Author: Andre.nosp@m.a.Di.nosp@m..Simo.nosp@m.ne@c.nosp@m.ern.c.nosp@m.h

Introduction

Package to generate RPC digits starting from Geant4 hits.

Class Overview

This package contains the RPC_Digitizer algorithm. It takes care of all the steps necessary for RPC hits digitization:

  - Conversion from SimID to OfflineID
  - Cluster simulation (According to experimental data)
  - Propagation time calculation.

Working principles

RPC hits are generated by the SensitiveDetector (SD) which assigns to them a Simulation Identifier (SimID), uniquely identifying the gas gap each hit is registered in. The position of the hit in the reference system of the gas gap is also stored, together with the time from the beginning of the event, i.e. the time of flight of the particle generating the hit. RPC hits are represented in the simulation code by instances of the class RPCHit. The digitization process takes care of adding to the hits the information necessary for further analysis (for example trigger algorithm simulation and track reconstruction). It translates any SimID to a Standard Offline Identifier (OID), which is used by the other ATHENA algorithms to uniquely identify RPC strips in the muon spectrometer. Using the position information provided by the hits, the digitization can properly calculate the propagation time of each electronic signal along its strip, add it to the time of flight of the hit and assign this global time to the digit. The information obtained is stored in a new instance of the class RPCDigit and posted in StoreGate for further processing.

Cluster simulation

When a particle generates an avalanche in an RPC, charge signals are induced (and detected) on the readout strips. A set of $n$ adjacent strips with signals is called a cluster of size n. In RPC operation, due to possible signal induction on more than one strip, cluster sizes are in general greater than 1, with an average cluster size at working point typically of 1.3. The hit production mechanism provided by the ATLAS Geant4 simulation does not include a tool for proper simulation of clusters. Thus a particle generates hits on only one strip, except when secondaries are produced and detected by neighboring strips. The digitization algorithm reproduces the observed cluster sizes by generating, when necessary, digits on strips adjacent to the one actually crossed by the particle. Cluster simulation is carried on in three steps:

- experimental distributions are used to decide, according to the impact
  point of the particle along the strip, whether the cluster size will be 1 or 2.
- experimental distributions are used to decide what the final size of the
  simulated cluster will be.
- digits are created according to the results of the above steps.

Particular attention needs to be paid to the way the additional digits are created around the one actually crossed by the muon. The decision of which strip to turn on is made according to experimental distributions.