Color Reconnection in W Pair Events

Guillaume Leibenguth

Université Catholique de Louvain

Belgium

DIS 2003, St. Petersburg

On behalf of the LEP collaborations

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Introduction

W pair production at LEPII:

Color reconnection:• W decay range ~ 0.1 fm• QCD hadronisation scale: 1 fm

e

W+

W-

e+

e-

WWWW

qqqq: 45%lvqq: 45 %

e+

e-

W+

W-

,Z

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Definition and Goal

Goal: CR main systematic effect on the W mass measurement in qqqq channel:

Mw (Stat) = 35 MeV Mw(CR) = 90 MeV (reduce the weight of this

channel in the combination from 45% to 10 %)

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Color Reconnection

Interconnections in WW qqqq are expectedin QCD, due to the large space time overlap

Perturbative effects of CR (hard

gluon exchange between quarks

from W decays) are suppressed:

CR implemented in

different hadronisation models

W

cN

2

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Color Reconnection Models

Pythia: (string remodelling), CR occurs for overlapping strings

• SK1: flux tube (lateral dimension), reconnection is based on the overlap O:

iKORECO eP 1 K being a free

parameter• SK2: strings are treated as vortex lines, reconnection if lines crosses• SK2’: like SK2 + condition that the string length is reduced

• GAL: generalised area law (usually called Rathsman)

Sjöstrand, Khoze (SK) models based on doubly resonant W!

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Color Reconnection Models (2)

Herwig CR:• CR occurs if cluster size can be reduced

Ariadne CR2 (Reconnection allowed if):• String length is reduced• After cascade evolved down to 2 GeV (i.e. ~ w)

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

WW multiplicity

Measure charged particle multiplicities in the fully hadronic (4q) and semi-leptonic (2q) channels, compare 4q – 2 (2q)

(note: different momentum cuts, i.e. no LEP combination)

s (GeV) 4q – 2(2q)

ALEPH 189-208 0.31 0.23 0.10

DELPHI 183-189 -0.26 0.60 0.38

L3 183-189 -0.29 0.26 0.30

OPAL 183-202 0.07 0.39 0.37

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Fragmentation Function

• CR is expected to change string potential and configuration

• Larger differences between fragmentation models are observed

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Particle Flow: Definition

In order to compare the particle production from the Ws, one has to take into account that the decay products are in different planes.

(The angle is defined with respect to the jet direction.)

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Particle Flow: Projection

:

…

Rescale angle:resc = j /j+1

Define RatioR = (A+B)/(C+D)

W1, W2 show the intra-jet region, CR changes inter-jet activities (i.e. region C and D)

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Particle Flow: Method

Integrate the most sensitive region,

use the ratio:

8.0

2.0

8.0

2.0

)(

)(

rrD

rC

rrB

rA

dddn

ddn

dddn

ddn

Rn =

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Results of Particle Flow Analysis

Distance between CR and no-CR = sensitivity

For LEP combination, normalize to r = Rdata/RnoCR

Fit: no-CR cannot be ruled out SK1 100% is excluded

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Results of Particle Flow Analysis

(Main uncertainty: hadronization modelling(0.0081), background (0.0031))

Rdata = smaller than predicted by usual Ariadne or Herwig!

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Reminder: W Mass Measurement

“Standard” analysis: direct reconstruction of all decay products. It assumes that both W bosons decay

independently.

W mass best statisticalvariable. Use it as a discriminating variable for Color Reconnection.

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Reduce CR bias …

Remove low energy particles Pcut

or „hybrid“ cone algorithm

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

… and measure Mw

•Mw alternative estimator

+ CR from particle flow combined:

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Problems: Remaining

( Mw(qqqq) , Mw (lvqq)) = 22 43 MeV (no FSI syst)

Ariadne 2 does not behave as expected:

A problem with the gluon energy parameter?

Use of the Z0 peak data.

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

At the Z0 Peak

Define rapidity:

Rapidity gap event:

Two populated regions are separated by an empty region

look for CR effect

)ln(2

1

//

//

pE

pEy

//pWhere is the 3-momentum component wrt thrust or jet axis

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Rapidity Gap: Analysis

Select 3 jets events, with defined gluon jet:

Cut on the smallest particle rapidity (a)

or on the largest rapidity difference (b)

)2/sin( minjetjet EK 7 GeV

Ejet < 35 GeV

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Results

Distribution of the charged particle multiplicity and total electric charge of the leading part of the jet

Ariadne 2-CR, Rathsman-CR:Large excess at Qleading= 0Re-tuning leads to large chi2Rathsman and Ariadne 2 are disfavored.

G. Leibenguth CR in W Pair Event DIS 2003, 25/04/03

Conclusions

(Mw(qqqq),Mw(lvqq)) = 22 43 MeV

No color reconnection effects

are observed at the level of ~ 100 MeV.

• Ariadne 2:

LEP collaborations are working on this issue.

• Final publications? Maybe this summer…