Colour Reconnection and Bose-Einstein Correlations in WW Decays Nuno Anjos LIP, Lisbon DELPHI Collaboration DIS 04, Štrbské Pleso, Slovakia, April 16 th,

Colour Reconnection and

Bose-Einstein Correlationsin WW Decays

Nuno Anjos

LIP, Lisbon

DELPHI Collaboration

DIS 04, Štrbské Pleso, Slovakia, April 16th, 2004

Cover

DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004

2/20Nuno Anjos

Outline

o Cross Talk in WW 4 jets

o DELPHI at LEP2

o Colour Reconnection (CR)

o Particle Flow

o The mW Method

o Bose-Einstein Correlations (BEC)

o Conclusions


3/20Nuno Anjos

CR possible betweensoft particles

Cross Talk in WW 4 jets

++

00

Interference is possible!

Electroweak phase

Is there any interference between the two evolving hadronic systems from the decays of the W’s?

1s

Perturbative QCD

CR supressed

1s

Fragmentation

~ 0.1 fm < ~ 1 fm < ~ 10-100 fm

e+ W+

W-e-

Stable Hadrons BEC possible

CR and BEC

o Fundamental probes of the dynamics of hadronisation o Contribute to the W mass systematic error in the fully hadronic channel

Colour

Reconnection90 GeV/c2

Bose-Einstein Corr.

35 GeV/c2

Total Syst. 101 GeV/c2

Stat. 36 GeV/c2

Q

Q

q

q

_

_

Overlap

--


4/20Nuno Anjos

The DELPHI Detector @ LEP 2

e- e+

4 well separated, high multiplicity jets, balanced event, no qq

radiative return.

About 5,000 WW 4q events collected:

DELPHI detector at LEP2

4 jet topology

Year (1996) 1997 1998 1999 1999 total L

s(GeV)

161-172

183 189 192-202

203-209

183-209

L (pb-1) (20) 53 157 227 223 660


5/20Nuno Anjos

CR: Colour rearrangement between different colour singlets

Example: Hadronic decay of B mesons

B J/+ X

CR in WW decays: some hadrons cannot be assigned to either W

CR is suppressed in the perturbative phase 1) Group structure of QCD requires the exchange of 2 gluons

2)CR may be possible during hadronisation 1) Average multiplicity and particle distributions

2) W mass

Colour Reconnection

BR exp ~ 1%BR full CR ~ 3-5%BR no CR ~ 0.3-0.5%


6/20Nuno Anjos

CR Phenomenological Modelso Sjöstrand & Khoze (PYTHIA):

o Type I: (SK I) flux tubes

o Probrecon

= 1-exp(-I Vol. Overlap)

o Type II, II’ : vortex lines

o Gustafson & Häkkinen: minimizes string length

o Lönnblad(ARIADNE): CR between dipoles, min. string length

o Webber et al. (HERWIG): CR if smaller cluster sizes

SK-I

.18 1.26 2.34 4.9 100 I0.66

MW ~ O(100 MeV)


7/20Nuno Anjos

The Particle Flow Method

oSelect only events with well- defined regions: low efficiency

oCount the particles between each pair of jets

oCompare the Inside W’s and Between W’s particle flow

Data events183-189 GeV

Expected Events MC

1343 1286.01.0

Eff.[%] Pur.[%] Pairing Eff. [%]

22 – 15 75-70 ~79

All results are preliminary!

No CR

CR

Cross-Talk between W systems should result in some depletion and/or enhancement of soft particles in some phase space regions


8/20Nuno Anjos

Measuring the Particle Flow

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

A

C

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’s and

Between W’s

Z qq()

WW qql

data at 189 GeV #( A+C )

#( B+D )( ) dd#( A+C )

#( B+D )

SK1 100%SK1 100%

no CR

SK1 100%

0.2 0.8

DELPHIpreliminary


9/20Nuno Anjos

The Ratio of Particle Flow

<R>189 = 0.979 0.032(sta) 0.030(sys)

In the systematic error, were considered: Bose-Einstein effects (0.7%) Fragmentation modelling (2.6%) Work Background subtraction/modelling (1.2%) Extrapolation to 189 GeV (1.4%)

R

8.0

2.0

8.0

2.0

DB regions

CA regions

rr

rr

dddn

dddn

R

DELPHI preliminary

Data: 183-209 GeV


10/20Nuno Anjos

Particle Flow Results

Comparison of LEP Particle Flow results

R

DELPHI preliminary

DELPHI Particle Flow results


11/20Nuno Anjos

The mW Method

MC prediction SK1

Bias of mW estimators:

Build mW estimators with low sensitivity to CR: mW(cone)

mW is unknown, but must be invariant

for different estimators

mW = mW(std)-mW(cone) contains information about

CR

Start with the standard mW analysis and mass estimator, mW(std)

DELPHI preliminary


12/20Nuno Anjos

mW ResultsData: 189-209 GeV

centr.

+1 sigma

MC prediction SK1

-1 sigma

2

Cross check :

mW(qqlv) = 8 56 MeV/c2

AR (noCR-CR) = 7 4 MeV/c2

HW (noCR-CR) = 20 4 MeV/c2

DELPHI preliminary

DELPHI preliminary

mw(std,Rcone= 0.5rad) = 59 35(sta) 21(sys) MeV/c2

MLBZ 11

Calibration 3

Background 3

BEI-BEA 8

BE0-BEI 16

Total Syst. 21

Stat. 35

Total 41

Syst.error


13/20Nuno Anjos

Combination of DELPHI Measurements

66.101.102.1)1(

SK

Correlation between the two methods is small

Combine both DELPHI measurements

2


14/20Nuno Anjos

)(/)()( 222 QQQR ref

Bose-Einstein Correlations

two-particle correlations

o Quantum-mechanical effect: Bose-Einstein statistics

o Symmetrisation of the wave function of n identical bosons

o Enhanced production of identical bosons close in phase-space

Inclusive two-particle density

Phase-space (projected into 1 dimension)

Normalised two-particle density

BEC in light quark Z decays identical to BEC inside a hadronically decaying W

Reference Sample: MC, Unlike sign, Mixed events

22221 4)( mMppQ

dQ

dn

NQ pairs

ev

1)(2


15/20Nuno Anjos

Measuring Inter-W BEC

Hadronic

events

Semi-hadronicEvents

Mix 2 Semi-hadronic events

Assuming independent WW decay

Observables

data-data comparison

BEC between separately hadronising strings?

No Inter-W BEC:0

DELPHI preliminary

No Inter-W BEC:1)(2)(2)( QQQ WWmix

WWW

)(2)(2)()( QQQQ WWmix

WWW

)(2)(2

)()(

QQ

QQD

WWmix

W

WW


16/20Nuno Anjos

Monte Carlo Simulation of BEC

- Andersson et al.• Based in Lund Area Law• No arbitrary input parameters• Reproduces BEC inside Z and W decays• No BEC between two strings • No mature MC

- LUBOEI• Has 2 input parameters: correlation strength and source size R• Reproduces BEC inside Z and W decays (BE ins)• Has a switch to allow inter-string BEC (BE full) mW35MeV• Implemented in PYTHIA (BE32 variant used here)

Global Models: Generate weights for a given event

Local Models: Reshuffle the 4-momentum of particles

- Kartvelishvili & Kvatadze

• Allows inter-string BEC (mW 15MeV)• Not tested yet


17/20Nuno Anjos

Measured BEC Distributions

like-sign

unlike-sign

D like-sign

D unlike-sign

550 pb-1 s = 189 – 209 GeV2567 qql events 3252 qqqq events

DELPHI preliminary

DELPHI preliminary

DELPHI preliminary


18/20Nuno Anjos

WW BEC Results

Measured correlation strength:

Systematic error:

Correlation strength

Sou

rce s

ize

Fit R and simultaneously

like-sign

Fixing R to BE full value

BE full

Data

-BE full-BE ins

DELPHI preliminary

DELPHI preliminary

= 0.241 0.075(sta) 0.038(sys)

)1)(1()( QeNQD RQ


19/20Nuno Anjos

LEP Combination of WW BEC Results

Combine observed fraction of BE full

Result of combination:23% of LUBOEI BE32

with BEC between W’s is observed

Corresponding W mass shift: 8 5 MeV/c2


20/20Nuno Anjos

Conclusions

CR and BEC investigated in WWqqQQ events in DELPHI at LEP2

Colour Reconnection:o Two uncorrelated observables are studied:

Comparing the particle flow between jets ( R ) Modifying the kinematics of the jets ( mW[std,cone] )

o Preliminary results indicate small amount of Colour Reconnectiono The observables are not sensitive to the ARIADNE modelo Work continues on fragmentation error in the particle flow

Bose-Einstein Correlations:o Model independent measurement of inter-W BEC developedo Mixed reference sample built from semi-hadronic data eventso Inter-W BEC in like sign pairs found at 2.9leveloLEP combination: 23% BE32 inter-W, mW=8 5 MeV/c2

Documents

Colour Reconnection and Bose-Einstein Correlations in WW Decays Nuno Anjos LIP, Lisbon DELPHI Collaboration DIS 04, Štrbské Pleso, Slovakia, April 16 th,