1 Bose-Einstein Correlations in hadronic W decays at LEP Nick van Remortel University of Antwerpen...
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Transcript of 1 Bose-Einstein Correlations in hadronic W decays at LEP Nick van Remortel University of Antwerpen...
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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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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 in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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
550 pb-1, s = 189 – 209 GeV2567 qql, 3252 qqqq events
Observe inter-W BEC at 2.9 level75% of LUBOEI BEfull
ALEPH
EPS ’03 note 2003-013
686 pb-1, s = 183 – 209 GeV2406 qql, 6154 qqqq events
Disagree 3.7 s with LUBOEI BEfullSome fits observe weak signal (1.7 )
OPAL
EPS ’03 note PN523
680 pb-1, s = 183 – 209 GeV4533 qql, 4470 qqqq events
Ambiguous results: (Q) prefers no inter-W BEC, D(Q) is inconclusive
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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
The D(Q) distribution
L3
DELPHI
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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
L3 analysis: Phys. Lett. B547 (2002) 139629 pb-1, s = 189 – 209 GeV3800 qql, 5100 qqqq events
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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
DELPHI Analysis: (Public note 2003-020 CONF 640)
550 pb-1, s = 189 – 209 GeV2567 qql, 3252 qqqq events
BEins Model
BEfull Model
DATA FIT
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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
New ALEPH results: (Public note 2003-013)
685 pb-1, s = 183 – 209 GeV2406 qql, 6154 qqqq events
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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
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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BEC in hadronic W decays at LEP
Nick van Remortel ISMD 2003, Krakow
New OPAL results: (Public note PN523)
680 pb-1, s = 183 – 209 GeV4533 qql, 4470 qqqq events