Motivation Color reconnection Standard variables

Motivation

Color reconnection

Standard variables

Particle production between jets,

Particle flow distributions

Summary

Color Reconnectionin Hadronic WW Events at

LEP

Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 1


Motivation

MW( ) = 80,442 ± 0,034stat ± 0,028syst GeV/c2

MW( ) = 80,460 ± 0,031stat ± 0,054syst GeV/c2

qqWW

qqqqWW

± 40 MeV/c2 uncertainty fromcolour reconnection in hadronic channel

2W cMeV4618qqqqqqM /)(

Interesting physics where we might learn something about the QCD vacuum


30°

primary quarks independent decay

perturbative crc non-perturbative crc


Dynamical rearrangement of color charges as distance of

decay vertices of W 0,1 fm

perturbative QCD*

Description of color reconnection before parton shower

2'

max'

210

2/

O

mN

kN

NN

N

Wq

CRCq

C

WsnoCRC

CRC

32

2

10

W

W

C

WSFCRC

mN

C

* T. Sjöstrand, V.A. Khoze, Phys. Rev. Lett. 72 (1994) 28.


Time scale of fragmentation corresponds 1fm !!!

Processes in partonshower and hadronisation might be important!!!

Simulation with phenomenological models!!!

These depend on the description of the QCD-vacuums!!!


phenomenological models:

models of Sjöstrand/Khoze (skI/skII/skII‘):framework of string fragmentation(strings overlap/cross, implemented in JETSET)

model of Gustafson/Häkkinen (GH):framework of string fragmentation(minimize total string length, implemented in JETSET)

models of Lönnblad (ar2/ar3):framework of dipole cascade model(minimize string length, implemented in ARIADNE)

model of Webber et.al. (hcr):framework of cluster fragmentation(minimize cluster s-t-distance, implemented in HERWIG)

new model of Rathsman:based on area law between string pieces, will be implemented


Investigate some standard variables

momentum distribution: )/2ln(ln spxp

Similiar analyses of all four experiments, no evidence for crc


The idea of the particle flow

Color reconnection leads to

rearrangements of particle production

Careful ! very simple picture !


The event selection

100o - 140o

100o - 140o< 100o

< 100o

A

B

D

C

•preselection based on energymeasured in calorimeters

•cuts on ISR events•jet clustering

•Low efficiency (> 15 %)•very clean topology


Particle production between jets

1 plane / 4 planes


Consider the ratio:

)(//1

)(//1

DCddnNBAddnN

Revt

evt

RdDCddnNBAddnN

Sevt

evtr

)(//1

)(//1

RdDCddnN

RdBAddnNR

evt

evt

n

)(//1

)(//1

and the two observables:

L3


ALEPH: Rn189GeV(0.2-0.8) = 0.820 0.038 0.026 =2.8

L3: Rn189GeV(0.3-0.7) = 0.771 0.049 0.029 =3.3

OPAL*: 1/ Rn189GeV(0.2-0.8) = 0.83 0.03 0.01 =3.1

*using charged tracks only

data

nn 0skIR100skIR

%)(%)(sensitivity

Slightly different selections, object definition and reconstruction, 1/4 planes, nevertheless the methodshows high sensitivity in framework of sk1 model (0-100%)(similiar numbers for Sr)


comparison of differentcolor reconnection models

at 189 GeV


HERWIG systematically higher than JETSET

large systematic uncertaintyfor Rn/Sr

further work on observablesto make them more robust against fragmentation effects


Summary

Low sensitivity of standard variables for color reconnection / no evidence is found

particle flow analysis is very promising with high sensitivity for effects in sk1 model,all experiments work on this method

most important uncertainty comes from fragmentation systematics, but still room for improvements in observables


Results are not yet conclusive butcombination of energies and experimentswill come soon to extract best description

ofthe hadronic final states in WW events

Motivation Color reconnection Standard variables

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