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

2. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 Intro/Aims 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 …) 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++? 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? Validation/comparison process now started:- first comparisons of fHerwig vs. Herwig++ for SUSY signal process presented here …

3. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 SUSY Comparison Process Chose ATLAS SU3 benchmark SUSY point for first comparisons : [m 0 =100GeV,m 1/2 =300GeV,A 0 =-300GeV,tan(Beta)=6,sign(mu)=+1] 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 Samples: Still to add details here about our Herwig++ JO

4. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 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/ ) SLHA Spectrum/Decay File Resulting spectrum+decay file - ISAJETSU3.spc - available in cvs: http://atlas-sw.cern.ch/cgi-bin/viewcvs-atlas.cgi/offline/Generators/Herwigpp_i http://atlas-sw.cern.ch/cgi-bin/viewcvs-atlas.cgi/offline/Generators/Herwigpp_i Herwig++ needs sparticle spectrum/decay file in SUSY les Houches Accord (SLHA) format [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))

5. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 Differences in defaults: fHerwig: two-loop alphas, scale = 2s’t’u’/(s’ 2 +t’ 2 +u’ 2 ) [s’=s-m 2 LSP,…] Herwig++: LO alphas, scale (for PDFs, alphas) = s ^ ^ SU3: Hard Process Cross Section SU3 Cross Section (pb) fHerwig18.6 Herwig++18.0 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) NOTE: Peter Richardson has sent us a “fix”, such that a scale of m T 2 =2stu/(s 2 +t 2 +u 2 ) 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 ^ ^^^ ^ ^ ^

6. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 SU3 Plots: Truth Jets (CN4) P T >20 GeV First Look: Herwig++ has more truth jets than fHerwig (– mainly concentrated at lowish-p T and central eta) BUT, from discussions with Peter, found out that… ->

7. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 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 NOTE: for “best physics”, SUSY showering should be switched ON … Herwig++ includes a treatment of SUSY showering (fHerwig has no such treatment): for comparison with fHerwig, switch it OFF (controlled in MSSM.model) SU3 Plots: Truth Jets (CN4) P T >20 GeV -> distributions become more similar

8. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 SU3 Plots: Final State Leptons First Look: Herwig++ gives more stable, final-state leptons than fHerwig (– mainly concentrated at lowish-p T ) [situation same for muons] (see backups for more plots) final-state electrons P T >20 GeV

9. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 SU3 Plots: dilepton masses P T >20 GeV 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!! ]

10. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 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”) However, this did not have significant impact on the lepton distributions (some visible impact was observed for leptons with p T <20GeV (see backups), but it’s insufficient to explain differences)

11. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 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?) P T >20 GeV Suggests differences due to b-quark hadronisation? - this is under investigation… -> good agreement found when b-decays excluded [effect of c- and tau-decays not significant (- see backups)]

12. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 SU3 Plots: Final State Photons P T >20 GeV Herwig++ also gives more stable final-state photons than fHerwig, and with a softer p T -spectrum

13. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 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

14. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 Summary http://atlas-sw.cern.ch/cgi-bin/viewcvs-atlas.cgi/offline/Generators/Herwigpp_i/share  Can we now easily use Herwig++ in Athena for SUSY production? Example jobOptions (jobOptions.herwigpp-SusySU3.py) are in cvs: [along with files ISAJETSU3.spc, MSSM.model - which are also needed]: 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 Yes!!  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.)

15. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 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? Herwig++ distributions all look reasonable, but some features wrt fHerwig still need to be fully understood (eg. - higher number of low-p T leptons,…) -> we are working (alongside the authors) on understanding these things Getting there!! (Also happy to take suggestions from the floor!!!)

16. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 Backups

17. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 SU3 Plots: Final State Leptons final-state muons FIRST LOOK: Herwig++ gives more stable, final-state leptons than fHerwig (– mainly concentrated at lowish-p T ) P T >20 GeV

18. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 Requiring Ks,lambda to be stable, does reduce number of low-P T leptons, but effect is not significant enough to explain differences SU3 Plots: Final State Leptons final-state electrons (no pT-cut) Herwig++ gives more stable, final-state leptons than fHerwig [situation same for muons]

19. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 SU3 Plots: Final State Leptons Test: Look at final-state leptons from the hard subprocess BUT: - exclude those from taus (differences between Herwig++ and TAUOLA?) P T >20 GeV More electrons still seen in Herwig++ [situation same for muons] final-state electrons from HP

20. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 SU3 Plots: Final State Leptons Test: Look at final-state leptons from the hard subprocess BUT: - exclude those from c decays P T >20 GeV final-state electrons from HP More electrons still seen in Herwig++ [situation same for muons]

21. HERWIG vs. Herwig++: SUSY comparison  Claire Gwenlan26 August 08 SU3 Plots: Final State Leptons Test: Look at final-state leptons from the hard subprocess BUT: - exclude those from b decays P T >20 GeV final-state electrons from HP Much closer agreement between fHerwig and Herwig++ [situation same for muons]