Color Reconnection in Hadronic WW Events at LEP Motivation Color reconnection Standard variables Particle production between jets, Particle flow distributions Summary Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 1

Motivation Interesting physics where we might learn something about the QCD vacuum MW( ) = 80,442 ± 0,034stat ± 0,028syst GeV/c2 MW( ) = 80,460 ± 0,031stat ± 0,054syst GeV/c2 ± 40 MeV/c2 uncertainty from colour reconnection in hadronic channel

primary quarks independent decay perturbative crc non-perturbative crc

Description of color reconnection before parton shower Dynamical rearrangement of color charges as distance of decay vertices of W  0,1 fm perturbative QCD* * T. Sjöstrand, V.A. Khoze, Phys. Rev. Lett. 72 (1994) 28.

Time scale of fragmentation corresponds  1fm !!! Processes in partonshower and hadronisation might be important!!! Simulation with phenomenological models!!! These depend on the description of the QCD-vacuums!!!

phenomenological models: models of Sjöstrand/Khoze (skI/skII/skII‘): framework of string fragmentation (strings overlap/cross, implemented in JETSET) model of Gustafson/Häkkinen (GH): framework of string fragmentation (minimize total string length, implemented in JETSET) models of Lönnblad (ar2/ar3): framework of dipole cascade model (minimize string length, implemented in ARIADNE) model of Webber et.al. (hcr): framework of cluster fragmentation (minimize cluster s-t-distance, implemented in HERWIG) new model of Rathsman: based on area law between string pieces, will be implemented

Investigate some standard variables momentum distribution: Similiar analyses of all four experiments, no evidence for crc

The idea of the particle flow Color reconnection leads to rearrangements of particle production Careful ! very simple picture !

The event selection A D C B preselection based on energy measured in calorimeters cuts on ISR events jet clustering 100o - 140o < 100o A B D C Low efficiency (> 15 %) very clean topology

Particle production between jets 1 plane / 4 planes

and the two observables: Consider the ratio: and the two observables:

OPAL*: 1/ Rn189GeV(0.2-0.8) = 0.83  0.03  0.01 =3.1 ALEPH: Rn189GeV(0.2-0.8) = 0.820  0.038  0.026 =2.8 L3: Rn189GeV(0.3-0.7) = 0.771  0.049  0.029 =3.3 OPAL*: 1/ Rn189GeV(0.2-0.8) = 0.83  0.03  0.01 =3.1 sensitivity Slightly different selections, object definition and reconstruction, 1/4 planes, nevertheless the method shows high sensitivity in framework of sk1 model (0-100%) (similiar numbers for Sr) *using charged tracks only

comparison of different color reconnection models at 189 GeV

HERWIG systematically higher than JETSET large systematic uncertainty for Rn/Sr further work on observables to make them more robust against fragmentation effects

Summary Low sensitivity of standard variables for color reconnection / no evidence is found particle flow analysis is very promising with high sensitivity for effects in sk1 model, all experiments work on this method most important uncertainty comes from fragmentation systematics, but still room for improvements in observables

Results are not yet conclusive but combination of energies and experiments will come soon to extract best description of the hadronic final states in WW events