Nick van RemortelUniversity of Antwerpen, Belgium

On behalf of the LEP experiments

WWBEC

in

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Measurement of BEC in High-Energy Physics

Two-particle correlations are studied by Two-particle densities,Often in the variable Q

Enhancement at small Q when BEC is there

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Why study BEC between different W’s in

Separation between W’s ~ 0.1 fm

Hadronization size: 0.5 to several fm

If two particle sources overlap: Inter-W BEC possible

Potential bias in direct measurement of MW

MW = 0 – 100 MeV

Colour Reconn.

Inter-W BEC

Syst. Uncert. On MW

Hadronisation models: In absence of CR no inter-W BEC is possible

Other syst.

Coherent particle production inside strings = independent

But maybe incoherent BEC à la HBT between two W’s

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Modeling of BEC in WW events

Global Reweighting Models:

V. Kartvelishvili and R. Kvatadze, Phys. Lett B 514 (2001) 7.

S. Todorova-Nova and J. Rames, hep-ph/9710280

Estimate matrix elements for permutation of identical bosons

In Lund Area Law:Predict small shifts in MW : ~ 10 MeV

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Local Models: LUBOEI

Pairs get shift Q, according to

Where

BE0

BE32

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

LEP WW statistics

~ 10k WW events in each experiment

45.6% are

43.9% are

Ranging in CM energies from 160 – 209 GeV

Selection Performances

4q : E ~ 80%, P ~ 85%2q: E ~ 62%, P ~ 96%

15% of qq() background has to be subtracted using Models

BEC inside one W

BEC between W’s

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Measurement of inter-W BEC

In case of independent WW decay

• S. V. Chekanov, E. A. De Wolf and W. Kittel, Eur. Phys. J. C6 (1999) 403.• E. A. De Wolf, hep-ph/0101243

Mix 2 semi-hadronic events

Two observables

• OPAL: still working on this method• L3: Phys. Lett. B547 (2002) 139.• ALEPH: EPS Budapest 2001• DELPHI: ICHEP Amsterdam 2002 and about to publish

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Results from L3

NO Inter-W BEC found

Big enhancement at low Q forInter-W BEC

Nearly flat and zero for no inter-W BEC

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Fit D’(Q):

D’(Q) = Ddata(Q)/DMC no inter-W (Q)

Big enhancement at low Q forInter-W BEC

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Results from ALEPH

(Q) = (Q)data – (Q)MC no BE

No D(Q) and no numbers given yet !

(Q) is consistent with zero

BUT

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Results from DELPHI

LUBOEI does not change charge multipl.

Enhancement at low Q is followed by dip at higher Q

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Increased sensitivity analysis

Fraction of pairs coming from different W’s

very low

SA

ME

WD

IFF

W

Give pair weights, using 2 extra variables

Typical gain in sensitivity : 20%

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Result

BEins Model

BEfull Model

DATA FIT

Fraction of pairs from different W’sis enhanced

By fitting D(Q) with

DELPHI finds a non-zero value of at 3 (stat + syst)

1.4 lower than BEfull

Final paper in preparation !

Nick van Remortel Moriond QCD 2003

BEC in WW events at LEP

Inter-W BEC, observed in DELPHI seems to be concentrated in very small Q region

Corresponds to a bigger R value than expected from MC with inter-W BEC

Good news for MW : smaller amount of pairs participate in

the effect

Interesting for BEC guru’s:Is it HBT or not ?

BEC in WW events at LEP

Nick van Remortel Moriond QCD 2003

Summary

Indications for inter-W Bose-Einstein Correlations seen by DELPHI Final publication is in preparation.

Not confirmed by L3, although raw data agree qualitatively well with DELPHI

Other LEP experiments are working hard to provide final results on the subject