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Monte Carlo Tuning: The HERA Experience

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. Modeling ep interactions. proton structure: pdf

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Monte Carlo Tuning: The HERA Experience

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  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 Q2 and small x

  2. 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. 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. 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. Where it started from…92 • first hadronic final state measurements with Lint= 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 Q2 (LEP) or W2 (Lepto 5. 2) • O(s) matrix element and parton shower (Lepto 6.1) • Color Dipole Model (Ariadne 4.03)!

  6. Where it got to…. • transverse energy flow from 1994 data L=2.7pb-1 • 3.2 < Q2 < 2200 GeV2 8·10-5 < x < 0.11 • increased precision requires improved understanding of Monte Carlo Models • fine tuning of MC parameters possible and necessary

  7. 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

  8. Lepto 6.5 • ME calculation reproduce cross-sections • QCD cascade: • DGLAP based leading-log parton showers • strong ordering of gluons in kt • 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

  9. 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  = pt(jet)2+Q2 • matrix element cut-off: PT2CUT=4 GeV2

  10. 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

  11. Ariadne 4.10 • QCD cascade: based on the color dipole model • gluon emission from independently radiating dipoles • no ordering of gluons in kT, BFKL emulation • gluon emission corrected to reproduceME 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

  12. proton remnant Transverse Energy Flow Q2 = 3.2 GeV2 14.1 GeV2 175 GeV2 2200 GeV2 x= 0.8 10-4 0.63 10-3 0.4 10-2 0.11 • peaking ET in “current jet” region with rising Q2 • plateau behavior at low Q2 A: 99/1p(10) 1.6 p(15) 0.5 p(25) 1.4 99/2p(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 G. Grindhammer et al. Data: H1 Eur.Phys.J C12 (2000)

  13. proton remnant Charged particle multiplicity Q2 = 7 GeV2 14 GeV2 32 GeV2 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 G. Grindhammer et al. Data: H1 Nucl.Phys.B 485 (1997)

  14. proton remnant Charged particles multiplicities Q2 = 7 GeV2 14 GeV2 32 GeV2 x= 1.6 10-4 0.64 10-3 2.1 10-3 • p*t > 1 GeV • only Ariadne and the high CLMAX parameter sets of Herwig give a good description G. Grindhammer et al. Data: H1 Nucl.Phys.B 485 (1997)

  15. Pt spectrum Q2 = 7 GeV2 14 GeV2 32 GeV2 x= 1.6 10-4 0.64 10-3 2.1 10-3 • 0.5 < * < 1.5 • difficulties at high pt for low Q2 • only Ariadne describes the full phase space G. Grindhammer et al. Data: H1 Eur.Phys.J C12 (2000)

  16. MC parameter tuning • N.H Brook et al.: • tuning on hadronic final state variables in various Q2 regions: • xP in current region of the Breit frame • ET flow in hadronic center of mass system • event shape variables: thrust TC and TZ, 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

  17. Ariadne: suppression of soft gluon emission for proton remnantP(10) • sensitive to di-jet cross-section • default parameter:Et spectra too hard at low Q2 • increasing P(10): • suppression of ET over whole  range • effect at low and high ET NH. Brook et al.

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

  19. Herwig: fragmentation parameters • LO s improves agreement • PSPLT: • increases ET flow • CLMAX: • broader jets • harder momentum spectra for charger particles • no parameter set has been found describing all aspects of DIS data

  20. Lepto: improved SCI • modified SCI (Lepto 6.5.2)  suppressing SCI at high Q2 • improved 2 by a factor ~5 • further improvement on (2+1) jet data varying PARL(8)=zpmin PARL(9)=ŝmin • But: other hadronic final state variables better described by default setting = 1/2(1-cos*)

  21. Jets at high Q2 • 640 < Q2 < 35000 GeV2 • MC models used with optimized parameters • zp, xp distributions most sensitive to differences in the models • best description of data by Ariadne modified Durham algorithm

  22. 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

  23. Parton Cascades at small x • DGLAP: • resummation of lnQ2 strong ordering in kT • BFKL: • resummation of ln 1/x no ordering in kT • 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

  24. 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

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

  26. 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?!

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