ALICE event generators

1. Yuri Kharlov IHEP, Protvino (for the ALICE collaboration) LCG Generator Russian team meeting SINP MSU 27 December 2004 ALICE event generators

2. ALICE simulation framework ALICE simulation framework is called ALIROOT is based on ROOT ALIROOT includes the whole chain of simulation from event generation to reconstruction. The role of the framework is shown here: The basic principles applied in the design of the AliRoot framework are re-usability and modularity: • Re-usability is the protection of the investment made by the programming physicists of ALICE. • Modularity allows replacement of parts of the system with minimal or no impact on the rest.

3. ALIROOT layout

4. Requirements for event generators • ALICE will study heavy-ion collisions at LHC energies. • Final-state particle multiplicity in Pb-Pb collisions at 5.5 AteV is predicted as 1400-8000 charged particles per unit rapidity. • Simulations should determine the sensitivity of the ALICE detector for new phenomena expected in HIC: • Particle properties modification in dense nuclear matter • Collective effects (flows, correlations) • Parton attenuation in passing through nuclear matter • Change of the particle yields in AA vs pp collisions

5. Problems with existing event generators • LHC energies are 30 times higher than RHIC and 300 times higher than SpS energies, so no tuning with experimental data is available yet. • Existing event generators give different predictions for the expected particle multiplicity, pt- and y-distributions and the dependence of different observables on pt and y at LHC energies. • Most of the physics signals, like hyperon production, high-pt observables, open charm and beauty, quarkonia, etc., even at lower energies are not exactly reproduced by the existing event generators. • Simulation of small cross-section observables would demand prohibitively long runs to simulate a number of events that is commensurable with the expected number of detected events in the experiment. • Existing event generators do not reproduce some observables of HIC like correlations, flows,etc.

6. However To allow for efficient simulations we have developed the offline framework such that allows for a number of options: • The simulation framework provides an interface to several external generators, like HIJING. • A simple event generator based on parametrised (pt,y) distributions can provide a signal-free event with multiplicity specified as a parameter. • Rare signals can be generated using the interface to external generators like PYTHIA or simple parameterisations of transverse momentum and rapidity spectra defined in function libraries. • The framework provides a tool to assemble events from different signal generators (event cocktails). • The framework provides tools to combine underlying events and signal events on the primary particle level (cocktail) or on the digit level (merging). • “Afterburners” are used to introduce particle correlations in a controlled way.

7. Event generators implementation in ALIROOT • To facilitate the usage of different generators an abstract generator interface called AliGenerator has been developed to provide the user with an easy and coherent way to study a variety of physics signals as well as a full set of tools for testing and background studies. This interface allows the study of full events, signal processes, and their mixture. • Several event generators are available via the abstract ROOT class that implements the generic generator interface, TGenerator. Through implementations of this abstract base class we wrap several FORTRAN Monte Carlo codes to access them from the AliRoot classes: • HIJING • PYTHIA • ISAJET • HERWIG • DPMJET • TPHIC • EPEMGEN • A set of parametrized generators are implemented which reproduce pt,y distributions of final-state particles. • It is customary in heavy-ion event generation to superimpose different signals on an event to tune the reconstruction algorithms. This is possible in AliRoot via the so-called cocktail generator.

8. AliGenerator: base class for all generators AliGenerator has the responsibility of generating primary particles of an event. Some realisations of this class do not generate the particles themselves but delegate the task to an external generator like PYTHIA through the TGenerator interface.

9. Parametrized generators AliGenParam is a realisation of AliGenerator that generates particles using parameterised pt and rapidity distributions. Instead of coding a fixed number of parameterisations directly into the class implementations, user defined parametrisation libraries AliGenLib can be connected at run time allowing for maximum flexibility.

10. Cocktail generators AliCocktail generator is a realisation of AliGenerator which does not generate particles itself but delegates this task to a list of objects of type AliGenerator that can be connected as entries AliGenCocktailEntry at run time. In this way different physics channels can be combined in one event.

11. Afterburner processors and correlation analysis The modularity of the event generator framework allows easy integration with the simulation steering class AliRun and with the objects that are responsible for changing the output of event generators or for assembling new events making use of the input of several events. These processors are generally called Afterburners. They are especially needed to introduce a controlled (parameterised) particle correlation into an otherwise uncorrelated particle sample. In AliRoot this task is further simplified by the implementation of a stack class AliStack that can be connected to both AliRun and AliGenerator. Currently, afterburners are used for the simulation of the two-particle correlations and azimuth flow signals.

12. Summary • ALIROOT is a solid self-consistent tool for the ALICE off-line. • ALIROOT is equipped by a wide range of event generators. • ALIROOT provides easy and transparent methods to incorporate any new event generators.