1. EvtGen in ATLAS/LHC Roger W.L. Jones James R. Catmore Maria Smizanska Lancaster University, UK

2. Introduction ATLAS is due to begin data-taking in 2007 ATLAS is due to begin data-taking in 2007 A wide and diverse physics programme is planned A wide and diverse physics programme is planned The collaboration is currently performing feasibility studies with simulation software, to assess the performance of the detector in measuring a range of quantities The collaboration is currently performing feasibility studies with simulation software, to assess the performance of the detector in measuring a range of quantities 100011110101110101 100101110110100011 101110100011101100 010101001111001010 100101110101011011 Generation SimulationDigitization Reconstruction AOD buildingPhysics analysis The Event Generation stage, at the start of the simulation chain, must produce realistic simulations of the particle decays if the feasibility studies & physics analyses are to be meaningful The Event Generation stage, at the start of the simulation chain, must produce realistic simulations of the particle decays if the feasibility studies & physics analyses are to be meaningful

3. Requirements for generation of beauty events The beauty sector is characterized by the large number of decay channels with a variety of different topologies and multiplicities The beauty sector is characterized by the large number of decay channels with a variety of different topologies and multiplicities A B-decay package should adequately account for A B-decay package should adequately account for Cascade decay chains with many nodes A large variety of spin configurations producing complex patterns in the multi-dimensional space of the decay angles Quantum interference (mixing, CP violation, resonant, non-resonant final states) which have an important impact on the phenomenology of the decay This led to the creation of dedicated B-decay packages, the most successful being EvtGen This led to the creation of dedicated B-decay packages, the most successful being EvtGen p B0SB0S K+K+ J/ + - p K-K-

4. EvtGen EvtGen is a C++ software package originally written for BaBar EvtGen is a C++ software package originally written for BaBar The code has a number of highly attractive features The code has a number of highly attractive features Implementation of spinor algebra to account for spin and to allow the accurate simulation of angular distributions User input mechanism allows the use of complex amplitudes to encapsulate the decay physics Interference effects are calculated by the code during the simulation Each node of the decay chain is treated independently to allow efficient and fast Monte Carlo generation Code is organized into a modular architecture, with different processes (models) encoded in separate classes Amplitude base class provides all the algebraic operations on complex numbers and matrices, as overloaded operators

5. Requirements: LHC vs B-factories In 2001 the LHC community decided to adapt the EvtGen code to the LHC environment In 2001 the LHC community decided to adapt the EvtGen code to the LHC environment Work performed within the LCG Project (Monte Carlo Generators) Involvement of LHCb, ATLAS and CMS physicists Differences between the B-factories and LHC necessitated several significant changes, culminating in the development of an LHC version of EvtGen Differences between the B-factories and LHC necessitated several significant changes, culminating in the development of an LHC version of EvtGen Differing collision regimes (e + e - versus proton-proton and different energies) LHC produces a complete set of B-hadron species and excited states EvtGen is also used beyond the scope of beauty studies (e.g. top, Higgs, SUSY – any decays leading to a b-jet)

6. LHC version of EvtGen The proton-proton collisions, up to and including production of hadrons are performed by standard LHC production packages (e.g. Pythia, Herwig etc) The proton-proton collisions, up to and including production of hadrons are performed by standard LHC production packages (e.g. Pythia, Herwig etc) Approximately half of the B-decays are produced in EvtGen; the rest are decayed with Pythia (string model) Approximately half of the B-decays are produced in EvtGen; the rest are decayed with Pythia (string model) Decay in Pythia is frozen for those particles which are to be decayed by EvtGen; the interface then picks up these undecayed particles An EvtGen decay table informs the code which amplitude should be used to decay a given particle, and gives the branching ratio for each process The user may provide his own decay table to over-ride the default The data is passed between the codes in HepMC format The data is passed between the codes in HepMC format Once Pythia and EvtGen have decayed the particles, the results are written in HepMC format and committed to transient or persistent storage Once Pythia and EvtGen have decayed the particles, the results are written in HepMC format and committed to transient or persistent storage

7. PYTHIALHC-EvtGen Transient data store Pythia_iEvtGen_i Job Options Decay tables List of particles whose decay is frozen in Pythia Frozen B-hadrons and decayed particles from Pythia B-hadron decay products from EvtGen Persistency External libraries invoked by interfaces External Internal

8. Physics modifications in LHC-EvtGen BaBar produces coherent BB pairs from Υ(4s) resonances BaBar produces coherent BB pairs from Υ(4s) resonances These are negligible in the LHC environment LHC-EvtGen contains models which account for incoherent decays of B-mesons BaBar-EvtGen is tuned to obtain b-decay multiplicities appropriate to BaBar - not the LHC BaBar-EvtGen is tuned to obtain b-decay multiplicities appropriate to BaBar - not the LHC In the LHC-EvtGen version, strong and electro-magnetic decays are left to Pythia The old version of JetSet included in BaBar-EvtGen is removed to enable compatibility with new the Pythia versions (which are used for pp collisions) Pythia does not account for spin degrees of freedom Pythia does not account for spin degrees of freedom LHC-EvtGen must impose polarization of non-zero spin particles before decay e.g production polarization studies

9. Example validation exercise The distribution of the angle of the decay planes of the final states of B s J/ ( ) (KK) is shown here, calculated by EvtGen and the theoretical model

10. Conclusions LHC-EvtGen version is being developed by the LHC experiments within LCG project LHC-EvtGen version is being developed by the LHC experiments within LCG project Relevant updates from BaBar-EvtGen are regularly included in the LHC version Relevant updates from BaBar-EvtGen are regularly included in the LHC version LHC-EvtGen has many users and is developing quickly LHC-EvtGen has many users and is developing quickly Users are from a wide variety of physics communities within the LHC Besides benefiting from BaBar experimental results which are uploaded into the LHC version, we also benefit from similar activities at the Tevatron Besides benefiting from BaBar experimental results which are uploaded into the LHC version, we also benefit from similar activities at the Tevatron