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Cover. Outline. Cross Talk in WW  4 jets DELPHI at LEP2 Colour Reconnection (CR) Particle Flow The D m W Method Bose-Einstein Correlations (BEC) Conclusions. Cross Talk in WW  4 jets.

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  1. Cover

  2. Outline • Cross Talk in WW 4 jets • DELPHI at LEP2 • Colour Reconnection (CR) • Particle Flow • TheDmWMethod • Bose-Einstein Correlations (BEC) • Conclusions

  3. Cross Talk in WW  4 jets Is there any interference between the two evolving hadronic systems from the decays of the W’s? Perturbative QCD Fragmentation Stable Hadrons BECpossible CRsupressed CRpossible betweensoft particles CRand BEC • Fundamental probesof the • dynamicsof hadronisation • Contribute to theW mass • systematic errorin the • fully hadronic channel Electroweak phase Q p+p+ e+ W+ _ p0p0 Q q p-p- e- W- _ ... q ~ 0.1 fm <~ 1 fm <~ 10-100 fm Overlap Interference is possible!

  4. The DELPHI Detector @ LEP 2 About5,000WW 4qeventscollected: DELPHI detector at LEP2 e+ e- 4 jet topology 4 well separated, high multiplicity jets, balanced event, no qq radiative return.

  5. Colour Reconnection CR: Colour rearrangement between differentcolour singlets Example: Hadronic decay ofB mesons B  J/Y + X CRinWW decays: some hadrons cannot be assigned to either W CR is suppressed in the perturbative phase 1) Group structure of QCDrequires the exchange of2 gluons 2) CRmay be possible during hadronisation 1)Average multiplicity and particle distributions 2) W mass BRexp ~1% BRfullCR ~3-5% BRnoCR ~0.3-0.5%

  6. CR Phenomenological Models DMW~O(100 MeV) • Sjöstrand & Khoze (PYTHIA): • Type I: (SK I) flux tubes • Probrecon • = 1-exp(-kIVol. Overlap) • Type II, II’ : vortex lines SK-I kI .18 0.66 1.26 2.34 4.9 100 • Gustafson & Häkkinen: minimizesstring length • Lönnblad(ARIADNE):CR between dipoles, min.stringlength • Webber et al. (HERWIG):CR if smaller cluster sizes

  7. The Particle Flow Method No CR CR Cross-Talk between W systemsshould result in some depletionand/orenhancement of soft particles in some phase space regions All results are preliminary! • Select only events withwell- definedregions:low efficiency • Count the particles betweeneach pair of jets • Compare the Inside W’s and Between W’sparticle flow

  8. Measuring the Particle Flow Project all charged particlesPi in the plane ( jet 1, jet 2) Inside W’sand Between W’s A Rescale the angles to have an equal amount of phase-space between the jets : FJ,i FJ,i . (1/FJK) D B C ( ) DELPHI preliminary data at 189 GeV #( A+C ) #( A+C ) d Z  qq(g) dF #( B+D ) #( B+D ) WW qqln no CR A B C D SK1 100% SK1 100% 0.2 0.8

  9. The Ratio of Particle Flow In the systematic error, were considered: • Bose-Einstein effects (0.7%) • Fragmentation modelling (2.6%) Work • Background subtraction/modelling (1.2%) • Extrapolation to 189 GeV (1.4%) <R>189 = 0.979 0.032(sta)  0.030(sys) Data: 183-209 GeV R DELPHI preliminary

  10. Particle Flow Results Comparison of LEP Particle Flow results DELPHI Particle Flow results DELPHI preliminary R

  11. The DmW Method Start with the standard mW analysis and mass estimator, mW(std) Bias of mW estimators: DELPHI preliminary MC prediction SK1

  12. DmW Results Dmw(std,Rcone= 0.5rad) = 59 35(sta)  21(sys) MeV/c2 Data: 189-209 GeV 2 MC prediction SK1 DELPHI preliminary +1 sigma centr. kI -1 sigma DELPHI preliminary Syst. error kI

  13. Combination of DELPHI Measurements Correlationbetween the twomethods is small Combine both DELPHI measurements 2 kI

  14. Bose-Einstein Correlations • Quantum-mechanical effect:Bose-Einstein statistics • Symmetrisation of thewave functionofn identical bosons • Enhanced production of identical bosonsclose in phase-space two-particle correlations BEC in light quark Z decays identical to BEC inside a hadronically decaying W Phase-space (projected into 1 dimension) Inclusive two-particle density Normalised two-particle density Reference Sample: MC,Unlike sign,Mixed events

  15. Measuring Inter-W BEC Observables BEC between separately hadronising strings? Assuming independent WW decay No Inter-W BEC:1 Hadronic events Semi-hadronic Events No Inter-W BEC:0 Mix 2 Semi-hadronic events DELPHI preliminary data-data comparison

  16. Monte Carlo Simulation of BEC Local Models:Reshuffle the 4-momentum of particles - LUBOEI • Has 2input parameters: correlation strength Land source size R • Reproduces BEC insideZand W decays (BEins) • Has a switch to allow inter-string BEC(BEfull)DmW35MeV • Implemented in PYTHIA (BE32 variant used here) Global Models:Generate weights for a given event - Andersson et al. • Based in Lund Area Law • No arbitraryinput parameters • Reproduces BEC insideZand W decays • No BEC between two strings • Nomature MC - Kartvelishvili & Kvatadze • Allows inter-string BEC(DmW15MeV) • Not tested yet

  17. Measured BEC Distributions 550 pb-1 s = 189 – 209 GeV 2567 qqln events 3252 qqqqevents DELPHI preliminary Dlike-sign Drlike-sign DELPHI preliminary Dunlike-sign DELPHI preliminary Drunlike-sign

  18. WW BEC Results Systematic error: Fixing R to BE full value DELPHI preliminary Fit R and L simultaneously DELPHI preliminary c2=9 -BE full -BE ins like-sign Source size c2=4 c2=1 Data BE full Measured correlation strength: L = 0.241 0.075(sta)  0.038(sys) Correlation strength

  19. LEP Combination of WW BEC Results Combine observed fraction of BE full Result of combination: 23% of LUBOEI BE32 with BEC between W’s is observed Corresponding W mass shift: 8  5 MeV/c2

  20. Conclusions • CRand BECinvestigated inWWqqQQevents in DELPHI at LEP2 • Colour Reconnection: • Two uncorrelated observables are studied: • Comparing theparticle flowbetween jets ( R ) • Modifying the kinematics of the jets ( DmW[std,cone] ) • Preliminary results indicatesmall amountof Colour Reconnection • The observables arenot sensitiveto theARIADNE model • Work continues onfragmentation error in theparticle flow • Bose-Einstein Correlations: • Model independent measurement of inter-W BEC developed • Mixed reference sample built from semi-hadronic data events • Inter-W BEC in like sign pairs found at 2.9slevel • LEP combination:23% BE32 inter-W, DmW=8  5 MeV/c2

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