HERWIG vs. HERWIG++ - SUSY: SU3 comparison -

1. HERWIG vs. HERWIG++- SUSY: SU3 comparison - ATLAS MC Generator Meeting Physics and Performance Week: 26-29 August 2008 Claire Gwenlan (Oxford), Zhonghua Qin (DESY) Andy Buckley (Durham), Alessandro Tricoli (RAL) with many thanks to Peter Richardson for helpful discussion!!!

2. Intro/Aims As well as some SM prod., fortran Herwig (fHerwig) currently used in ATLAS as main generator for SUSY signal processes  are we ready to begin the transition to Herwig++? Validation/comparison process now started:- first comparisons of fHerwig vs. Herwig++ for SUSY signal process presented here … • QUESTIONS: • Can we now easily use Herwig++ in Athena for SUSY production? • What is current functionality in Herwig++ cf. fHerwig (for SUSY?) • Can we reproduce fHerwig results with Herwig++ - or at least understand where any differences are coming from? This is still very much work in progress!!(as you will see, several observed features remain to be understood … and we are currently working together with the authors in this …) HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

3. SUSY Comparison Process Chose ATLAS SU3 benchmark SUSY point for first comparisons:[m0=100GeV,m1/2=300GeV,A0=-300GeV,tan(Beta)=6,sign(mu)=+1] Samples: • fHerwig samples for reference: • - jobOption: CSC.005403.SU3_jimmy_susy.py • format: official validation sample AODs, produced with jobTransform • csc_evgenTruthJetMet_trf.py (“sampleB’s”) within Athena 14.2.0.2 Still to add details here about our Herwig++ JO HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

4. SLHA Spectrum/Decay File Herwig++ needs sparticle spectrum/decay file in SUSY les Houches Accord (SLHA) format Created ours to be equivalent to dc1new.txt, as used in fHerwig reference -> used ISAJET7.64+ISAWIG(versions used in order to correspond to dc1new.txt) -> … with ISALHA.f and ISALHD.f as add-ons to get spectrum and decay files in SLHA format (available from http://home.fnal.gov/~skands/slha/ ) Resulting spectrum+decay file - ISAJETSU3.spc - available in cvs:http://atlas-sw.cern.ch/cgi-bin/viewcvs-atlas.cgi/offline/Generators/Herwigpp_i [corresponding SLHA files for other ATLAS benchmark points also created, and will be checked into cvs soon…] (files were created corresponding to both “old” and ‘new”-styles (using versions ISAJET7.64 and 7.75 respectively)) HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

5. Differences in defaults:fHerwig: two-loop alphas, scale = 2s’t’u’/(s’2+t’2+u’2) [s’=s-m2LSP,…]Herwig++: LO alphas, scale (for PDFs, alphas) = s ^ ^ ^^^ ^ ^ ^ NOTE: Peter Richardson has sent us a “fix”, such that a scale of mT2=2stu/(s2+t2+u2) is used in Herwig++ instead of s (we have not had time to look at this yet, but it should make the cross sections agree even more closely) – this scale will also be included in the next Herwig++ release ^ SU3:Hard Process Cross Section From jobOptions, as described on previous slides:(fHerwig: CSC.005403.SU3_jimmy_susy.py; Herwig++: jobOptions.herwigpp-SusySU3.py)  good agreement, within known differences between generators (NB. Implementation of two-loop alphas does exist in Herwig++, but default is LO) HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

6. SU3 Plots: Truth Jets (CN4) First Look:Herwig++ hasmore truth jetsthan fHerwig(– mainly concentrated at lowish-pT and central eta) BUT, from discussions with Peter, found out that… -> PT>20 GeV HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

7. SU3 Plots: Truth Jets (CN4) … Herwig++ includes a treatment of SUSY showering(fHerwig has no such treatment):for comparison with fHerwig, switch it OFF (controlled in MSSM.model) -> distributions become more similar PT>20 GeV NOTE: for “best physics”, SUSY showering should be switched ON Since there are some remaining differences, have also performed some checks (not shown here) with eg. different UE parameters, … - but variations looked at so far give very similar results HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

8. SU3 Plots: Final State Leptons final-state electrons First Look:Herwig++ gives more stable, final-state leptons than fHerwig(– mainly concentrated at lowish-pT ) [situation same for muons] (see backups for more plots) PT>20 GeV HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

9. SU3 Plots: dilepton masses First Look: higher number of final-state leptons in Herwig++ can also be seen in dilepton masses [but spectrum file looks sensible - i.e. dilepton edge in the right place!!] PT>20 GeV HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

10. However, this did not have significant impact on the lepton distributions (some visible impact was observed for leptons with pT<20GeV (see backups), but it’s insufficient to explain differences) SU3 Plots: Final State Leptons Test:Stable Particles?at the moment, unable to set minimum lifetime for particles to be set stable in Herwig++ (“pltcut” for fHerwig) (This is coming in next Herwig++ release …) In our first iterations we ignored it - effect on final state leptons? [ATLAS common cut: pltcut=3.33x10-11(used to make Ks and Lambda particles stable)] -> For now, can explicitly set these particles stable in Herwig++ To set stable, add lines to JO:cmds.append(“set /Herwig/Particles/K_S0:Stable Stable”) cmds.append(“set /Herwig/Particles/Lambda0:Stable Stable”) HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

11. SU3 Plots: Final State Leptons Some Other Tests: look at final-state leptons from HP BUT:- 1. exclude those from taus (differences between Herwig++ and TAUOLA?)- 2. exclude those from c,b decays (differences in heavy quark hadronisation?) -> good agreement found when b-decays excluded[effect of c- and tau-decays not significant (- see backups)] PT>20 GeV Suggests differences due to b-quark hadronisation? - this is under investigation… HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

12. SU3 Plots: Final State Photons Herwig++ also gives more stable final-state photons than fHerwig,andwith a softer pT-spectrum PT>20 GeV HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

13. SU3 Plots: Missing-Et Some differences (particularly at small MET and small SumET) remaining to be understood (- presumably due to the observed differences in particle multiplicities) MET SumET HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

14. Summary • Can we now easily use Herwig++ in Athena for SUSY production? Yes!! Example jobOptions (jobOptions.herwigpp-SusySU3.py) are in cvs: [along with files ISAJETSU3.spc, MSSM.model - which are also needed]: http://atlas-sw.cern.ch/cgi-bin/viewcvs-atlas.cgi/offline/Generators/Herwigpp_i/share Can be used now (e.g. use with athena jobTransforms/jobOptions …)NOTE: should checkout and compile the following tags to get most recent versions: Generators/Herwigpp_i-00-01-02, External/Herwigpp-02-02-01, External/ThePEG-01-03-00 • What is current functionality in Herwig++ c.f. fHerwig (for SUSY)? Herwig++ already has (most of)* the functionality of fHerwig for SUSY AND also has additional features not present in fHerwig (most particularly, an implementation of SUSY showering) (Cont.) HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

15. (Also happy to take suggestions from the floor!!!) Summary • * Only found a couple of things that we currently use, which are not yet implemented: • pltcut (“minimum lifetime for particles to be set stable”) • (:- however, it is possible to set any particular particles stable) • spin correlations for three-body (or four-body) decays • (in current release these are decayed according to phase space) • Both of these things will be available in next release Next Herwig++ release: estimated for end of September (then, all functionality of fHerwig should be present - and more!) • Can we reproduce fHerwig results with Herwig++ - or at least understand where any differences are coming from? Getting there!! Herwig++ distributions all look reasonable, but some features wrt fHerwig still need to be fully understood (eg. - higher number of low-pT leptons,…) -> we are working (alongside the authors) on understanding these things HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

16. Backups HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

17. SU3 Plots: Final State Leptons final-state muons FIRST LOOK:Herwig++ gives more stable, final-state leptons than fHerwig(– mainly concentrated at lowish-pT ) PT>20 GeV HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

18. SU3 Plots: Final State Leptons final-state electrons (no pT-cut) Herwig++ gives more stable, final-state leptons than fHerwig Requiring Ks,lambda to be stable, does reduce number of low-PT leptons, but effect is not significant enough to explain differences [situation same for muons] HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

19. SU3 Plots: Final State Leptons final-state electrons from HP Test: Look at final-state leptons from the hard subprocess BUT: - exclude those from taus (differences between Herwig++ and TAUOLA?) PT>20 GeV More electrons still seen in Herwig++ [situation same for muons] HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

20. SU3 Plots: Final State Leptons final-state electrons from HP Test: Look at final-state leptons from the hard subprocess BUT: - exclude those from c decays PT>20 GeV More electrons still seen in Herwig++ [situation same for muons] HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan

21. SU3 Plots: Final State Leptons final-state electrons from HP Test: Look at final-state leptons from the hard subprocess BUT: - exclude those from b decays Much closer agreement between fHerwig and Herwig++ [situation same for muons] PT>20 GeV HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan