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 Q2 and small x

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 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 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 UB RunII MC

Where it started from…92• first hadronic final state

measurements with 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 Q2 (LEP) or W2 (Lepto 5. 2)

• (s) matrix element and parton shower (Lepto 6.1)

• Color Dipole Model (Ariadne 4.03)!

6 UB RunII MC

Where it got to….

• transverse energy flow from 1994 data =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 UB RunII MC

Monte Carlo Generators for HERA Physics

Proceedings of the workshop 1998/99Editors: A.T. Doyle, G. Grindhammer, G. Ingelman, H. Jung

Email: heramc@desy.de

•Foreword

•Contents

•Working Group Reports:•QCD cascades•Fixed higher order programs for DIS and photoproduction•Transition from photoproduction to DIS•Non-perturbative QCD•Diffraction•Heavy flavours•QED radiative effects•Exotics

The printed version of the proceedings is available as DESY-PROC-1999-02. Copies can be picked up in the DESY library or ordered from Maren Högemeier. © DESY 1999, all rights reserved.

Additional reports are still welcome to be included in the electronic version of the proceedings. Instructions for writing/submitting contributions and for conveners.

http://www.desy.de/~heramc/proceedings last update 25-10-1999

[DESY LOGO]

8 UB RunII MC

Inclusive hadronic final stateG. 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 UB RunII MC

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

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 = pt(jet)

2+Q2

– matrix element cut-off: PT2CUT=4 GeV2

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 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 kT, 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 UB RunII MC

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

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 ET in “current jet” region with rising Q2

•plateau behavior at low Q2proton

remnant

G. G

rind

ham

mer

et

al.

Data: H1 Eur.Phys.J C12 (2000)

14 UB RunII MC

Charged particle multiplicity

Data: H1 Nucl.Phys.B 485 (1997)

proton remnant

G. G

rind

ham

mer

et

al.

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

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. G

rind

ham

mer

et

al.

proton remnant

Q2 = 7 GeV2 14 GeV2 32 GeV2

x= 1.6 10-4 0.64 10-3 2.1 10-3

Data: H1 Nucl.Phys.B 485 (1997)

16 UB RunII MC

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. G

rind

ham

mer

et a

l.

Data: H1 Eur.Phys.J C12 (2000)

17 UB RunII MC

MC parameter tuningN.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

18 UB RunII MC

Ariadne: suppression of soft gluon emission for proton remnant P(10)

NH

. Br o

ok e

t al.

• 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

19 UB RunII MC

Ariadne: gluon emission outside suppression cut-off P(25)

N. H

. Bro

ok

e t a

l .

•decreasing P(25): -larger changes at high ET

-effect larger in fwd region

• less sensitive to ET flow

default tuned

P(10) 1.0 1.6P(15) 1.0 0.5

P(25) 2.0 1.4

20 UB RunII MC

Herwig: fragmentation parameters• LO s improves agreement

PSPLT: increases ET flowCLMAX: •broader jets•harder momentum spectra for charger particles

•no parameter set has been found describing all aspects of DIS data

21 UB RunII MC

Lepto: improved SCI

= 1/2(1-cos*)

• 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)=zp

min

PARL(9)=ŝmin

• But: other hadronic final state variables better described by default setting

22 UB RunII MC

Jets at high Q2

212

2

2

mQ

Q

modified Durham algorithm

• 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

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 UB RunII MC

Parton Cascades at small xDGLAP: 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

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 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 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?!