1 Bose-Einstein Correlations in hadronic W decays at LEP Nick van Remortel University of Antwerpen Belgium

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Bose-Einstein Correlations in hadronic W decays at LEP

Nick van Remortel

University of AntwerpenBelgium

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BEC in hadronic W decays at LEP

Nick van Remortel ISMD 2003, Krakow

Some generalities

BEC is a quantum-mechanical effect, arising from the symmetrisation of the wave function of n identical bosons.

Enhanced production of identical bosons close in phase-space

I consider only two-particle correlations, where

Inclusive two-particle density

Phase-space projected into 1 dimension

Normalised two-particle density

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Large variety of reference samples: MC events, opposite charge pairs, mixed tracks, etc.

Many boson (and fermion) species analysed: charged pions, neutral pions, kaons, protons, lambda’s. Sometimes dedicated particle ID performed, sometimes not.

Data are usually fitted to extract correlation strength and source size: Gaussian, exponential, power law, Hedgeworth, Laguerre, …

Different techniques may lead to different strengths and sizes, although in many cases the underlying physics is the same.

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What is known about BEC at LEP

Higher order correlations between like-sign pions exist: Coherent nature of particle production

Production source is not spherical, but elongated along the event axis:

DELPHI

Spatial and temporal characteristics of hadronisation region

Incoherent particle production

L: Phys. Lett. B540 (2002) 185.

D: Phys. Lett. B471 (2000) 460.O: Eur. Phys. J. C16 (2000) 423.

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Correlations between neutral pions are observed

Test lund string predictions

BEC in light quark Z0 decays areidentical to BEC inside a hadronically decaying W

Fragmentation of Z and W are identical

So far inconclusiveO: Phys. Lett. B559 (2003) 131.

L: Phys. Lett. B524 (2002) 55.

L: Phys. Lett. B547 (2002) 139.

And many more …

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Open questions

Is particle production coherent or incoherent ?

• In the Lund string picture in principle coherent• 3 particle BEC somehow in contradiction ?• If completely coherent, why do we see strong BEC ?• BEC in High energy collisions is not to be compared with HBT in astronomy

Are there BEC between separately hadronising strings (i.e. Ws)?

• In the Lund picture in principle not

• If not, why strong BEC in Heavy Ion collisions ?

• If yes, are they the same as BEC inside a string ?

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BEC between particles from different Ws

Distance between W’s ~ 0.1 fm

Hadronisation-volume: 0.5 to several fm

If the 2 production sources overlap: Inter-W BEC possible

Potential bias in direct measurement of MW

MW = 0 – 100 MeV

Colour Reconn.

Inter-W BEC

Syst. Unc. On MW

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

Coherent particle production in strings = independent

Maybe incoherent BEC à la HBT between two W’s

Other.

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Monte carlo simulation of BEC

Global Models:

In Lund Area Law:

Local Models:Reshuffle the 4-momentum of particles

Generate weights for a given event

BE0

BE32

LUBOEI

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Lund Area Law: (Nucl. Phys. B513 (1998) 627.)

• Does not have (arbitrary) input parameters• Reproduces 2 particle BEC inside Z0 decay (and inside Ws)• Measured source size corresponds to region of homegenity along the string• Can explain the elongated source and predicts differences between neutral and charged pions• Does not allow for BEC between two strings

LUBOEI: (Eur. Phys. J. C2 (1998) 165.)

• Has 2 input parameters: strength and size R• Reproduces 2 particle BEC inside Z0 decay (and inside Ws)• Does not predict elongated sources• Predicts correlations between unlike-sign pion pairs• Has a switch to allow inter-string BEC (predicted MW35MeV)

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

A recent alternative:

A global reweighting model that can allow for inter-string BEC (predicted MW15MeV)

Not widely used or tested, why ?

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Measurement of an inter-W BEC signal

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

Inter-W BEC can be investigatedby comparing data with data model-independent !

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Status of the LEP analyses

L3

Phys. Lett. B547 (2002) 139

629 pb-1, s = 189 – 209 GeV3800 qql, 5100 qqqq events

Consistent with no inter-W BECDisagree 3.8 with LUBOEI BEfull

DELPHIEPS ’03 note 2003-020 CONF 640


Observe inter-W BEC at 2.9 level75% of LUBOEI BEfull

ALEPH

EPS ’03 note 2003-013


Disagree 3.7 s with LUBOEI BEfullSome fits observe weak signal (1.7 )

OPAL

EPS ’03 note PN523


Ambiguous results: (Q) prefers no inter-W BEC, D(Q) is inconclusive

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The (Q) distribution

DELPHI

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The D(Q) distribution

L3

DELPHI

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LEP-wide combination of WW BEC results

Measurements cannot be directly combined. Observed fraction of BEfull model is used.

Combined results observe 23%of the LUBOEI BE32 modelWith inter-W BEC.

This results in a small W mass uncertainty due to BEC: 8 5 MeV

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Conclusions

Data from the four LEP experiments are not as inconsistent as it seems: All observe a small excess at very low Q values ! It is clear that in all cases the LUBOEI model overestimates the effect, although it describes BEC inside a Z0 very well. Could it be that BEC between strings is not the same as inside a string ?? Number of particle pairs coming from different strings, resulting in small Q values is very small Data need to be combined directly (after unfolding) I fear this will not be done. There are other systems with two independently fragmenting strings three-jet Z0 events.

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L3 analysis: Phys. Lett. B547 (2002) 139629 pb-1, s = 189 – 209 GeV3800 qql, 5100 qqqq events

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D’(Q) = D(Q)data / D(Q)MCnoBE

L3 does NOT observe BEC between different Ws and disagrees with the LUBOEI Befull

model at the level of 3.8 sigma

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DELPHI Analysis: (Public note 2003-020 CONF 640)


BEins Model

BEfull Model

DATA FIT

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Data tend to prefer a larger source size than the Befull MC, tuned to Z0 data.

Might be an indication of HBT-like BEC at work

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New ALEPH results: (Public note 2003-013)


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When fixing the source size, ALEPH is consistent with no inter-W BEC. Free source size gives small signal. Again larger R preferred.

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New OPAL results: (Public note PN523)


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OPAL is consistent with both scenarios in D(Q) analysis, (Q) prefers no inter-W BEC.

Documents

1 Bose-Einstein Correlations in hadronic W decays at LEP Nick van Remortel University of Antwerpen Belgium