Motivation
Color reconnection
Standard variables
Particle production between jets,
Particle flow distributions
Summary
Color Reconnectionin Hadronic WW Events at
LEP
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 1
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 2
Motivation
MW( ) = 80,442 ± 0,034stat ± 0,028syst GeV/c2
MW( ) = 80,460 ± 0,031stat ± 0,054syst GeV/c2
qqWW
qqqqWW
± 40 MeV/c2 uncertainty fromcolour reconnection in hadronic channel
2W cMeV4618qqqqqqM /)(
Interesting physics where we might learn something about the QCD vacuum
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 3
30°
primary quarks independent decay
perturbative crc non-perturbative crc
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 4
Dynamical rearrangement of color charges as distance of
decay vertices of W 0,1 fm
perturbative QCD*
Description of color reconnection before parton shower
2'
max'
210
2/
O
mN
kN
NN
N
Wq
CRCq
C
WsnoCRC
CRC
32
2
10
W
W
C
WSFCRC
mN
C
* T. Sjöstrand, V.A. Khoze, Phys. Rev. Lett. 72 (1994) 28.
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 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!!!
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 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
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 7
Investigate some standard variables
momentum distribution: )/2ln(ln spxp
Similiar analyses of all four experiments, no evidence for crc
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 8
The idea of the particle flow
Color reconnection leads to
rearrangements of particle production
Careful ! very simple picture !
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 9
The event selection
100o - 140o
100o - 140o< 100o
< 100o
A
B
D
C
•preselection based on energymeasured in calorimeters
•cuts on ISR events•jet clustering
•Low efficiency (> 15 %)•very clean topology
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 10
Particle production between jets
1 plane / 4 planes
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 11
Consider the ratio:
)(//1
)(//1
DCddnNBAddnN
Revt
evt
RdDCddnNBAddnN
Sevt
evtr
)(//1
)(//1
RdDCddnN
RdBAddnNR
evt
evt
n
)(//1
)(//1
and the two observables:
L3
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 12
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 13
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
*using charged tracks only
data
nn 0skIR100skIR
%)(%)(sensitivity
Slightly different selections, object definition and reconstruction, 1/4 planes, nevertheless the methodshows high sensitivity in framework of sk1 model (0-100%)(similiar numbers for Sr)
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 14
comparison of differentcolor reconnection models
at 189 GeV
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 15
HERWIG systematically higher than JETSET
large systematic uncertaintyfor Rn/Sr
further work on observablesto make them more robust against fragmentation effects
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 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
Thomas Ziegler, Johannes Gutenberg Universität, Mainz Moriond conference, 17-24 march 2001 17
Results are not yet conclusive butcombination of energies and experimentswill come soon to extract best description
ofthe hadronic final states in WW events