EvtGen in ATLAS/LHCEvtGen in ATLAS/LHC

Roger W.L. JonesRoger W.L. Jones

James R. CatmoreJames R. Catmore

Maria SmizanskaMaria Smizanska

Lancaster University, UKLancaster University, UK

IntroductionIntroduction

ATLAS is due to begin data-taking in 2007ATLAS is due to begin data-taking in 2007 A wide and diverse physics programme is plannedA wide and diverse physics programme is planned The collaboration is currently performing feasibility studies with simulation The collaboration is currently performing feasibility studies with simulation

software, to assess the performance of the detector in measuring a range of software, to assess the performance of the detector in measuring a range of quantitiesquantities

100011110101110101100101110110100011101110100011101100010101001111001010100101110101011011

Generation Simulation Digitization

ReconstructionAOD buildingPhysics analysis

The The Event GenerationEvent Generation stage, at the start of the simulation chain, must produce realistic 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 meaningfulsimulations of the particle decays if the feasibility studies & physics analyses are to be meaningful

Requirements for generation of Requirements for generation of beauty eventsbeauty events

The beauty sector is characterized by the large number of decay The beauty sector is characterized by the large number of decay channels with a variety of different topologies and multiplicitieschannels with a variety of different topologies and multiplicities

A B-decay package should adequately account forA 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 This led to the creation of dedicated B-decay packages, the most successful being successful being EvtGenEvtGen

p B0S

K+J/

+

-

pK-

Bphys t a(t) B0 b(t) B 0

EvtGenEvtGen

EvtGen is a C++ software package originally written for EvtGen is a C++ software package originally written for BaBarBaBar

The code has a number of highly attractive featuresThe 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

Requirements: LHC vs “B-factories”Requirements: LHC vs “B-factories”

In 2001 the LHC community decided to adapt the In 2001 the LHC community decided to adapt the EvtGen code to the LHC environmentEvtGen 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 Differences between the B-factories and LHC

necessitated several significant changes, culminating necessitated several significant changes, culminating in the development of an in the development of an LHC versionLHC version of EvtGen 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)

LHC version of EvtGenLHC version of EvtGen

The proton-proton collisions, up to and including production of The proton-proton collisions, up to and including production of hadrons are performed by standard LHC production packages hadrons are performed by standard LHC production packages (e.g. Pythia, Herwig etc) (e.g. Pythia, Herwig etc)

Approximately half of the B-decays are produced in EvtGen; the Approximately half of the B-decays are produced in EvtGen; the rest are decayed with Pythia (string model)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 formatThe data is passed between the codes in HepMC format Once Pythia and EvtGen have decayed the particles, the results Once Pythia and EvtGen have decayed the particles, the results

are written in HepMC format and committed to transient or are written in HepMC format and committed to transient or persistent storagepersistent storage

PYTHIA LHC-EvtGen

Transient data store

Pythia_i EvtGen_i

Job Options

Decay tables

List of particles whose decay is frozen

in Pythia

Frozen B-hadronsFrozen B-hadronsand decayed particles

from Pythia

B-hadron decay products

from EvtGen

Persistency

External libraries invoked by interfaces

External

Internal

Physics modifications in Physics modifications in LHC-EvtGen LHC-EvtGen

BaBar produces coherent BB pairs from BaBar produces coherent BB pairs from ΥΥ(4s) resonances(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 BaBar-EvtGen is tuned to obtain b-decay multiplicities appropriate to BaBar - not the LHCappropriate 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 freedomPythia 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

Example validation exerciseExample validation exercise

The distribution of the

angle of the decay

planes of the final states of

BsJ/() (KK)

is shown here, calculated by

EvtGen and the

theoretical model

ConclusionsConclusions

LHC-EvtGen version is being developed by the LHC LHC-EvtGen version is being developed by the LHC experiments within LCG projectexperiments within LCG project

Relevant updates from BaBar-EvtGen are regularly Relevant updates from BaBar-EvtGen are regularly included in the LHC versionincluded in the LHC version

LHC-EvtGen has many users and is developing LHC-EvtGen has many users and is developing quicklyquickly

Users are from a wide variety of physics communities within the LHC

Besides benefiting from BaBar experimental results Besides benefiting from BaBar experimental results which are uploaded into the LHC version, we also which are uploaded into the LHC version, we also benefit from similar activities at the Tevatronbenefit from similar activities at the Tevatron