Evt Gen in ATLAS Evt Gen in ATHENA

Evt. Gen in ATLAS • • Evt. Gen in ATHENA Standard use of Evt. Gen in ATLAS Use cases needed intervention in Evt. Gen ATLAS strategy for Evt. Gen development 13. 5. 2004 M. Smizanska, LCG Generators meeting, CERN

Evt. Gen in ATHENA, since Nov 2003 atlas/offline/external/Evt. Gen/source code …lib. Evt. Gen. a copy of July 2003 Babar release + LHCb Pythia 6 instead of Jetset + incoherent oscillations + CDF Bs & B-baryons ( this is already inside Ba. Bar version) ATHENA: Externals/Evt. Gen/…/requirements points to lib. Evt. Gen. a ATLAS interface to Evt. Gen: ATHENA: Generators/Evt. Gen_i/…/Evt. Decay top algorithm, interface to Evt. Gen. Can be combined with algorithms Pythia, Pythia. B, Single. Particle. Gun. 13. 5. 2004 M. Smizanska, LCG Generators meeting, CERN

A. Standard use of Evt. Gen in ATLAS both: use(default) and develop new code in Evt. Gen Standard use of Evt. Gen in ATLAS: 1. Pythia: • We produce b-event , fragmentation, hadronization (repeated) • We let strong and electromagnetic B-decays in Pythia – reason - not all excited B states and decays are known to Evt. Gen Ø Can be done in Evt. Gen, however need carefull balance between fragmentation and excited states decays – to be tuned vs b-jet multiplicities. Bar has done the tuning of Evt. Gen to B multiplicities but they have only with Bd-mesons. LHC need to include Bs and B-baryon sector! • We leave all weak decays for Evt. Gen 2. Interface: • pass all un-decayed B to decay in Evt. Gen • add the decay to Hep. Mc tree 13. 5. 2004 M. Smizanska, LCG Generators meeting, CERN

B. Use cases needed intervention in Evt. Gen 1. Lb production polarization and Lb →L J/y • As we produce Lb in Pythia – no sense of using production-polarization model • no tools to express polarization in Pythia. • it would be highly untrivial to follow a polarization from b quark… • Ø So we introduced Lb polarization ‘by hand’ in Evt. Gen. Should be done to all baryons – however before that: all strong and Elmag B-decays should go into Evt. Gen. • We wrote a decay model for Lb → L J/y with all 6 helicity amplitudes. This model can be included at some stage (after validation) into Evt. Gen. Ø 13. 5. 2004 M. Smizanska, LCG Generators meeting, CERN

B. Use cases needed intervention in Evt. Gen, cont 2. Bs → J/y f • • • Ø SVVHELAMP model is available in Evt. Gen ATLAS validated it by comparing to direct MC generation using probability density fcn In first stage we have left only strong decay sector(helicity amplitudes) in validation procedure on both sides Evt. Gen does not provide interference of weak amplitudes D B=2(mixing) and D B=1. This is a general feature of Evt. Gen needs to be re-written. We introduced a weak interference for particular decay Bs → J/yf Need to be rewritten to allow for interference of several weak amplitudes in any B-decay – need to foresee contributions from New Model amplitudes 13. 5. 2004 M. Smizanska, LCG Generators meeting, CERN

Our strategy for Evt. Gen development 1. 2. 3. 4. 5. 6. 7. ATLAS wants that any changes of Evt. Gen due to LHC needs are incorporated into official Ba. Bar package. This means we do not want to split from Ba. Bar package. The motivation for point 1. is that Ba. Bar is producing data and they tune Evt. Gen accordingly. ATLAS would be happy if CDF, D 0 also agree with point 1. CDF, D 0 are also going to produce new data and can tune Evt. Gen as well. Currently ATLAS use the Evt. Gen code taken from LCHb, although we have (slightly) different interface code. Changes in Evt. Gen being prepared by ATLAS are relevent for LHCb and CMS. When fully validated we will propose them to LHCb and CMS. If agreed we may submit them to Ba. Bar code manager. It will be usefull if changes already done by LHCb concerning Pythia are included into Ba. Bar code. However the code describing B oscillations needs to be revised. We believe that any changes to Evt. Gen package should be incorporated by Ba. Bar Evt. Gen code manager. 13. 5. 2004 M. Smizanska, LCG Generators meeting, CERN