1. Ursula Bassler, LPNHE-Paris, RUN II MC workshop 1 Monte Carlo Tuning: The HERA Experience Monte Carlo Models for DIS events Description of inclusive hadronic final state Parameter tuning for Ariadne, Herwig, Lepto Jets at high Q 2 and small x

2. 2 UB RunII MC Modeling ep interactions proton structure: pdf hard interaction: LO ME calculation at O(  S ) QCD radiation: Parton Shower Models, Color Dipole Model hadronisation: String or Cluster fragmentation

3. 3 UB RunII MC Parton Density Functions strong constraints from structure function measurements pdf’s determined with global fit programs: MRST, CTEQ  hadronic final state is a good probe for QCD models independent of pdf’s.

4. 4 UB RunII MC MC Models at HERA: MC Models used for DIS: Lepto, Ariadne, Herwig, Rapgap MC Models used for  p: Pythia, Phojet MC Models at Small x: LDCMC, Smallx, Cascade MC Models for diffraction: Rapgap, Lepto SCI, Ridi, DiffVM

5. 5 UB RunII MC Where it started from…92 first hadronic final state measurements with L int= 1.6 nb -1 transverse energy flow in the laboratory frame w.r.t.  and  e comparison to various models: Leading Log Parton Showers with max. virtuality scale Q 2 (LEP) or W 2 (Lepto 5. 2) O(  s ) matrix element and parton shower (Lepto 6.1) Color Dipole Model (Ariadne 4.03)!

6. 6 UB RunII MC Where it got to…. transverse energy flow from 1994 data L=2.7pb -1 3.2 < Q 2 < 2200 GeV 2 8·10 -5 < x < 0.11  increased precision requires improved understanding of Monte Carlo Models  fine tuning of MC parameters possible and necessary

7. 7 UB RunII MC

8. 8 UB RunII MC Inclusive hadronic final state G. Grindhammer et al: Comparison of energy flow and particle spectra in the hadronic CMS Lorentz transformation from lab frame Ariadne, Lepto, Rapgap and Herwig compared for various parameter sets ** p

9. 9 UB RunII MC Lepto 6.5 ME calculation reproduce cross-sections QCD cascade: – DGLAP based leading-log parton showers – strong ordering of gluons in k t fragmentation: – JETSET - string model parameters: – “Soft Color Interaction” between partons from hard interaction and proton remnant – “Generalized Area Law”: allows interactions between color string pieces

10. 10 UB RunII MC Rapgap 2.06/48 originally developed for description of diffractive events takes into account direct and resolved virtual photon contributions QCD cascade/fragmentation: – similar to Lepto parameters: – resolved process scale  = p t(jet) 2 +Q 2 – matrix element cut-off: PT2CUT=4 GeV 2

11. 11 UB RunII MC Herwig 5.9 QCD cascade: – coherent parton cascade with LO ME corrections – LO shower, but NLO  S running fragmentation: – cluster fragmentation parameters: – strongly constraint from e + e - data – CLMAX: maximum cluster mass – PSPLT: cluster splitting

12. 12 UB RunII MC Ariadne 4.10 QCD cascade: based on the color dipole model –gluon emission from independently radiating dipoles –no ordering of gluons in k T, BFKL emulation –gluon emission corrected to reproduce ME O(  s ) fragmentation: JETSET parameters: –PARA(10): suppression of soft gluon emission for proton remnant –PARA(15): for the struck quark –PARA(25): gluon emission outside suppression cut

13. 13 UB RunII MC Transverse Energy Flow Q 2 = 3.2 GeV 2 14.1 GeV 2 175 GeV 2 2200 GeV 2 x= 0.8 10 -4 0.63 10 -3 0.4 10 -2 0.11 A: 99/1 p(10) 1.6 p(15) 0.5 p(25) 1.4 99/2 p(10) 1.2 p(15) 1.0 p(25) 1.0 sgsr sgsc prob H: LO: CLMAX 3.35 PSPLT 1.0 96: CLMAX 5.5 PSPLT 0.65 99/1: CLMAX 3.0 PSPLT 1.2 99/2: CLMAX 5.0 PSPLT 1.0 peaking E T in “current jet” region with rising Q 2 plateau behavior at low Q 2 proton remnant G. Grindhammer et al. Data: H1 Eur.Phys.J C12 (2000)

14. 14 UB RunII MC Charged particle multiplicity Data: H1 Nucl.Phys.B 485 (1997) proton remnant G. Grindhammer et al. Q 2 = 7 GeV 2 14 GeV 2 32 GeV 2 x= 1.6 10 -4 0.64 10 -3 2.1 10 -3 reasonable descriptions can be found for all models Herwig shows large variations depending on input parametrs

15. 15 UB RunII MC Charged particles multiplicities p* t > 1 GeV only Ariadne and the high CLMAX parameter sets of Herwig give a good description G. Grindhammer et al. proton remnant Q 2 = 7 GeV 2 14 GeV 2 32 GeV 2 x= 1.6 10 -4 0.64 10 -3 2.1 10 -3 Data: H1 Nucl.Phys.B 485 (1997)

16. 16 UB RunII MC P t spectrum Q 2 = 7 GeV 2 14 GeV 2 32 GeV 2 x= 1.6 10 -4 0.64 10 -3 2.1 10 -3 0.5 <  * < 1.5 difficulties at high p t for low Q 2 only Ariadne describes the full phase space G. Grindhammer et al. Data: H1 Eur.Phys.J C12 (2000)

17. 17 UB RunII MC MC parameter tuning N.H Brook et al.: tuning on hadronic final state variables in various Q 2 regions: x P in current region of the Breit frame E T flow in hadronic center of mass system event shape variables: thrust T C and T Z, jet broadening Bc, jet mass  C fragmentation function differential and integrated jet shapes di-jet production cross-sections charged particle distributions  compute combined  2 for all variables  difficulties in describing simultaneously jets and charged particle distributions

18. 18 UB RunII MC Ariadne: suppression of soft gluon emission for proton remnant P(10) NH. Brook et al. sensitive to di-jet cross-section default parameter: Et spectra too hard at low Q 2 increasing P(10): -suppression of E T over whole  range -effect at low and high E T

19. 19 UB RunII MC Ariadne: gluon emission outside suppression cut-off P(25) N.H. Brook et al. decreasing P(25): -larger changes at high E T -effect larger in fwd region less sensitive to E T flow default tuned P(10) 1.0 1.6 P(15) 1.0 0.5 P(25) 2.0 1.4

20. 20 UB RunII MC Herwig: fragmentation parameters LO  s improves agreement PSPLT: increases E T flow CLMAX: broader jets harder momentum spectra for charger particles no parameter set has been found describing all aspects of DIS data

21. 21 UB RunII MC Lepto: improved SCI = 1/2(1-cos  * ) modified SCI (Lepto 6.5.2  )  suppressing SCI at high Q 2 improved  2 by a factor ~5 further improvement on (2+1) jet data varying PARL(8)=z p min PARL(9)=ŝ min But: other hadronic final state variables better described by default setting

22. 22 UB RunII MC Jets at high Q 2 modified Durham algorithm 640 < Q 2 < 35000 GeV 2 MC models used with optimized parameters z p, x p distributions most sensitive to differences in the models best description of data by Ariadne

23. 23 UB RunII MC Jets in Charged Current Events event selection in same kinematic region, but smaller cross-section similar behavior of jets than in Neutral Current stronger deviations seen for LEPTO w.r.t to data and other models

24. 24 UB RunII MC Parton Cascades at small x DGLAP: resummation of lnQ 2  strong ordering in k T BFKL: resummation of ln 1/x  no ordering in k T CCFM: color coherence  strong angular ordering  additional transverse energy in forward direction produced for BFKL and CCFM approach BFKL/CCFM in MC models: Ariadne, LDCMC, Smallx,Cascade

25. 25 UB RunII MC Forward Jets at small x rise of jet cross- section with decreasing x, underestimated by MC Models Lepto/Herwig and LDCMC predict smaller cross-sections Ariadne and Rapgap show reasonable agreement

26. 26 UB RunII MC CCFM evolution - Cascade H.Jung, G.P Salam CCFM equation implemented in backward evolution schema forward jets: -good description for H1 cross-section -above ZEUS measurement

27. 27 UB RunII MC Conclusions MC tuning at HERA not yet to the precision of LEP, but –hadronic environment probed with a lepton –ongoing progress in understanding of various aspects in hadronic final state –further high precision measurements ARIADNE gives overall a good picture of DIS events useful experience for hadron colliders?!