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Colour Reconnection in WW events

Jorgen D’Hondt

University of Brussels - DELPHI Collaboration

- WWqqQQ events collected at LEP2
- Colour Reconnection effect at LEP2
- Model dependent measurement(s)
- Preliminary conclusions

XXXII International Symposium on Multiparticle Dynamics

1/12

Known example :

hadronic decay of B mesons

B J/Y + X

BR.exp ~1%

BR.fullCR ~3-5%

BR.noCR ~0.3-0.5%

G.Gustafson, U.Petterson and P.Zerwas, Phys.Lett. B209 (1988) 90

4 jet topology

e+

e-

DELPHI detector at LEP2

Main interest :

- Probe the interaction of two fragmenting strings
- Causes largest systematic error on W-mass

2/12

- W decay vert. ~0.1 fm distance
- hadronisation scale ~1 fm

large space-time overlap

- suppressed in parton shower :
- due to group structure of QCD at least 2 gluons must be emitted
- W width restrict the energy range of primary gluons from q1Q4 and q2Q3

~ (as2/MW)GW/Nc2

- CR effect could occur in the confined region ???

as<<1

as1

T.Sjöstrand and V.A.Khoze, Z.Phys. C62 (1994)

Models to emulate the CR effect

3/12

CR implemented in existing fragmentation models :

- PYTHIA : string reconfiguration if they overlap or cross in space-time
- SK1(lateral flux tube) : via event string overlap O : pCR = 1-exp(-·O)
- SK2 (vortex line with core) : reconnection if cores cross
- SK2’ : SK2 + only if string length is reduced
- ARIADNE : rearrangement of colour dipoles to reduce the string length (mass)
- AR2 : only after soft gluon radiation (Eg < GW)
- AR3 : allowed everywhere (also in pertubative phase)
- HERWIG : rearrangement of colour dipoles changing the size of the clusters

SK1 mW shift

- Latest preliminary predictions for the W-mass
- PYTHIA (SK1) ~ 50 MeV/c2
- ARIADNE (AR2) ~ 70 MeV /c2
- HERWIG ~ 40 MeV/c2
- Statistical uncertainty (LEP2) ~ 30 MeV /c2

K ~ 0.66

For each model one can study the effect on the structure of the WW events with Monte Carlo simulation and compare it with data.

PCR

4/12

Most sensitive observables

design an observable sensitive to for example k (SK1)

New!

Particle flow

W-mass measurements

Design an observable to measure a possible

enhancement of particles in the inter-W

regions…

CR is an important systematic uncertainty

on mW measured in WW qqqq events,

but we designed another mW estimator

which does not have this feature...

simply count particles

(background sensitive)

jet kinematics of the event

(background insensitive)

sensitivity ~ 2.7s

(for full SK1)

sensitivity ~ 4.3s

(for full SK1)

Eff. ~ 12%, Pur. ~ 87%

Eff. ~ 90%, Pur. ~ 71%

The correlation between both observables was found to be negligibly small

...all results are preliminary

5/12

# WW 4q selected (mW observable)

WW event

up to 208.8 GeV

‘98

‘99

‘00

‘97

‘96

indirect

design limit

Up to 5000 WWqqQQ events expected by DELPHI

background

sWW 18 pb

W-mass : # ~ 6000

(used data ‘98-’00)

background

Particle flow : # ~ 720

(used data ‘97-’00)

6/12

Influence on W mass estimator (MeV/c2) for different values of k(SK1)

DELPHI preliminary

mW is assumed to be unknown but it must be invariant for different estimators

mW(std)-mW(cone)

from MC as function of k

MC prediction

Correlation between W mass estimators ~ 83 %

uncertainty on the difference is small

7/12

2

MC prediction SK1

2 sigma

1 sigma

centr.

k

MeV/c2

first time this is done

with LEP data !!

Monte Carlo prediction if there is Colour Reconnection

different behaviour

8/12

Project all charged particles Pi

in the plane ( jet 1, jet 2)

Inside W andBetween W’s

D

Rescale the angles to have an

equal amount of phase-space

between the jets :

FJ,iFJ,i . (1/FJK)

C

A

B

(

)

data at 189 GeV

#( A+C )

#( A+C )

1

d

Z qq(g)

dF

Nev

#( B+D )

#( B+D )

WW qqln

no CR

A

B

C

D

SK1 100%

SK1 100%

0.2

0.8

9/12

In the systematic error, were considered:

- Bose-Einstein effects (2%)
- Fragmentation modelling (1%)
- Background subtraction/modelling (0.5%)
- Generators/Tunings (0.3%)

Data @183-209 :

<R189> = 0.900 0.031 0.021

( extrapolated to 189 GeV )

R

Monte Carlo prediction :

10/12

no correlation assumed

between the two

measurements

(good approximation)

2

68% CL for k

[0.3 , 3.3]

central value

1.3

68% CL for PCR

[ 14% , 75% ]

central value

44%

11/12

- Colour reconnection models are investigated in WWqqQQ events
- Two uncorrelated observables are designed :
- Integrating the particle flow between jets ( R )
- Using the kinematics of the jets ( DmW[std,cone] )
- Preliminary results prefer a small amount of Colour Reconnection

(also the LEP combined measurement, cfr. talk of Nigel Watson)

- The observables are not sensitive to the ARIADNE model !?

Therefore one cannot decrease

the systematic uncertainty on mW

- Preliminary shift on the W-mass
- PYTHIA (SK1) ~ 50 MeV/c2
- ARIADNE (AR2) ~ 70 MeV/c2
- HERWIG ~ 40 MeV/c2
- Statistical uncertainty (LEP2) ~ 30 MeV/c2

More work needed onARIADNE!!

12/12

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