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

2. 2 BEC in hadronic W decays at LEP Nick van RemortelISMD 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

3. 3 BEC in hadronic W decays at LEP Nick van RemortelISMD 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.

4. 4 BEC in hadronic W decays at LEP Nick van RemortelISMD 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.

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

6. 6 BEC in hadronic W decays at LEP Nick van RemortelISMD 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 ?

7. 7 BEC in hadronic W decays at LEP Nick van RemortelISMD 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 M W  M W = 0 – 100 MeV Colour Reconn. Inter-W BEC Syst. Unc. On M W 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.

8. 8 BEC in hadronic W decays at LEP Nick van RemortelISMD 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 BE 0 BE 32 LUBOEI

9. 9 BEC in hadronic W decays at LEP Nick van RemortelISMD 2003, Krakow Lund Area Law: (Nucl. Phys. B513 (1998) 627.) Does not have (arbitrary) input parameters Reproduces 2 particle BEC inside Z 0 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 Z 0 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  M W  35MeV)

10. 10 BEC in hadronic W decays at LEP Nick van RemortelISMD 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  M W  15MeV) Not widely used or tested, why ?

11. 11 BEC in hadronic W decays at LEP Nick van RemortelISMD 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 investigated by comparing data with data  model-independent !

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

13. 13 BEC in hadronic W decays at LEP Nick van RemortelISMD 2003, Krakow The  (Q) distribution DELPHI

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

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

16. 16 BEC in hadronic W decays at LEP Nick van RemortelISMD 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 Z 0 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 Z 0 events.

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

18. 18 BEC in hadronic W decays at LEP Nick van RemortelISMD 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

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

20. 20 BEC in hadronic W decays at LEP Nick van RemortelISMD 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

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

22. 22 BEC in hadronic W decays at LEP Nick van RemortelISMD 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.

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

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