1. 1 Some validation of MC@NLO, AlpGen and TopRex Stan Bentvelsen Feb 18 th, 2004

2. P 2 Resume of MC@NLO MC@NLO:  Matching NLO calculations of QCD process with parton shower MC simulation Fully exclusive events generated  Hard emissions treated as in NLO  Soft emissions handled by MC shower (Herwig)  No ‘double counting’ between these two  Running in ATLAS: Create event file using ‘runNLO’ program (extern)  Contains kinematic of hard NLO process Interface to Herwig via McAtNLO_i

3. P 3 Release 7.6.0 Herwig_i-00-01-29  Points to herwig6.504 Excellent – this is needed for heavy gauge boson production in McAtNlo Bug: problem with ‘timel’ routine; herwig job cannot be longer than 1000 CPU seconds McAtNlo_i-00-00-04  Interface outdated and buggy - need to be updated to comply to MCATNLO vsn 2.3 (or 2.2)  Inputini.F  Initmcatnlo.F Works fine for top-production Does not work for events with W,Z (JPR=13,14) and others? Private version working against 6.5.0

4. P 4 Resume: Weights Weights: ±w  ‘unweighted’ events, up to a sign! (practically weight ±1)  Efficient event generation possible NLO distributions (without MC showering) are non- physical 86.5%13.5% tt production cross section MCatNLO:842 pb HERWIG: 458 pb PYTHIA:490 pb (nb: no consistent pdf’s!) ‘standard’ tt production process -1706

5. P 5 Resume: Comparison to LO generators All distributions normalised to 1 Pt(tt system)  Herwig & MCatNLO agree at low Pt,  At large Pt MCatNLO ‘harder’  PYTHIA completely off Same distribution on linear scale

6. P 6 MCATNLO practicalities Event Weights  Stored in HepMC::GenEvent weight()  Not being picked up by Atlfast My private AtlfastAlgs::StandardNtupleMaker is getting weights from herwig common (uh! Ugly!) Event files  What is granularity of event files? How many events in one file?  Who is making these for DC2?

7. P 7 Unit conversion External MCATNLO file  Units in GeV Conversion  Conversion to MeV done in filling HepMC common block Atlfast  Not updated to MeV yet  Not been able to fully check MCATNLO with Atlfast in 7.6.0 Jet-finding, thresholds, etc: need modification of StandardOptions values

8. P 8 AlpGen ‘multiparton’ generator Many hard processes – with extra n-jets (‘light jets’)  E.g.: tt+n-jets, W+n-jets  Exact (LO) matrix element  Alpgen generates file with hard scattering External files To be fed into Herwig/Pythia shower MC’s  GNU compiler bug, noticeable in optimized complex number arithmetic. To be safe, use optimization flag, -O2 (default for AlpGen version v1.4)  Works fine under gcc 2.96

9. P 9 AlpGen in 7.6.0 Somewhat confusing structure:  Package AlpGen_i-00-00-04 Is not updated to Alpgen V1.3 What is the status of this package?  Package Herwig_i-00-01-29 Interface to AlpGen files in routine atoher_65.F Called via UPINIT Has been updated to Alpgen v 1.3 (excellent!) Can the AlpGen_i package be deleted from release?

10. P 10 Alpgen: tt+1jet Inputs M top =175 1 extra light jet Jet: P t >10, |  | 0.4 Initial grid 3 * 200000 Events: 40.10 6 Produced 60 samples Production  Un-weighting to single lepton (e, ,  ) decay  Effective  : 293 pb 1.9 10 6 events generated (8 10 -4 generation efficiency) 18.1% (351000) events pass first selection  E T miss >20 GeV, lepton (e,  ) P t >20 and >=4 jets P t >40 Example run in AlpGen

11. P 11 AlpGen tt+1-jet production tt-system alpgen affected by extra gluon Previously problems, now solved! Histograms normalized to unity Extra jet:  Pt-min = 10 GeV  |η| < 2.5  R>0.4 Alpgen looks ok!

12. P 12 W+jets background Most important background for top: W+n jets  Leptonic decay of W, and n=4 extra jets  In Pythia only relevant process: qq’  W (+q(g) ) No ‘hard’ matrix element for 4 extra jets I.e.: 3 or 4 extra jets need to be generated by  Shower  Fragmentation+ Decays  Detector response+ Reconstruction  MC@NLO has NLO qq’  W+X No ‘hard’ matrix element for 4 extra jets Generated 350k events, only 1 event passed first selection  lepton (e,  ) P t >20 and >=4 jets P t >40  Alpgen has ‘hard’ matrix element for 4 extra jets

13. P 13 NIKHEF data processing facility Due to small generation efficiencies in Alpgen: Use local NIKHEF LCG grid Currently 30% of total LCG grid This will change soon  Total 240 CPU’s Mix of PIII: 0.8, 1.2, 2.0 and 2.6 GHz machines AlpGen jobs running! Total submitted jobs: 2303 Total GHzHrs (equivalent hours on 1 GHz machine): 15469 (!) AlpGen jobs take lot of CPU

14. P 14 Alpgen: W+4jets Un-weighting to W lepton (e, ,  ) decay  Production 1 Effective  : 4390 pb  108401 unweighted events generated (3.6 10 -6 efficiency)  2.57% (2784) events pass first top selection  Production 2 Effective  : 2430 pb  380740 unweighted events generated (2.6 10 -5 efficiency)  3.41% (13002) events pass first selection W+4 extra light jets Jet: Pt>10, |  | 0.3 No lepton cuts Initial grid: 200000*3 Events: 150·106 Jobs: 198 W+4 extra light jets Jet: Pt>10, |  | 0.4 No lepton cuts Initial grid: 200000*3 Events: 150·106 Jobs: 98

15. P 15 Top signal + background Luminosity: 10 pb -1  MC@NLO signal  Alpgen1 sample Luminosity: 150 pb -1  MC@NLO signal  Alpgen2 sample

16. P 16 TopRex generator Request from top-group to include TopRex  Relevant top physics: Handling of spin-correlations in top-decays Simulation of single-top processes Rare top decays, processes, FCNC,…  Showering and decay handled by Pythia  Used in many (fortran based) Top analyses  Used by CMS Slabospitsky, Sonnenschein http://sirius.ihep.su/~spitsky/toprex/toprex.html

17. P 17 TopRex generator 2 packages:  External/TopRex Pointer to toprex installation in /afs/cern.ch/atlas/offline/toprex  “out of the box” toprex installation  Perhaps need some cosmetic update (dir structure)  Generators/TopRex_i Interface package ala Pythia_i “toprex.dat” file to select process, switches, etc All this is available and working and checked against release 6.5.0  Tbd: linking against GENSER version of Pythia Can we have this available for DC2 ??

18. P 18 TopRex Motivation:  Study top-spin correlations  Need detailed simulation to check these subtle effects

19. P 19 Summary MCATNLO  Update interface to vsn 2.2/2.3  Make sure weights are handled correctly AlpGen  All in place – if AlpGen_i package removed  CPU demanding event generation TopRex  All is there – need to be injected in Atlas software. Urgent requests from top-group Logistics of externally generated event files