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Low-p T Multijet Cross Sections

Low-p T Multijet Cross Sections. John Krane Iowa State University. Part I: Data vs MC, interpreted as physics Part II: Data vs MC, interpreted as a tuning problem. MC Workshop Oct. 4 2002, Fermilab. Motivation. High-p T inclusive jet spectra appear to be well described by NLO QCD

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Low-p T Multijet Cross Sections

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  1. Low-pT Multijet Cross Sections John Krane Iowa State University Part I: Data vs MC, interpreted as physics Part II: Data vs MC, interpretedas a tuning problem MC Workshop Oct. 4 2002, Fermilab

  2. Motivation • High-pT inclusive jet spectra appear to be well described by NLO QCD Possible exceptions include kT algorithm analysis, possibly also the ratio of cross sections at 630/1800 GeV, large-h dijets (BFKL). But only at 1 or 2 s,not actual disagreement • Originally, this was a search for BFKL effects, which could produce extra jets in low-Q2 events John Krane -- Iowa State University

  3. The Analysis • Jets with ET > 20 GeV, usual jet and event cuts,efficiencies applied but no unsmearing • Study inclusive samples of events having at least:1-jet, 2-jets, 3-jets, 4-jets • Compare to normalized Pythia + GEANT and Herwig + GEANT, tune if necessary The usual sample John Krane -- Iowa State University

  4. points=data, histo=Pythia Data and Pythia Inclusive xsec looks fine Multijet xsecs exhibit deviations from Pythia Let’s pretend it’s physics John Krane -- Iowa State University

  5. (D-T)/T Solid lines: energy scale Ålum uncertainty Dash: smearing uncertainty Dotted: total error in ratio John Krane -- Iowa State University

  6. points=data, histo=Herwig Data and Herwig Started generating jets at 0.5 GeV Multijet xsecs exhibit similar deviations John Krane -- Iowa State University

  7. (Data and Pythia) Vector sum pT Define • The more jets in the event, the more imbalance in energy Could this be ISR, with pT lost down the Beampipe? Events > 250 are the excess in 3+ jet events >150 in 4+ John Krane -- Iowa State University

  8. (Data and Pythia) Angles in 3-jet events Find which jet is “the third one” by isolating the two jets with minimal SpT Many back-to-backin the data Usually, third jet is near one of the first two, but more so in Pythia John Krane -- Iowa State University

  9. (Data and Herwig) Angles in 3-jet events Find which jet is “the third one” by isolating the two jets with minimal SpT Min SpT not bad… Third jet is often at 90 degrees, often composed of underlying event E John Krane -- Iowa State University

  10. Early impressions of these results • Signs point to initial state radiation effects in data • DGLAP style? • BFKL style? • …or a need to tune the MC John Krane -- Iowa State University

  11. “Try tuning Pythia, also compare to Herwig;see what works…” • Herwig defaults also did poorly • Many iterations required • Only compared to distributions shown today (and a few other very similar ones…) Pythia works if PARP(83) = 0.32 (from 0.5) Fraction of matterin the proton “core” Herwig works if PTMIN = 3.7 GeV pT generation threshold Does this changeunderlying eventin some way? Both higher and lowervalues do worse! A Multiplepartonscattering parameter Didn’t try Jimmy… John Krane -- Iowa State University

  12. Data and Tuned MC Points = Data Solid = Pythia Dash = Herwig John Krane -- Iowa State University

  13. (D-T)/T No remaining deviations from data Is this because there were no ISR effects? Answer lies in the validity of our tuning John Krane -- Iowa State University

  14. Vector sum pT • Tuned MC reproduced the small “shoulder”in addition to the 3+ and 4+ John Krane -- Iowa State University

  15. Angles with Pythia (and Jetrad) Points = Data Solid = Pythia (error bands and…) Dash-dot = Jetrad John Krane -- Iowa State University

  16. Pythia’s CKIN(3) showsno such sensitivity… Angles with Herwig Cross section shapes verystrange if ptmin<3.7 GeV Points = Data Solid = Herwig Peak (from “underlying event jets”) becomes enormous if ptmin>3.7 GeV Dot = Herwig with cut on merged jets John Krane -- Iowa State University

  17. Conclusions • Results not entirely satisfying • Would like to make definitive statements about ISR • …or provide solid tuning suggestions • Instead, we found sensitivity to several params. • Think the multiple parton scattering is constrainedby other data, we provide a new handle • We don’t understand the Herwig tuning at all • Our decision: publish the data, leave tuning to experts with a more global view • Tuning isn’t really our forté • If we do it, we probably want a second paper out of it! John Krane -- Iowa State University

  18. Backup Slides John Krane -- Iowa State University

  19. Cone Algorithm Details (Run I) • Draw a cone around a “seed” • Calc sum ET, and ET-weighted position • Draw new cone here and recalculate sum ET, position • Reiterate until stable John Krane -- Iowa State University

  20. CH hadrons FH  EM Energy Scale calorimeter jet Correction back to “the particle level” • Remove noise, underlying event,extra pp interactions • Correct for detector response • Undo misassignment of particle energies to jets p K particle jet Time parton jet John Krane -- Iowa State University

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