1. CLAS12 Simulation Analysis and Optimization K.Dergachev and G.P.Gilfoyle University Of Richmond - Department of Physics Introduction Jefferson Laboratory (JLab) is a US national laboratory built to probe the atomic nucleus and to unravel the quark-gluon nature of matter. Our best theory of the strong interaction that binds nuclei together is Quantum Chromodynamics (QCD) which is not well understood in the nuclear environment. In this project we develop and optimize simulations of the proposed CLAS12 detector as part of the JLab 12-GeV Upgrade. CEBAF The Continuous Electron Beam Accelerating Facility (CEBAF) is the heart of the JLab complex and produces an extremely precise electron beam. The accelerator is 7/8-mile around and sits in a racetrack-shaped tunnel 25 feet underground. It is a superconducting linear accelerator capable of producing electrons with energies up to 6 GeV. The electron beam makes up to five laps around the track before being steered into one of the three existing experimental halls; Halls A, B, and C in the figure below. Figure 1: Jefferson Lab CLAS – 6 GeV The CEBAF Large Acceptance Spectrometer (CLAS) in Hall B is used to detect electrons, protons, neutrons, pions, photons, and other subatomic particles. Drift chambers determine the path of charged particles after bending in a toroidal magnetic field produced by superconducting electric coils. Cerenkov counters separate electrons from negative pions. Scintillators determine time of flight, and calorimeters measure energy. The information from these components is used to determine particle momentum and energy and to identify each particle. Figure 2: CLAS Upgrade Rationale JLab was built to study the transition from the proton-neutron-meson picture of atomic nuclei to one based on quarks, gluons, and QCD. The original accelerator was designed and built with an eye to the future to eventually increase the beam energy. In the time since, a compelling scientific case has been made to upgrade CEBAF and the end stations to more fully explore this transition region. The JLab 12-GeV Upgrade is now the highest priority project in nuclear physics in the United States. Figure 4: CLAS 12 Event Simulation CLAS12 To take full advantage of the new physics opportunities with the upgraded CEBAF, a new detector called CLAS12 will be built in Hall B. It will retain many design features of the original CLAS, but will have a new superconducting magnet coils and additional improvements. Higher luminosity for hydrogen targets (10-fold increase over CLAS). Improve small-angle acceptance for electrons and large-angle acceptance for recoil baryons. Increase momentum range of electron-pion separation. Better scintillator segmentation. Add another Cerenkov counter. Add central detector with dual purpose solenoid magnet design. Replace current mini-torus for Moller electron shielding. Provide magnetic field for large angle momentum analysis. Figure 3: CLAS 12 Simulation Results Preliminary results from Sim12 are shown in Figures 4-6. Figure 4 shows the event display for a single electron. The light blue components are the drift chambers, the yellow ones are the torus magnet coils, the dark blues elements are the electromagnetic calorimeters in the forward and central detectors. The electron path is the straight, red line (the magnetic field is off here) which shows secondary electrons knocked lose by the passage of the scattered electron beam. Plugins Plug-ins for event generators have been developed to separate the event generation from the simulation. Developers can test different event generation schemes without recompiling Sim12. The existing types are: TXT – Text-based format supporting unlimited particles per event. Each particle is characterized by Particle Data Group identification number, vertex position (in cm), and 3-momentum (in GeV/c). File extension is.txt. LUND – Text-based format that follows the LUND scheme for describing the contents of an event (T.Sjostrand, Comp. Phys. Comm. 82 (1994) 74). For particle decays only the final particles (children) are kept. CLAS12 Simulation A detailed simulation of the CLAS12 (named Sim12) is necessary to validate the design decisions of the upgraded detector, understand the detector response, and eventually for acceptance calculations (acceptance is the effectiveness of the detector at measuring different events). A group at JLab has already begun developing a GEANT4-based, object-oriented simulation package. GEANT4 - A software toolkit from CERN for the simulation of the passage of particles through matter. XML-based geometry - The geometry of CLAS12 will be described with the Extensible Markup Language (XML). XML is simple, flexible format used to define the structure of information and to exchange data between different processes, sites, etc. Plug-ins for event generation - There are already a variety of event generators available to members of the Collaboration. Plug-ins will be used to pass simulated event data to Sim12. This technique creates a clean separation between event generation and simulation. Developers do not have to rebuild Sim12 to test a new event generator. Hit generation schemes – A library of generic detector types have been written to give greater control to the user. The choice of hit generation is made in the geometry files so the method can be altered without modifying the Sim12 code. Output data – Output from Sim12 is written in a form called EVIO that is compatible with the CEBAF Online Data Acquisition (CODA). Utilities exist to produce ROOT histograms from EVIO files produced by Sim12. Installation and Run-Time Optimization For Sim12 to be a useful, it must be quick to implement for new users. We have written procedures, scripts, and documentation to speed installation and optimize running. Installation – Sim12 requires a large number of software libraries shown in the table below. Table 1. List of software packages for Sim12. A detailed installation guide was written to guide new users (http://clasweb.jlab.org/wiki/index.php/Getting_Started_Sim12_- Offsite). Compile time is still an issue (up to 2 hours), so binary packages were created allowing users to simply extract the files from an archive for the Suse 10.2 and Fedora Core 6/7 Linux distributions. Run-time – Event Simulations are typically slow (a few Hz) and the graphical user interface (GUI) for CLAS12 is computationally intensive. The GUI was operated in either immediate mode or stored mode. The former has no limitation on data size, and the latter is fast for visualizing large data repetitively, and so is suitable for animation. In stored mode, the OpenGL function calls that have been executed are kept in a display list where they can be executed after creation without the overhead of any processing that was performed to obtain those function calls. This enables much faster rendering of OpenGL at a cost of increased memory use. The histogram to the left shows a simulation of the energy lost by a sample of 13,500 scattered electrons that passed through region 1 of the CLAS12 drift chambers. The region 1 drift chambers consists of six layers (layer one is the closest to the target). As the electrons move through the drift chamber gas, they lose increasing amounts of energy The histogram to the right shows a simulation of the electron vertex position as a function of the position along the beam axis (z) and perpendicular to it (y). The target is treated as a point giving rise to the large peak at the origin. The events downstream of the target are beam interactions with air or secondary interactions by small-angle electrons. Figure 5 Figure 6 A B C SIM12JGEANT4HDDS EVIODAWNMOTIF XERXES-CGEANT4CLHEP