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Bose-Einstein Correlations in hadronic W decays at LEP

This study examines the quantum-mechanical effect of Bose-Einstein correlations in hadronic W decays at LEP. It explores the production of identical bosons and their correlations in phase space, using various techniques to extract correlation strength and source size. The analysis also investigates higher order correlations and correlations between different particles. Open questions regarding particle production coherence and inter-string correlations are discussed. The status of the LEP analyses is presented.

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Bose-Einstein Correlations in hadronic W decays at LEP

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

  2. BEC in hadronic W decays at LEP 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 Nick van Remortel ISMD 2003, Krakow

  3. BEC in hadronic W decays at LEP 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. Nick van Remortel ISMD 2003, Krakow

  4. DELPHI BEC in hadronic W decays at LEP What is known about BEC at LEP Higher order correlations between like-sign pions exist: Coherent nature of particle production Incoherent particle production L: Phys. Lett. B540 (2002) 185. Production source is not spherical, but elongated along the event axis: Spatial and temporal characteristics of hadronisation region D: Phys. Lett. B471 (2000) 460. O: Eur. Phys. J. C16 (2000) 423. Nick van Remortel ISMD 2003, Krakow

  5. BEC in hadronic W decays at LEP Correlations between neutral pions are observed Test lund string predictions So far inconclusive O:Phys. Lett. B559 (2003) 131. L:Phys. Lett. B524 (2002) 55. BEC in light quark Z0 decays are identical to BEC inside a hadronically decaying W Fragmentation of Z and W are identical L:Phys. Lett. B547 (2002) 139. And many more … Nick van Remortel ISMD 2003, Krakow

  6. BEC in hadronic W decays at LEP 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 ? Nick van Remortel ISMD 2003, Krakow

  7. BEC in hadronic W decays at LEP BEC between particles from different Ws Distance between W’s ~ 0.1 fm If the 2 production sources overlap: Inter-W BEC possible Potential bias in direct measurement of MW Inter-W BEC Other. D MW = 0 – 100 MeV Colour Reconn. Hadronisation-volume: 0.5 to several fm Syst. Unc. On MW Hadronisation models: In absence of CR, no inter-W BEC is possible Coherent particle production in strings = independent Maybe incoherent BECà la HBT between two W’s Nick van Remortel ISMD 2003, Krakow

  8. BE0 BE32 BEC in hadronic W decays at LEP Monte carlo simulation of BEC Generate weights for a given event Global Models: In Lund Area Law: Reshuffle the 4-momentum of particles Local Models: LUBOEI Nick van Remortel ISMD 2003, Krakow

  9. BEC in hadronic W decays at LEP 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 l 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 DMW35MeV) Nick van Remortel ISMD 2003, Krakow

  10. BEC in hadronic W decays at LEP A recent alternative: V. Kartvelishvili and R. Kvatadze, Phys. Lett B 514 (2001) 7. A global reweighting model that can allow for inter-string BEC (predicted DMW15MeV) Not widely used or tested, why ? Nick van Remortel ISMD 2003, Krakow

  11. BEC in hadronic W decays at LEP Measurement of an inter-W BEC signal • 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 Inter-W BEC can be investigated by comparing data with data  model-independent ! Nick van Remortel ISMD 2003, Krakow

  12. BEC in hadronic W decays at LEP Status of the LEP analyses L3 Phys. Lett. B547 (2002) 139 629 pb-1, s = 189 – 209 GeV 3800 qqln, 5100 qqqq events Consistent with no inter-W BEC Disagree 3.8 s with LUBOEI BEfull DELPHI EPS ’03 note 2003-020 CONF 640 550 pb-1, s = 189 – 209 GeV 2567 qqln, 3252 qqqq events Observe inter-W BEC at 2.9 s level 75% of LUBOEI BEfull ALEPH EPS ’03 note 2003-013 686 pb-1, s = 183 – 209 GeV 2406 qqln, 6154 qqqq events Disagree 3.7 s with LUBOEI BEfull Some fits observe weak signal (1.7 s) OPAL EPS ’03 note PN523 680 pb-1, s = 183 – 209 GeV 4533 qqln, 4470 qqqq events Ambiguous results: Dr(Q) prefers no inter-W BEC, D(Q) is inconclusive Nick van Remortel ISMD 2003, Krakow

  13. DELPHI BEC in hadronic W decays at LEP The Dr(Q) distribution Nick van Remortel ISMD 2003, Krakow

  14. DELPHI L3 BEC in hadronic W decays at LEP The D(Q) distribution Nick van Remortel ISMD 2003, Krakow

  15. BEC in hadronic W decays at LEP 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 model With inter-W BEC. This results in a small W mass uncertainty due to BEC: 8  5 MeV Nick van Remortel ISMD 2003, Krakow

  16. BEC in hadronic W decays at LEP 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. Nick van Remortel ISMD 2003, Krakow

  17. BEC in hadronic W decays at LEP L3 analysis: Phys. Lett. B547 (2002) 139 629 pb-1, s = 189 – 209 GeV 3800 qqln, 5100 qqqq events Nick van Remortel ISMD 2003, Krakow

  18. BEC in hadronic W decays at LEP 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 Nick van Remortel ISMD 2003, Krakow

  19. BEfull Model DATA FIT BEins Model BEC in hadronic W decays at LEP DELPHI Analysis: (Public note 2003-020 CONF 640) 550 pb-1, s = 189 – 209 GeV 2567 qqln, 3252 qqqq events Nick van Remortel ISMD 2003, Krakow

  20. BEC in hadronic W decays at LEP 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 Nick van Remortel ISMD 2003, Krakow

  21. BEC in hadronic W decays at LEP New ALEPH results: (Public note 2003-013) 685 pb-1, s = 183 – 209 GeV 2406 qqln, 6154 qqqq events Nick van Remortel ISMD 2003, Krakow

  22. BEC in hadronic W decays at LEP When fixing the source size, ALEPH is consistent with no inter-W BEC. Free source size gives small signal. Again larger R preferred. Nick van Remortel ISMD 2003, Krakow

  23. BEC in hadronic W decays at LEP New OPAL results: (Public note PN523) 680 pb-1, s = 183 – 209 GeV 4533 qqln, 4470 qqqq events Nick van Remortel ISMD 2003, Krakow

  24. BEC in hadronic W decays at LEP OPAL is consistent with both scenarios in D(Q) analysis, Dr(Q) prefers no inter-W BEC. Nick van Remortel ISMD 2003, Krakow

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