Color Reconnection in W Pair Events Guillaume Leibenguth Université Catholique de Louvain Belgium DIS 2003, St. Petersburg On behalf of the LEP collaborations

G. LeibenguthCR in W Pair EventDIS 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+ W-W- e+e+ e-e- WW qqqq: 45% lvqq: 45 % e+e+ e-e- W+W+ W-W- ,Z

G. LeibenguthCR in W Pair EventDIS 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. LeibenguthCR in W Pair EventDIS 2003, 25/04/03 Color Reconnection Interconnections in WW qqqq are expected in 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

G. LeibenguthCR in W Pair EventDIS 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: 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. LeibenguthCR in W Pair EventDIS 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. LeibenguthCR in W Pair EventDIS 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  0.23  0.10 DELPHI  0.60  0.38 L  0.26  0.30 OPAL  0.39  0.37

G. LeibenguthCR in W Pair EventDIS 2003, 25/04/03 Fragmentation Function CR is expected to change string potential and configuration Larger differences between fragmentation models are observed

G. LeibenguthCR in W Pair EventDIS 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. LeibenguthCR in W Pair EventDIS 2003, 25/04/03 Particle Flow: Projection : … Rescale angle:  resc =  j /  j+1 Define Ratio R = (A+B)/(C+D) W1, W2 show the intra-jet region, CR changes inter-jet activities (i.e. region C and D)

G. LeibenguthCR in W Pair EventDIS 2003, 25/04/03 Particle Flow: Method Integrate the most sensitive region, use the ratio: Rn =

G. LeibenguthCR in W Pair EventDIS 2003, 25/04/03 Results of Particle Flow Analysis Distance between CR and no-CR = sensitivity For LEP combination, normalize to r = R data /R noCR Fit: no-CR cannot be ruled out SK1 100% is excluded

G. LeibenguthCR in W Pair EventDIS 2003, 25/04/03 Results of Particle Flow Analysis (Main uncertainty: hadronization modelling(0.0081), background (0.0031)) R data = smaller than predicted by usual Ariadne or Herwig!

G. LeibenguthCR in W Pair EventDIS 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 statistical variable. Use it as a discriminating variable for Color Reconnection.

G. LeibenguthCR in W Pair EventDIS 2003, 25/04/03 Reduce CR bias … Remove low energy particles Pcut or „hybrid“ cone algorithm

G. LeibenguthCR in W Pair EventDIS 2003, 25/04/03 … and measure Mw Mw alternative estimator + CR from particle flow combined:

G. LeibenguthCR in W Pair EventDIS 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. LeibenguthCR in W Pair EventDIS 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 Where is the 3- momentum component wrt thrust or jet axis

G. LeibenguthCR in W Pair EventDIS 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) 7 GeV E jet < 35 GeV

G. LeibenguthCR in W Pair EventDIS 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 Q leading = 0 Re-tuning leads to large chi2 Rathsman and Ariadne 2 are disfavored.

G. LeibenguthCR in W Pair EventDIS 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…