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Color R econnection in W Pair Events

Color R econnection in W Pair Events. Guillaume Leibenguth Universit é Catholique de Louvain Belgium. On behalf of the LEP collaborations. DIS 2003, St. Petersburg. Introduction. W pair production at LEPII: Color reconnection: W decay range ~ 0.1 fm QCD hadronisation scale: 1 fm. e +.

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Color R econnection in W Pair Events

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

  2. Introduction W pair production at LEPII: Color reconnection: • W decay range ~ 0.1 fm • QCD hadronisation scale: 1 fm e+ e+ W+ ,Z W+ e W- e- e- W- qqqq: 45% lvqq: 45 % WW WW CR in W Pair Event

  3. 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 %) Definition and Goal CR in W Pair Event

  4. 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 CR in W Pair Event

  5. 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! CR in W Pair Event

  6. 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) CR in W Pair Event

  7. 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) preliminary CR in W Pair Event

  8. Fragmentation Function • CR is expected to change string potential and configuration • Larger differences between fragmentation models are observed CR in W Pair Event

  9. 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.) Particle Flow: Definition CR in W Pair Event

  10. 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) CR in W Pair Event

  11. Integrate the most sensitive region, use the ratio: Particle Flow: Method Rn = CR in W Pair Event

  12. Results of Particle Flow Analysis For LEP combination, normalize to r = Rdata/RnoCR Distance between CR and no-CR = sensitivity Fit: no-CR cannot be ruled outSK1 100% is excluded CR in W Pair Event

  13. Results of Particle Flow Analysis (Main uncertainty: hadronization modelling(0.0081), background (0.0031)) Rdata = smaller than predicted by usual Ariadne or Herwig! CR in W Pair Event

  14. Reminder: W Mass Measurement “Standard” analysis: direct reconstruction of all decay products. It assumes that both W bosons decayindependently. W mass best statistical variable. Use it as a discriminating variable for Color Reconnection. CR in W Pair Event

  15. Reduce CR bias … Remove low energy particles Pcut or „hybrid“ cone algorithm CR in W Pair Event

  16. … and measure Mw + CR from particle flow combined: • Mw alternative estimator CR in W Pair Event

  17. 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. CR in W Pair Event

  18. 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 CR in W Pair Event

  19. Rapidity Gap: Analysis Select 3 jets events, with defined gluon jet: 7 GeV Ejet < 35 GeV Cut on the smallest particle rapidity (a) or on the largest rapidity difference (b) CR in W Pair Event

  20. 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= 0 Re-tuning leads to large chi2 Rathsman and Ariadne 2 are disfavored. CR in W Pair Event

  21. 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… CR in W Pair Event

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