulation basically contains:

positions of beginning and end of a G4 step inside the TRT straw (in G4 language, PreStepPoint and PostStepPoint)
energy deposition inside the TRT straw

The TRT digitization is divided into several steps:

Resimultion of ionisation process
Because the straws constitute very thin layers of active gas, the standard G4 ionisation functions are not appropriate for the detailed simulation of the ionisation process. As a consequence, the first step of the digitization is a re-simulation of energy loss by charged particles. This is done using the Photon Absorption Ionisation model as implemented in the TRT_PAI_Process package. The ionisation process creates clusters distributed along the particle trajectory.
For photons interacting with the Xenon gas, no re-simulation is done. The deposited energy forms one cluster at the point of interaction.
Modelling of the drift chamber
Clusters are converted to deposits, i.e. signals reaching the front end electronics. This process includes several effects/steps:
- The number of primary drift electrons is calculated from the cluster energy
- Electrons are drifted to the wire in the presence of the magnetic field (different effects in barrel/end cap). Stocastic recapturing of drift electrons in the drift gas. Optionally a drift time spread is included
- Wire propagation delays (direct and reflected signals) can be optinally modelled
Modelling of the front-end electronics
For each straw:
- Energy deposits are filled (as delta functions) into a vector in the appropriate time bins.
- This vector is convulated with the signal shaping functions (separate for low and high threshold signals)
- The signals are then discriminated against low and high thresholds in appropriate time slices and the output digit constructed.

In addition, the package handles detector noise, which is modelled after the detector performance in the 2004 combined test beam.

Class Overview

The TRTDigitization package contains of following classes:

TRTDigitization: main class for the TRT digitization.
TRTProcessingOfStraw: Handles the complete digitization straw-by-straw.
TRTGasGain: Gas gain as function of drift distance.
TRTSignalShape: Provides signal shaping for the electronics signals.
TRTElectronicsProcessing: conversion of signals from deposits on signal wires to digits - the 27 bit output from the front-end electronics
TRTDigit: the digit for a single straw.
TRTTimeCorrection: Calculates the time correction from wire propagation delays.
TRTDigCondBase: communication with CondDB - the conditions data base.
TRTDigCondDase::StrawState: The status of an individual straw.
TRTDigCondFakeMap: Fake straw conditions map until we know the real detector.
TRTDigSettings: containing parameters and settings for TRT digitization.
TRTElectronicsNoise: Together with TRTNoise, this controls the simulation of electronics noise.
TRTNoise: Together with TRTElectronicsNoise, this controls the simulation of electronics noise.