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Multijets in e+e- annihilations Hasko Stenzel

Multijets in e+e- annihilations Hasko Stenzel. Outline. Jet rates and algorithms energy evolution measurements of α s 4-jet analyses color factors measurements of α s quark mass effects Multijet event shapes NP-analysis out-of-plane momentum non-global variables beyond 4-jets.

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Multijets in e+e- annihilations Hasko Stenzel

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  1. Multijets in e+e- annihilations Hasko Stenzel

  2. Outline • Jet rates and algorithms • energy evolution • measurements of αs • 4-jet analyses • color factors • measurements of αs • quark mass effects • Multijet event shapes • NP-analysis • out-of-plane momentum • non-global variables • beyond 4-jets ! mostly perturbative QCD e+e- multi-jets H.Stenzel

  3. Jet algorithms Jet definition based on a pair-wise track metric JADE algorithm Durham/kt algorithm Cambridge ordering parameter e+e- multi-jets H.Stenzel

  4. Jet rates • Corrections applied for: • background subtraction (LEPII) • ISR/FSR QED corrections • acceptance/efficiency • detector resolution Corrected distributions represent the hadron level. Hadronisation corrections are applied to perturbative predictions (e.g. for fits). Detector corrections Hadr. corrections Hadronisation uncertainty claimed ~2% at LEPI < 1 % at LEPII Exp. uncertainties e+e- multi-jets H.Stenzel

  5. Energy dependence of jet observables As in the case of event-shape Variables, jet rates, mean jet multiplicities and differential jet rates are evolving with energy through the running of αs. mean jet multiplicity 3-jet fraction e+e- multi-jets H.Stenzel

  6. Measurements of αs from jet observables • Observables derived from jet rates: • differential two jet rate • mean jet multiplicity mean jet multiplicity differential 2-jet rate Fits to the data with perturbative prediction O(αs2)+NNLA, LogR matching (mostly in the 3-jet region) e+e- multi-jets H.Stenzel

  7. Results from an OPAL analysis • Observables derived from jet rates: • differential two jet rate • mean jet multiplicity • Cambridge and Durham • Measurements at LEPI and LEPII e+e- multi-jets H.Stenzel

  8. 4-jet observables • 4-jet observables have been used extensively to determine • QCD color factors • αs (fixing color factors to QCD) • using NLO calculations O(αs3)+NNLA L3 e+e- multi-jets H.Stenzel

  9. QCD color factors in parton splitting 2 CF 2 CA 2 nfTR QCD : SU(3) CF=4/3, CA=3, TR=1/2 e+e- multi-jets H.Stenzel

  10. Color factors from 4-jets Leading order diagrams for 4-jet production e+e- multi-jets H.Stenzel

  11. Observables in 4-jet system Angles between the 4 jets, e.g. Bengtsson-Zerwas-Angle: angle between (p1 x p2) and (p3 x p4), ie. angle between planes P1 and P2 spanned by the two high and the two low energetic jets Complete set of observables: p1 p3 P2 p4 P1 p2 + R4(ycut) + y23 e+e- multi-jets H.Stenzel

  12. NLO fits to angular variables and jet rates Including NLLA resummation e+e- multi-jets H.Stenzel

  13. Results of simultaneous fit to αs and color factors e+e- multi-jets H.Stenzel

  14. Determination of αs from 4-jet observables (fixing color factors to QCD) • OPAL recently analysed • 4-jet rate • D-parameter • Tminor at LEPI and LEPII e+e- multi-jets H.Stenzel

  15. OPAL results on αs from 4-jet observables The uncertainty for αs from 4-jet observables is reduced wrt 3-jet case: e+e- multi-jets H.Stenzel

  16. ALEPH/DELPHI results on αs from R4 DELPHI tested the energy dependence of αs from 4-jets at E=150 GeV: e+e- multi-jets H.Stenzel

  17. Testing b-quark mass effects in the 4-jet rate DELPHI analysed flavour tagged 4-jet ratios: in order to extract the b-quark mass: preliminary result: e+e- multi-jets H.Stenzel

  18. non-perturbative analysis of 4-jet observables L3 analysis of mean value of D-parameter <D-parameter> Results: e+e- multi-jets H.Stenzel

  19. other 4-jet observables … measured at LEPI & LEPII but not analysed in the scope of power corrections … e+e- multi-jets H.Stenzel

  20. out-of-plane momentum for near-to-planar configurations … with different ycuts … e+e- multi-jets H.Stenzel

  21. non-global variables … sensitive to local radiation in restricted phase space regions: single jet mass and single jet broadening e+e- multi-jets H.Stenzel

  22. beyond 4-jets 5- and more jet rates, yij multijet resolution parameters e+e- multi-jets H.Stenzel

  23. Conclusion • Multijet observables (mostly 4-jets) have given access to • precision measurements of αs • color factors • test of quark mass effects many variables have been measured • D-parameter, Tminor, Oblateness, y34 • out-of-plane momenta • non-global variables few of them have been analysed for power corrections • D-parameter The potential of ≥5-jet configurations is to be explored. e+e- multi-jets H.Stenzel

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