Nick van Remortel

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Nick van Remortel University of Antwerpen,
Belgium On behalf of the LEP experiments
BEC in
2
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
3
BEC in WW events at LEP
Why study BEC between different Ws in
Separation between Ws 0.1 fm
If two particle sources overlap Inter-W BEC
possible
Potential bias in direct measurement of MW
Inter-W BEC
D MW 0 100 MeV
Other syst.
Colour Reconn.
Hadronization size 0.5 to several fm
Syst. Uncert. On MW
Hadronisation models In absence of CR no
inter-W BEC is possible
Coherent particle production inside strings
independent
But maybe incoherent BEC à la HBT between two Ws
Nick van Remortel
Moriond QCD 2003
4
BEC in WW events at LEP
Modeling of BEC in WW events
Global Reweighting Models
Estimate matrix elements for permutation of
identical bosons
Predict small shifts in MW 10 MeV
In Lund Area Law
S. Todorova-Nova and J. Rames, hep-ph/9710280
V. Kartvelishvili and R. Kvatadze, Phys. Lett B
514 (2001) 7.
Nick van Remortel
Moriond QCD 2003
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BEC in WW events at LEP
Local Models LUBOEI
Pairs get shift dQ, according to
Where
Nick van Remortel
Moriond QCD 2003
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BEC in WW events at LEP
BEC inside one W
LEP WW statistics
10k WW events in each experiment

Ranging in CM energies from 160 209 GeV
Selection Performances 4q E 80, P 85 2q
E 62, P 96
15 of qq(g) background has to be subtracted
using Models
BEC between Ws
Nick van Remortel
Moriond QCD 2003
7
BEC in WW events at LEP
Measurement of inter-W BEC

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

In case of independent WW decay
Two observables
Mix 2 semi-hadronic events

• 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
8
BEC in WW events at LEP
Results from L3
Big enhancement at low Q for Inter-W BEC
Nearly flat and zero for no inter-W BEC
NO Inter-W BEC found
Nick van Remortel
Moriond QCD 2003
9
BEC in WW events at LEP
Big enhancement at low Q for Inter-W BEC
D(Q) Ddata(Q)/DMC no inter-W (Q)
Fit D(Q)
Nick van Remortel
Moriond QCD 2003
10
BEC in WW events at LEP
Results from ALEPH
Dr(Q) Dr(Q)data Dr(Q)MC no BE
Dr(Q) is consistent with zero
BUT
No D(Q) and no numbers given yet !
Nick van Remortel
Moriond QCD 2003
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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
12
BEC in WW events at LEP
Increased sensitivity analysis
g
Q
Fraction of pairs coming from different Ws
very low
SAME W
Give pair weights, using 2 extra variables
Typical gain in sensitivity 20
DIFF W
Nick van Remortel
Moriond QCD 2003
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BEC in WW events at LEP
Result
BEfull Model
DATA FIT
By fitting D(Q) with
DELPHI finds a non-zero value of L at 3 s (stat
syst) 1.4 s lower than BEfull
Final paper in preparation !
BEins Model
Nick van Remortel
Moriond QCD 2003
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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
gurus Is it HBT or not ?
Nick van Remortel
Moriond QCD 2003
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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

Nick van Remortel
Moriond QCD 2003