Colour Reconnection in WW events
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Colour Reconnection in WW events. Jorgen D’Hondt University of Brussels - DELPHI Collaboration. WW qqQQ events collected at LEP2 Colour Reconnection effect at LEP2 Model dependent measurement(s) Preliminary conclusions. XXXII International Symposium on Multiparticle Dynamics. 1/12.

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Colour Reconnection in WW events

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Colour reconnection in ww events

Colour Reconnection in WW events

Jorgen D’Hondt

University of Brussels - DELPHI Collaboration

  • WWqqQQ events collected at LEP2

  • Colour Reconnection effect at LEP2

  • Model dependent measurement(s)

  • Preliminary conclusions

XXXII International Symposium on Multiparticle Dynamics

1/12


Colour reconnection in ww events

Colour Reconnection

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


Colour reconnection in ww events

Colour Reconnection in WW

  • 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

as1

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

Models to emulate the CR effect

3/12


Colour reconnection in ww events

Simulating the CR effect

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


Colour reconnection in ww events

CR

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


Colour reconnection in ww events

Achievements of LEP2

# WW  4q selected (mW observable)

WW event

up to 208.8 GeV

‘98

‘99

‘00

‘97

‘96

indirect

design limit

Up to 5000 WWqqQQ 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


Colour reconnection in ww events

W-mass observables

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


Colour reconnection in ww events

Data @189-209 :

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


Colour reconnection in ww events

Particle flow observable

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,iFJ,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


Colour reconnection in ww events

Preliminary results

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


Colour reconnection in ww events

Combination SK1

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 in ww events

Conclusion

  • Colour reconnection models are investigated in WWqqQQ 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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