Jorgen D’HondtJorgen D’HondtUniversity of Brussels - DELPHI University of Brussels - DELPHI

CollaborationCollaboration

WWWWqqQQqqQQ events collected at LEP2 events collected at LEP2 Colour Reconnection effect at LEP2Colour Reconnection effect at LEP2 Model dependent measurement(s)Model dependent measurement(s) Preliminary conclusionsPreliminary conclusions

XXXII International Symposium on Multiparticle Dynamics

1/12

Known example :

hadronic decay of B mesons

B B J/ J/+ X+ X

Main interest : • Probe the interaction of two fragmenting strings • Causes largest systematic error on W-mass

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

e- e+

2/12

DELPHI detector at LEP2DELPHI detector at LEP2

4 jet topology4 jet topology

BR.exp ~1%BR.fullCR ~3-5%BR.noCR ~0.3-0.5%

W decay vert. ~ 0.10.1 fm distance hadronisation scale ~ 11 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

~ (s2/MW) W/Nc

2

CR effect could occur in the confined region ???

s<<1 s1

Models to emulate the CR effect

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

3/12

PYTHIA : string reconfiguration if they overlap or cross in space-time

– SK1SK1 (lateral flux tube) : via event string overlap O : ppCRCR = 1-exp(- = 1-exp(-·O)·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 < W)

– AR3 : allowed everywhere (also in pertubative phase) HERWIG : rearrangement of colour dipoles changing the size of the clusters

CR implemented in existing fragmentation models :

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

with data.

SK1 mSK1 mW W shiftshift

PPCRCR

KK ~ 0.66 ~ 0.66

Latest preliminary predictions for the W-mass• PYTHIA (SK1) ~ 5050 MeV/c2

• ARIADNE (AR2) ~ 7070 MeV /c2

• HERWIG ~ 4040 MeV/c2

Statistical uncertainty (LEP2) ~ 3030 MeV /c2

4/12

Particle flowParticle flow W-mass measurementsW-mass measurementsCR is an important systematic uncertainty on mW measured in WW qqqq events, but we designed another mW estimator which does not have this feature...

Design an observable to measure a possibleenhancement of particles in the inter-Wregions…

New!New!

design an observable sensitive to for example (SK1)

CR

The correlation between both observables was found to be negligibly small

simply count particlessimply count particles(background sensitive)(background sensitive)

jet kinematics of the eventjet kinematics of the event(background insensitive) (background insensitive)

Eff. ~ 12%, Pur. ~ 87%Eff. ~ 12%, Pur. ~ 87% Eff. ~ 90%, Pur. ~ 71%Eff. ~ 90%, Pur. ~ 71%

...all results are preliminary

sensitivity ~ 2.7 (for full SK1)

sensitivity ~ 4.3 (for full SK1)

5/12

# WW 4q selected (mW observable)

‘00

‘99‘98

‘97‘96

design limit

WW 18 pb

indirect

Up to Up to 50005000 WW WWqqQQ events qqQQ events expected by DELPHIexpected by DELPHI

background

background

WW event up to 208.8 GeV

W-mass : # ~ 60006000(used data ‘98-’00)

Particle flow : # ~ 720720(used data ‘97-’00) 6/12

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

estimators

mmWW(std)-m(std)-mWW(cone)(cone)

from MC as function of

Correlation between W mass estimators ~ 83 %

uncertainty on the difference is small

Influence on W mass estimator (MeV/c2) for different values of (SK1SK1)

MC predictionMC prediction

DELPHI preliminary

7/12

Data @189-209 : Data @189-209 :

MeV/c2

Monte Carlo prediction if there is Colour Reconnection

centr.

1 sigma

2 sigma

different behaviourdifferent behaviour

MC prediction SK1MC prediction SK1

first time this is donefirst time this is donewith LEP data !!with LEP data !!

2

8/12

Project all charged particles PPii in the plane ( jet 1, jet 2)

AC

D

B

Rescale the angles to have an

equal amount of phase-space

between the jets :

J,i J,i . (1/JK)

A B C D

Inside W Inside W and Between W’sBetween W’s

Z qq()

WW qql

data at 189 GeV #( A+C )

#( B+D )( ) dd1

Nev

#( A+C )

#( B+D )

SK1 100%SK1 100%

no CRno CR

SK1 100%SK1 100%

0.2 0.8

9/12

8.0

2.0

8.0

2.0

DB regions1

CA regions1

dddn

N

dddn

NR

ev

ev

In the systematic error, were considered: Bose-Einstein effects (2%) Fragmentation modelling (1%) Background subtraction/modelling (0.5%) Generators/Tunings (0.3%)

<R189> = 0.900 0.031 0.021

Data @183-209 : Data @183-209 :

RMonte Carlo prediction :

( extrapolated to 189 GeV )

10/12

no correlation assumedno correlation assumedbetween the twobetween the twomeasurements measurements

(good approximation)(good approximation)

68% CL for [[0.3 , 3.30.3 , 3.3]]

central value1.31.3

2

68% CL for

PPCRCR

[ [ 14% , 75%14% , 75% ] ]

central value44%44%

11/12

12/12

Colour reconnection models are investigated in WWWWqqQQqqQQ events

Two uncorrelated observables are designed : Integrating the particle flow between jets ( RR ) Using the kinematics of the jets ( mmWW[std,cone][std,cone] )

Preliminary results prefer a small amount of Colour Reconnectionsmall amount of Colour Reconnection (also the LEP combined measurement, cfr. talk of Nigel Watson)

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

Preliminary shift on the W-mass• PYTHIA (SK1) ~ 5050 MeV/c2

• ARIADNE (AR2) ~ 7070 MeV/c2

• HERWIG ~ 4040 MeV/c2

Statistical uncertainty (LEP2) ~ 3030 MeV/c2

Therefore one cannot decreasethe systematic uncertainty on mW

More work needed onMore work needed on ARIADNEARIADNE !!!!