EvtGen in ATLAS James R Catmore, University of Lancaster, UK Mária Smižanská, University of Lancaster, UK Malte Muller, now at University College London, UK

Contents  About EvtGen  Why are ATLAS B-physics people interested in it?  Why should other people be interested in it?  Overview of ATLAS EvtGen Activities  Technical work  Physics validation  Progress  Further work

About EvtGen  Decay model package  Originated in BaBar; authors Anders Ryd, David Lange et al  Tuned against new data from BaBar, Belle, CLEO  Valuable validation work performed by B-factories  Large collaboration of experiments has accepted EvtGen as their principal B-decay package – LHCb, ATLAS, CMS, CDF, D0, BaBar, Belle  Agreement achieved on development and alteration of code by laboratories other than BaBar  BaBar incorporates changes to code on request

About EvtGen II  EvtGen has several highly desirable technical features for B-physics simulations  Works with complex amplitudes  Interference effects  CP-violation  Uses spinor algebra  Polarizations  Helicity states  Correct angular distributions  Novel and highly efficient algorithm for handling cascade decays Correct generation of B-events is extremely important for the triggering and selection of physics processes decaying to B- hadrons (Higgs, SUSY, exotics….)

About EvtGen III  EvtGen provides a rich selection of decay models  A model in EvtGen is a class which provides a recipe for a complex amplitude  Each spin configuration has its own model  e.g., S  VV, S  SS, S  TS, S  VS, T  SS, V  SS etc  A calculation is performed using complex amplitudes and spinor algebra.  Cascade decays are handled “node-wise” with a spin density matrix for each node for generating the angular distribution  Nodes are independent, hence a different model can be applied to each node

EvtGen Algorithm

EvtGen Algorithm II

EvtGen Algorithm III

EvtGen Algorithm IV

ATLAS EvtGen Activities I  Wholly UK (Lancaster) activity  Mária Smižanská [research officer] – software development  James Catmore [PhD student] – software development and physics validation  Malte Muller [CERN summer student] – initial technical work  Collaboration with LHCb, CDF, D0  Software development  Adapting to CMT context  Development of interface between ATHENA and EvtGen, made according to requirements for  Dedicated B-physics studies  General B-decay facility to be used by top, SUSY, Higgs, exotics groups

ATLAS EvtGen Activities II  Physics validation  Lancaster team is participating in EvtGen validation as part of a world-wide collaboration  Significant work performed by CLEO, BaBar, Belle, teams tuning EvtGen to their data  EvtGen will be a valuable source of new data from the labs involved in its development  Often new data will reach the EvtGen data files before it is published in the literature  New theory implemented in EvtGen decay models  Current Lancaster validation work involves the testing of EvtGen models against data from dedicated Monte Carlo generators

EvtGen in ATHENA Event Generator (PYTHIA) Conversion to HepMC B-mesons HepMC B-mesons PDG Decay files HepMC Event Record HepMC decay products User input EvtGen

Technical Development  ATHENA release  copy of latest (July 2003) Babar release  LHCb incoherent oscillations  CDF Bs & B-baryons  Structure  atlas/offline/external/EvtGen/ source code …libEvtGen.a  Externals/EvtGen/…/requirements  points to libEvtGen.a  Generators/EvtGen_i/…/EvtDecay  EvtDecay top algorithm, interface to EvtGen.  Can be combined with algorithms Pythia, PythiaB, SingleParticleGun

Technical Development II  Generators/EvtGen_i/share/PythiaEvtGen.txt  Concatenate algorithms Pythia + EvtDecay  includes ‘StopBweakDecays.txt’ datacards to stop all weak Pythia B-decays  Pythia:  produce event (81,82) but user can have any Pythia production  strong B-decays  store HepMC event in transient store  EvtDecay:  reads HepMC from transient store  pass all B to decay in EvtGen  add the decay into HepMC

Technical Development III  Generators/EvtGen_i/share/PythiaBEvtGen.txt  Concatenates algorithms PythiaB+EvtDecay  same as previous  PythiaB provides in addition:  Pre-selection of Pythia events with b-quark in fiducial volume  repeated hadronization to speed-up production

Physics Validation  EvtGen  We have validated the base helicity model for S  VV, SVV_HELAMP  Strong sector only (no mixing, CP- violation)  Used it to generate angular distributions for the decay B d  J/  (  +  ) K 0* (K +  - )  Tested against dedicated Monte Carlo  Dedicated Monte Carlo  Uses probability density function calculated directly using Helicity Formalism  Implemented in accept- reject Monte Carlo program (M.Smižanská)

The Decay B0dB0d K 0* J/  ++ -- K+K+ --

Decay Angles   J/   K*

Angular Distribution  Derived directly from the Helicity Formalism  which is the normal method of calculating angular distributions of decays  For our S  V (l + l - ) V (S 1 S 2 ) decay the probability density function for decay angles (  1,  2,  ) is

i

SVV_HELAMP

Results

Further work for Lancaster team  The requirement: prepare first production release of EvtDecay for use in Release  Successful start of a long program, a lot to do…..  Technical Development  User interfaces  Event selection  Kinematic cuts  Physics Validation  Exclusive b-decays  Strong sector helicity model  Incoherent mixing models  CP violation, effects on angular distributions for B and anti- B decays  b-jets  No work done (except B d  X tests made by BaBar) ·Needed for top, Higgs, SUSY groups  Assessment of HLT performance in light of the above  

Conclusions  Software development now at an advanced stage  Physics validation underway  A useful and successful beginning – but much work remains to be done