Introduction

The main goals of modern nuclear physics is the investigation of hadron properties, such as effective masses, decay widths, electromagnetic form factors etc., inside nuclear matter under extreme conditions of high density and high temperature. The aim of the new High Acceptance DiElectron Spectrometer (HADES)  this is a better understanding of the various processes contributing to di-lepton production in hot and compressed nuclear matter, leading ultimately to a search for signals of the partial restoration of the chiral symmetry of QCD. 

The complete HADES analysis and simulation software packages are:

Root

CERN Library

Pluto

Hydra

Hgeant

Oracle

UrQMD

 Root

Root  is an object oriented framework based software with main emphasis on physics data analysis. Root written in C++, contains an efficient hierarchical OO database, advanced statistical analysis (multi dimensional histogramming, fitting, minimization, cluster finding algorithms) and visualization tools. The user interacts with ROOT via a graphical user interface, the command line or batch scripts. The command and scripting language is C++ and large scripts can be compiled and dynamically linked in. ROOT provides a basic framework that offers a common set of features and tools for the domains of data analysis, data acquisition, event reconstruction and generation, detector simulation and related fields.

The backbone of the ROOT architecture is a layered class hierarchy, which grouped in frameworks and divided in categories. This hierarchy is organized in a mostly single root class library, that is most of the classes inherit from a common base class Tobject.

For many years, the data flow model in HEP has been:

Raw Data Tapes à Data Summary Tapes à Mini-DST's

The introduction of Ntuple in the PAW framework has proven to be very successful. Many experiments of high-energy and heavy-ion are using Ntuples as a convenient replacement for mini-DSTs. The PAW Ntuples, however, were restricted to very simple data objects, collection of single variables or array. ROOT introduce of a new concepts, which called Trees. Trees provide the functionality of the Ntuples and much more. The Tree architecture extends the concept of the ntuple to all complex objects or data structures found in RAW Data tapes and DST's. The idea is that the same data model, same language, same style of queries can be used for all data sets in one experiment. Trees are designed to support not only complex objects, but also a very large number of them in a large number of files.

CERN library

 The CERN Program Library is a large collection of general-purpose programs maintained and offered in both source and object code form on the CERN central computers. The most popular applications based on CERNLIB are PAW, PYTHIA and GEANT 3.21. Most of these programs were developed at CERN and are therefore oriented towards the needs of a physics research laboratory that is general mathematics, data analysis, detectors simulation, data-handling etc... applicable to a wide range of problems.

The library contains several thousand subroutines and complete programs, which are grouped together by logical affiliation into several hundred-program packages. 80% of the programs are written in FORTRAN and the remainder in assembly code, or C usually with a FORTRAN version also available. The language supported is currently Fortran 77.

Pluto

The Pluto program is frequently used for event generation in Heavy-ion Physics, which was developed for the simulation of the HADES experiments. It's designed within the ROOT environment, and makes use of ROOT and CLHEP-library resources. The building functionality of ROOT as an analysis environment, including tree structure, provides powerful tools that are fully exploited, enabling complex operations such as the calculation of spectral functions from first principles for hadronic resonances with multiple decay modes.

Pluto is a collection of C++ classes, and figure 1. shows the structure of Pluto.

Typical simulations may be executed with a few lines of input, with no expertise required on the part of the user. The output may be analyzed on line, or further processed with GEANT. The package includes models that address:

1.     Resonance widths and mass distributions in the nuclear medium

2.     Dalitz decays and direct dilepton channels

3.      Anisotropic angular distributions for selected channels

4.     NN elastic scattering

5.     Thermal distributions are implemented, enabling multi-hadron decays of hot fireballs

Figure 1.: The class structure of Pluto

 An important effect that must be taken into account for realistic simulations is the deviation of resonance shapes from fixed-width Breit-Wigner distributions, which is typically modeled as a mass-dependence in the resonance width. This is particularly important for resonances with large widths, such as the , , N*, and * resonance excitations for which the effect is largest.

Hydra

For the HADES analysis the Hydra packages has been written . It's a C++ analysis packages in the ROOT  environment. Hydra contains classes for the full event reconstruction and analysis. The reconstruction is divided into several steps:

1.    Reading event information from database.

2.    Retrieve all the parameters from a database describing the detector setup (geometry parameters, calibration parameters).

3.    Detecting particles hits in the sub-detectors using pattern recognition techniques.

4.    Identify particles.

5.    Fitting reconstructed tracks to obtain the particles' momenta.

The main goal of the reconstruction program is the reconstruction of measured or simulation HADES events. HADES is a spectrometer with a six-fold axial symmetry, which is divided into six identical sectors. Each sector holds several components: the START, RICH, MDC, Magnet, TOF wall, TOFINO wall and Pre-Shower. Furthermore, each detector can be made up of several modules.

Program's design

The reconstruction program for HADES involves some basic concepts:

l     Data (real, simulated)

l     Input/Output procedures

l     Parameters (geometry of detectors, calibration of the detectors, unpacking information, etc.)

l     Algorithms, tasks

These concepts have been incorporated in the following set of classes:

l    Fundamental class: Hades is the class, which encapsulates the whole reconstruction program, providing methods to control the different tasks, which can be realized. 

l    Classes that contain data: from the point of view of physics, an event holds all the information collected by the different detectors in the spectrometer regarding one interaction between one beam particle and the target. An event is represented by a HEvent object, HEvent being an abstract class. This allows deriving from HEvent other classes, which correspond, to the different kinds of event.

l    Classes to manage input/output of data: since the data can come from different sources one must provide a generic interface.

l    Classes to contain and to manage all the numerical information needed to process the data: To run an analysis, several numerical parameters are needed, as for example, calibration parameters or geometry positions of detectors. The parameters are valid for very different time scales. Once a detector is built, some parameters are fixed for the whole lifetime of this detector. Some parameters might change seldom, others quite often.

l    Classes to perform tasks: For each events one need to accomplish various tasks represented by HTask objects.   

Hgeant  

Hgeant describes the HADES simulation package. The simulation is based on the CERN software GEANT 3.2, which is used

1.    To define and represent the detector geometry, i.e. volumes and media

2.    To track particles through an experimental setup for the simulation of detector response

3.    To generate hit ROOT based on a realistic modeling of the physical processes occurring along the tracks. Following this, the hit ROOT are digitized and analyzed in the framework of the HADES ROOT environment.

The geometry definitions are retrieved from a relational database, but in addition input from text files is provided.

The HGeant system allows the user to:

1.     Describe an experimental setup by a structure of geometrical volumes.

2.     Accept events simulated by UrQMD or Pluto;

3.     Transport particles through the various regions of the setup, taking into account geometrical volume boundaries and physical effects according to the nature of the particles themselves, their interactions with matter and the magnetic field 

4.     Record particle trajectories and the response of the sensitive detectors

5.      Visualise the detectors and the particle trajectories

Oracle interface

Oracle is a commercial, very powerful relational database using SQL as query language. It is accessible from all Unix and Windows platforms at GSI. The access to the data stored in the Oracle database for HADES at GSI is available via WWW worldwide. It is possible to access the actual data online, with no need of making temporary copies or export files. The data is accessed directly from the Oracle database. All the parameters (geometry parameters, calibration parameters) of the HADES experiment are stored in the HADES Oracle database.

The UrQMD Model

 The Ultra-relativistic Quantum Molecular Dynamics model (UrQMD)  is microscopic model for simulating heavy ion collisions in the energy range from SIS to RHIC. It represents a Monte Carlo solution of a large set of coupled equations for the time evolution of the various phase space densities of particle species. Main goals are to gain understanding about the following physical phenomena within a single transport model:

l     Creation of dense hadronic matter at high temperatures

l     Properties of nuclear matter, Delta & Resonance matter

l     Creation of mesonic matter and of anti-matter

l     Creation and transport of rare particles in hadronic matter.

l     Creation, modification and destruction of strangeness in matter

l     Emission of electromagnetic probes

 A detail model description can be found in the following two articles

 

  1. Micrpscopic Model for Ultra-relativistic Heavy Ion Collisions

S. A. Bass, M. Belkacem, M. Bleicher, M. Brandstetter, L. Bravina, C. Ernst, L. Gerland, M. Hofmann, S. Hofmann, J. Konopka, G. Mao, L. Neise, S. Soff, C. Spieles, H. Weber, L. A. Winckelmann, H. Stocker, W. Greiner, Ch. Hartnack, J. Aichelin and N. Amelin: Microscopic Models for Ultrarelativistic Heavy Ion Collisions.

Prog. Part. Nucl. Phys. 41 (1998) 225-370

  1. Relativistic Hadron Hadron Collisions and the Ultra-relativistic Quantum Molecular Dynamics Model (UrQMD)

    M. Bleicher, E. Zabrodin, C. Spieles, S.A. Bass, C. Ernst, S. Soff, L. Bravina, M. Belkacem, H. Weber, H. Stocker, W. Greiner: Relativistic Hadron-Hadron Collisions in the Ultra-Relativistic Quantum Molecular Dynamics Model

    J. Phys. G25 (1999), 1859-1896