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V+jets and V+hf in HERWIG

Overview of Z/γ+bjet analysis at Tevatron, insights on b-quark origin, evolution patterns, and Herwig++ updates. Discussion on backward evolution, gluon splitting, and future research directions.

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V+jets and V+hf in HERWIG

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  1. V+jets and V+hf in HERWIG Mike Seymour CERN/University of Manchester Berkeley Workshop March 27th 2008

  2. V+jets and V+hf in HERWIG • V+jets and V+hf in HERWIG • some musings on Andy’s Z+bjet HERWIG results • V+jets and V+hf in Herwig++ • Status reports • HERWIG • Herwig++ • Rivet/CEDAR

  3. V+jet studies in HERWIG • See talks of: • AndreaMessina • Andy Mehta • Jason Nielsen • Michelangelo Mangano (?) • Nils Lavesson • etc (?)

  4. Z/γ+bjet from Tevatron • First a consistency check: • HERWIG6.510, CTEQ6L, inclusive Z (on shell) • parton level: apply +10% correction to σ(Z+b)/σ(Z) • inclusive kt algorithm, R=0.7, massive jets • 1 jet containing b quark with pt>20, |y|<1.5 • and the answer is…

  5. Z/γ+bjet from Tevatron • First a consistency check: • HERWIG6.510, CTEQ6L, inclusive Z (on shell) • parton level: apply +10% correction to σ(Z+b)/σ(Z) • inclusive kt algorithm, R=0.7, massive jets • 1 jet containing b quark with pt>20, |y|<1.5 • and the answer is… 0.20% GOOD !

  6. Z/γ+bjet from Tevatron • Where do these b’s come from? • 0.07% from gluon splitting • ~0.01% from backward evolution to a b! • 0.12% from bb->Z • this is 0.69% of qq->Z, but only 18% of these have a b jet

  7. Z/γ+bjet from Tevatron • Where do the rest of the b’s disappear to? • 60% of b’s do not backward evolve to a gluon: forced (non-perturbative evolution)

  8. Z/γ+bjet from Tevatron • but the other 40% look reasonable:

  9. Z/γ+bjet from Tevatron • Suppose we artificially force b quarks to be evolved back to a gluon perturbatively • i.e. replace forced emission by “abort and reshower” exception •  HERWIG would give 0.38%

  10. Z/γ+bjet from Tevatron • Aside 1: MLM mentioned distortion from ‘abort and reshower’ exceptions: • I find 2.8% of bb->Z events have one • small • acts in opposite direction

  11. Z/γ+bjet from Tevatron • Aside 2: gluon splitting contribution: • LEP measures ~0.3% Z->qq+(g->bb) • PYTHIA predicts ~0.16% • HERWIG predicts ~0.26% • (v. old numbers - need to be checked)

  12. Z/γ+bjet from Tevatron • Conclusion: • More to be understood in backward evolution of b partons, interplay between • modern pdfs: VFNS, current mass • parton showers: massless evolution with constituent mass as infrared cut-off

  13. V+jets and V+hf in Herwig++

  14. HERWIG status • Version 6.510 released October 31st 2005 • Not tuned to data (like all versions 6.1 onwards) ! • Steadily growing list of bugs: • VBF -> Higgs forward-backward asymmetry • B baryon decays • … • Very few promised new features • gg->Higgs ME corrections • W/Z spin correlations in HW/Z production • New release promised “real soon now”

  15. Herwig++ status • Version 2.1.4 released March 3rd 2008 • Version 2.2 coming soon • third digit ~ monthly, second digit ~ twice yearly • Similar quality physics in most areas • Hadronization • Underlying event (Jimmy) • Better in some • Parton shower mass effects (quasi-collinear) • Secondary hadron decays • BSM Simulation (Feynman rules input, spin corr.) • Full tune to e+e- data and Tevatron U.E

  16. Herwig++ status • Complete Physics and Manual • arXiv:0803.0883 [hep-ph] • Hooks for generalized CKKW/MLM matching built in (but nothing hangs from them yet…) • Version 3.0 foreseen for this Fall: complete replacement for HERWIG

  17. CEDAR/Rivet • Combined E-Data Analysis Resource for high-energy physics • HEPdata (database of experimental results) • HEPForge • development environment used by ~ general purpose MCs • LHAPDF • HEPML, RivetGun, AGiLe, … (utilities/interfaces) • JetWeb (web front end for Rivet) • Rivet (object oriented replacement for HZtool) • Experiment-independent implementation of hadron-level analysis for MC standardized comparison

  18. MCnet opportunities 2008: • CTEQ-MCnet school • Short-term studentships: for th. and exptl. students to spend 3-6 months with MC authors in: • CERN • Durham/Cambridge • Karlsruhe • Lund • UCL • on a project of their choice • Next closing date: • March 31st

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