Simulation in ALICE Andreas Morsch For the ALICE

Simulation in ALICE Andreas Morsch For the ALICE Offline Project 2003 Conference for Computing in High Energy and Nuclear Physics Andreas Morsch, CERN EP/AIP CHEP 2003

Topics ● ALICE Simulation Strategy ● Simulation in the Ali. Root Framework ● Virtual MC ● Event Generator Interfaces Andreas Morsch, CERN EP/AIP CHEP 2003

ALICE Simulation Strategy (1) ● ● Coherent simulation framework for detector and physics performance studies in the Ali. Root Framework based on ROOT – Physics simulation – Detailed detector response simulation – Fast simulation Use of Transport MC transparent to the user – Virtual MC Interface ● Maximum reuse of user code ● One single development line (in C++) Andreas Morsch, CERN EP/AIP CHEP 2003

ALICE Simulation Strategy (2) ● ● Event Generator Interface tailored to the needs of the Heavy Ion Physics Community – Soft Uncorrelated Background ⊕ Correlations ⊕ Hard Processes – Maximum of flexibility and user configuration capabilities Monte Carlo Truth – Full history from primary particles (partons) to hits available after simulation. Andreas Morsch, CERN EP/AIP CHEP 2003

Root based Event Display Andreas Morsch, CERN EP/AIP CHEP 2003

Root Based Interface to G 3 Geometry and Physics Andreas Morsch, CERN EP/AIP CHEP 2003

Ali. Root Components Used in Simulation User code for detector simulation. External Packages Transport MC User code for primary event generation TFluka FLUKA Andreas Morsch, CERN EP/AIP CHEP 2003

Virtual Monte Carlo (VMC) See presentation by Ivana Hrivnacova See presentation by Andrei Gheata Andreas Morsch, CERN EP/AIP CHEP 2003

Virtual Monte Carlo Status ● TGeant 3 – ● ● Used in production TGeant 4 – Used for Geant 4 physics validation – See talk by Isidro Gonzalez TFluka – Under development – Full chain from primary particle generation, transport, hits running – See also talk by Alberto Fasso on FLUKA physics Andreas Morsch, CERN EP/AIP CHEP 2003

Simulation of Heavy Ion Collisions ● ● Shortcomings of existing generators – None of the existing generators do give detailed account of the expected multiplicities, pt and rapidity dependence at LHC energies – Most of the hard probes (heavy flavor, jets. . . ) are not properly reproduced by existing generators. – Existing generators do not provide for event topologies like momentum correlations, azimuthal flow etc. The small cross-section of hard processes would demand prohibitively long runs to simulate a number of events that is commensurable with the expected number of detected events in the experiment Andreas Morsch, CERN EP/AIP CHEP 2003

The ALICE Approach ● ● ● The simulation framework provides an interface to external generators, like HIJING. A parameterised “signal free” underlying event with multiplicity as a parameter is provided. Rare signals can be generated using – External generators like PYTHIA – Libraries of parameterised pt and rapidity distributions The framework provides a tool to assemble events from different signal generators – On the primary particle level (cocktail) – On the digit level (merging) After-Burners are used to introduce particle correlations. Andreas Morsch, CERN EP/AIP CHEP 2003

Event Generator Interfaces Interface to external generators Interface to parametrisations and decayer Andreas Morsch, CERN EP/AIP CHEP 2003

Event Generator Interfaces Container class for generators Andreas Morsch, CERN EP/AIP CHEP 2003

Afterburner Processors ● ● Introduction of correlations in otherwise uncorrelated events – 2 particle correlations – Flow – Assembling of new events Design of classes involved in event generation (Ali. Run, Ali. Stack, Ali. Generator) supports requirements for Afterburner – Example: Several objects of type Ali. Stack containing the input events can be connected to the Afterburner (of type Ali. Generator) to fill a stack connected to Ali. Run (output event) Andreas Morsch, CERN EP/AIP CHEP 2003

Fast Simulation ● As many approaches as subdetectors. ● Two main applications in ALICE: – Very fast simulation ● – Smearing of track parameters from paramterisation obtained from slow simulation Combined slow and fast simulation ● Detailed simulation of inner detector system (vertexing) and fast simulation of TPC response (momentum resolution) Andreas Morsch, CERN EP/AIP CHEP 2003

Conclusions ● ● The ALICE Offline Project has developed a coherent simulation framework for detector and physics performance studies in the Ali. Root Framework based on ROOT The main simulation components are – – Virtual MC Interface ● TGeant 3 (production) ● TGeant 4 (physics validation) ● TFluka (implementation ongoing) Event generator interfaces tailored to the needs of the heavy ion community Andreas Morsch, CERN EP/AIP CHEP 2003