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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

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

slide2

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
parton density functions
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.
slide4

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
where it started from 92
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)!
where it got to
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
inclusive hadronic final state
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

slide9

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
rapgap 2 06 48
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
slide11

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
slide12

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
slide13

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)

charged particle multiplicity

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)

slide15

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)

slide16

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)

slide17

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

slide18

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.

slide19

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.

herwig fragmentation parameters
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
lepto improved sci
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*)

jets at high q 2
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

jets in charged current events
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
slide24

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
forward jets at small x
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
ccfm evolution cascade
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

conclusions
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?!