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Some validation of MC@NLO, AlpGen and TopRex

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

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Some validation of MC@NLO, AlpGen and TopRex

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

  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. 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• Bug: problem with ‘timel’ routine; herwig job cannot be longer than 1000 CPU seconds • McAtNlo_i-00-00-04 • Interface outdated and buggy - need to be updated to comply to MCATNLO vsn 2.3 (or 2.2) • Inputini.F • Initmcatnlo.F • Works fine for top-production • Does not work for events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. 13.5% 86.5% Resume: Weights ‘standard’ tt production process -1706 • Weights: ±w • ‘unweighted’ events, up to a sign!(practically weight ±1) • Efficient event generation possible • NLO distributions (without MC showering) are non-physical tt production cross section MCatNLO: 842 pb HERWIG: 458 pb PYTHIA: 490 pb (nb: no consistent pdf’s!)

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

  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. 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. 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. 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. Alpgen: tt+1jet • Inputs • Mtop=175 • 1 extra light jet • Jet: Pt>10, ||<2.5, R>0.4 • Initial grid 3 * 200000 • Events: 40.106 • Produced 60 samples • Production • Un-weighting to single lepton (e,,) decay • Effective : 293 pb • 1.9 106 events generated (8 10-4 generation efficiency) • 18.1% (351000) events pass first selection • ETmiss>20 GeV, lepton (e, ) Pt>20 and >=4 jets Pt>40 Example run in AlpGen

  11. 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 tt+1-jet production Alpgen looks ok!

  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, ) Pt>20 and >=4 jets Pt>40 • Alpgen has ‘hard’ matrix element for 4 extra jets

  13. 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 NIKHEF data processing facility AlpGen jobs take lot of CPU • Total submitted jobs: 2303 • Total GHzHrs (equivalent hours on 1 GHz machine): 15469 (!) AlpGen jobs running!

  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, ||<3.0, R>0.3 • No lepton cuts • Initial grid: 200000*3 • Events: 150·106 • Jobs: 198 • W+4 extra light jets • Jet: Pt>10, ||<2.5, R>0.4 • No lepton cuts • Initial grid: 200000*3 • Events: 150·106 • Jobs: 98

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

  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. 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. TopRex • Motivation: • Study top-spin correlations • Need detailed simulation to check these subtle effects <CosΘ+ · CosΘ->

  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

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