Colour Reconnection and Bose-Einstein Correlations in WW Decays Nuno Anjos LIP, Lisbon DELPHI Collaboration DIS 04, Štrbské Pleso, Slovakia, April 16 th, 2004 Cover

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos Outline oCross Talk in WW  4 jets oDELPHI at LEP2 oColour Reconnection (CR) oParticle Flow oThe D m W Method oBose-Einstein Correlations (BEC) oConclusions

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos CR possible between soft particles Cross Talk in WW  4 jets ++++ 0000 Interference is possible! Electroweak phase Is there any interference between the two evolving hadronic systems from the decays of the W’s? Perturbative QCD CR supressed Fragmentation ~ 0.1 fm < ~ 1 fm < ~ fm e+e+ W+W+ W-W- e-e- Stable Hadrons BEC possible CR and BEC o Fundamental probes of the dynamics of hadronisation o Contribute to the W mass systematic error in the fully hadronic channel  Colour Reconnection90 GeV/c 2 Bose-Einstein Corr.35 GeV/c 2 Total Syst.101 GeV/c 2 Stat.36 GeV/c 2 Q Q q q _ _ Overlap ----

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos The DELPHI LEP 2 e-e- e+e+ 4 well separated, high multiplicity jets, balanced event, no qq radiative return. About 5,000 WW  4q events collected: DELPHI detector at LEP2 4 jet topology Year(1996) total L  s(GeV) L (pb -1 )(20)

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos CR: Colour rearrangement between different colour singlets Example: Hadronic decay of B mesons B  J/  + X CR in WW decays: some hadrons cannot be assigned to either W CR is suppressed in the perturbative phase 1) Group structure of QCD requires the exchange of 2 gluons 2) CR may be possible during hadronisation 1) Average multiplicity and particle distributions 2) W mass Colour Reconnection BR exp ~ 1% BR full CR ~ 3-5% BR no CR ~ %

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos CR Phenomenological Models o Sjöstrand & Khoze (PYTHIA) : o Type I: (SK I)  flux tubes o Prob recon = 1-exp(-  I Vol. Overlap) o Type II, II’ :  vortex lines o Gustafson & Häkkinen: minimizes string length o Lönnblad( ARIADNE ): CR between dipoles, min. string length o Webber et al. ( HERWIG ): CR if smaller cluster sizes SK-I II 0.66  M W ~ O(100 MeV)

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos The Particle Flow Method oSelect only events with well- defined regions: low efficiency oCount the particles between each pair of jets oCompare the Inside W’s and Between W’s particle flow Data events GeV Expected Events MC  1.0 Eff.[%]Pur.[%]Pairing Eff. [%] 22 – ~79 All results are preliminary! No CR CR Cross-Talk between W systems should result in some depletion and/or enhancement of soft particles in some phase space regions

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos Measuring the Particle Flow Project all charged particles P i in the plane ( jet 1, jet 2) A C D B Rescale the angles to have an equal amount of phase-space between the jets :  J,i   J,i. (1/  JK ) ABCD Inside W’s and Between W’s Z  qq(  ) WW  qql data at 189 GeV #( A+C ) #( B+D ) () dd d #( A+C ) #( B+D ) SK1 100% no CR SK1 100% DELPHI preliminary

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos The Ratio of Particle Flow 189 =  (sta)  (sys) In the systematic error, were considered: l Bose-Einstein effects (0.7%) l Fragmentation modelling (2.6%) Work l Background subtraction/modelling (1.2%) l Extrapolation to 189 GeV (1.4%) R DELPHI preliminary Data: G eV

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos Particle Flow Results Comparison of LEP Particle Flow results R DELPHI preliminary DELPHI Particle Flow results

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos The D m W Method MC prediction SK1 Bias of m W estimators: Build m W estimators with low sensitivity to CR: m W (cone) m W is unknown, but must be invariant for different estimators D m W = m W (std)-m W (cone) contains information about CR Start with the standard m W analysis and mass estimator, m W (std) DELPHI preliminary

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos D m W Results Data: G eV  centr. +1 sigma MC prediction SK1 -1 sigma 22  Cross check :  m W (qqlv) = 8  56 MeV/c 2 AR (noCR-CR) = 7  4 MeV/c 2 HW (noCR-CR) = 20  4 MeV/c 2 DELPHI preliminary DELPHI preliminary  m w (std,R cone = 0.5rad) = 59  35(sta)  21(sys) MeV/c 2 MLBZ11 Calibration3 Background3 BEI-BEA8 BE0-BEI16 Total Syst.21 Stat.35 Total41 Syst. error

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos Combination of DELPHI Measurements Correlation between the two methods is small Combine both DELPHI measurements  22

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos Bose-Einstein Correlations two-particle correlations o Quantum-mechanical effect: Bose-Einstein statistics o Symmetrisation of the wave function of n identical bosons o Enhanced production of identical bosons close in phase-space Inclusive two-particle density Phase-space (projected into 1 dimension) Normalised two-particle density BEC in light quark Z decays identical to BEC inside a hadronically decaying W Reference Sample: MC, Unlike sign, Mixed events

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos Measuring Inter-W BEC Hadronic events Semi-hadronic Events Mix 2 Semi-hadronic events Assuming independent WW decay Observables data-data comparison BEC between separately hadronising strings? No Inter-W BEC:0 DELPHI preliminary No Inter-W BEC:1

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos Monte Carlo Simulation of BEC - Andersson et al. Based in Lund Area Law No arbitrary input parameters Reproduces BEC inside Z and W decays No BEC between two strings No mature MC - LUBOEI Has 2 input parameters: correlation strength  and source size R Reproduces BEC inside Z and W decays (BE ins) Has a switch to allow inter-string BEC (BE full)   m W  35MeV Implemented in PYTHIA (BE 32 variant used here) Global Models: Generate weights for a given event Local Models: Reshuffle the 4-momentum of particles - Kartvelishvili & Kvatadze Allows inter-string BEC (  m W  15MeV) Not tested yet

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos Measured BEC Distributions  like-sign  unlike-sign D like-sign D unlike-sign 550 pb -1  s = 189 – 209 GeV 2567 qql events 3252 qqqq events DELPHI preliminary DELPHI preliminary DELPHI preliminary

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos WW BEC Results Measured correlation strength: Systematic error: Correlation strength Source size Fit R and  simultaneously like-sign          Fixing R to BE full value BE full Data -BE full -BE ins DELPHI preliminary DELPHI preliminary  =  0.075(sta)  0.038(sys)

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos LEP Combination of WW BEC Results Combine observed fraction of BE full Result of combination: 23% of LUBOEI BE 32 with BEC between W’s is observed Corresponding W mass shift: 8  5 MeV/c 2

DIS 2004, Štrbské Pleso, Slovakia, April 16, /20 Nuno Anjos Conclusions CR and BEC investigated in WW  qqQQ events in DELPHI at LEP2 Colour Reconnection: o Two uncorrelated observables are studied:  Comparing the particle flow between jets ( R )  Modifying the kinematics of the jets (  m W [std,cone] ) o Preliminary results indicate small amount of Colour Reconnection o The observables are not sensitive to the ARIADNE model o Work continues on fragmentation error in the particle flow Bose-Einstein Correlations: o Model independent measurement of inter-W BEC developed o Mixed reference sample built from semi-hadronic data events o Inter-W BEC in like sign pairs found at 2.9  level oLEP combination: 23% BE 32 inter-W,  m W =8  5 MeV/c 2