1. 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, march

2. 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

3. primary quarks
independent decay
perturbative crc
non-perturbative crc

4. 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.

5. 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!!!

6. 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

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

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

9. 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

10. Particle production between jets
1 plane / 4 planes

11. and the two observables:
Consider the ratio:
and the two observables:

13. OPAL*: 1/ Rn189GeV(0.2-0.8) = 0.83  0.03  0.01 =3.1
ALEPH: Rn189GeV( ) =   =2.8
L3: Rn189GeV( ) =   =3.3
OPAL*: 1/ Rn189GeV( ) = 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

14. comparison of different color reconnection models at 189 GeV

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

16. 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

17. 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