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Recent jet measurements at the Tevatron

Recent jet measurements at the Tevatron. Sofia Vallecorsa University of Geneva. Outlines. The Tevatron and the experiments Jet Recostruction Inclusive jet cross section Cone algorithm (CDF,D0) K T algorithm (CDF) Boson+ jets W+jets (CDF) Z+jets (D0) Heavy flavour

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Recent jet measurements at the Tevatron

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  1. Recent jet measurements at the Tevatron Sofia Vallecorsa University of Geneva

  2. Outlines • The Tevatron and the experiments • Jet Recostruction • Inclusive jet cross section • Cone algorithm (CDF,D0) • KT algorithm (CDF) • Boson+ jets • W+jets (CDF) • Z+jets (D0) • Heavy flavour •  - tagged jets (D0) • Inclusive b-jet cross section (CDF) • bb correlation (CDF) • Conclusions Pavia, 19-21 Aprile

  3. The Tevatron Highest energy collider currently running • Peak luminosity ~1.8 1032 cm-2 s-1 • Integrated lumininosity ~ 25 pb-1/week ~ 1.6 fb-1 already delivered ~1.2 fb-1 on tape

  4. CDF and D0 Both detectors: • Silicon Microvertex Tracker • Calorimeter • Muon Chambers • High speed Trigger/DAQ CDF: L2 trigger on displaced vertex Excellent tracking resolution D0: Excellent  ID and acceptance Excellent tracking acceptance || < 2-3

  5. Jets at Tevatron JETS: collimated flows of hadrons Measurements  @HADRON LEVEL Theory prediction  @PARTON LEVEL • Cone based algorithms MidPoint (new RunII ) Infrared safe and well defined • Merging pairs of particles according to their relative pt Kt (recently used @ CDF) Infrared and collinear safe • TO COMPARE: • Need a common and unambiguous • definition for theory and experiments. • Jet reconstruction algorithms: • Jet corrections Pavia, 19-21 Aprile

  6. Hadronic showers EM showers Jet corrections • Calorimeter jets: complex detector behavior. • correct for detector resolution and efficiency • correct for pile-up interactions (~up to 6 extra interactions) • Hadron jets: model dependent correction • Underlying event subtraction • Remove fragmentation/hadronization effects • MC based -> need to be tuned on data by using different observables • Parton jets: model dependent correction • Gluon radiation, energy loss (MC based) Pavia, 19-21 Aprile

  7. Inclusive jet cross section Pavia, 19-21 Aprile

  8. data/theory – 1, % Inclusive Jet Production: Run I • Run I • Cone jet finding algorithm • Apparent excess at high pT, but within the overall systematic errors • Is it New Physics or parton distribution function ? • Between Run I and Run II • Machinery for improved jet finding algorithms: - MidPoint Cone Algorithm - kT Algorithm PDFs are further tuned Pavia, 19-21 Aprile

  9. Inclusive jet cross section L=1fb-1 • MidPoint algorithm cone 0.7 • Inclusive calorimetric trigger L3 Et>20,50,70,100 • Central jets 0.1<|y|0.7 Sensitive to UE+Hadronisation effects for PT<100 GeV/c • DATA: dominated by • JES uncertainties (2-3%) • NLO:dominated by • high X gluon PDFs Good agreement with NLO Pavia, 19-21 Aprile

  10. Inclusive jet cross section • MidPoint algorithm R = 0.7 • 2 regions in rapidity explored |yjet|< 0.4, 0.4 <|yjet|< 0.8 JES gives biggest contr. to uncertainty L = 380 pb-1 Good agreement with NLO prediction (NLOJET++)

  11. Inclusive jet cross section L~1fb-1 Measurement extended over 8 orders of magnitude Very good agreement with NLO • KT algorithm D=0.7 • Inclusive calorimetric trigger L3 Et >5,20,50,70,100 • Central jets 0.1<|y|<0.7, pt>54 GeV/c NLO corrected to hadron level

  12. Inclusive jet cross section • Forward jets measurements constrain gluon distribution in a kinematic region where no effect from new physics is expected -> help to distinguish between new physics and PDF if any excess is found in the central region • KT jets D=0.7 • 5 region in rapidity: • |Y| < 0.1 • 0.1 < |Y| < 0.7 • 0.7 < |Y| < 1.1 • 1.1 < |Y| < 1.6 • 1.6 < |Y| < 2.1 Pavia, 19-21 Aprile

  13. Boson + jets Pavia, 19-21 Aprile

  14. W+jets production • Background to top and Higgs Physics • Testing ground for pQCD in multijet environment • Key sample to test LO and NLO ME+PS predictions L = 320 pb-1 • Restrict W : • W  ev, |e|< 1.1 • JETCLU jets (R=0.4): • ETjets>15 GeV, |jet|< 2. • Uncertainties dominated by background subtraction and Jet Energy Scale LO predictions normalized to data integrated cross sections  Shape comparison only Pavia, 19-21 Aprile

  15. W+jets production Differential cross section w.r.t. di-jet invariant mass in the W+2 jet inclusive sample Differential cross section w.r.t. di-jet DR in the W+2 jet inclusive sample LO predictions normalized to data integrated cross sections  Shape comparison only More exhaustive comparisons expected soon!!! Pavia, 19-21 Aprile

  16. Z+jets production L = 343 pb-1 • Same motivations as W + jets • (Z) ~ (W) / 10, but Ze+e- cleaner • Central electrons (||<1.1) • MidPoint jets: • R = 0.5, pT > 20 GeV/c, |yjet|<2.5 Z+j MCFM: NLO for Z+1p or Z+2p  good description of the measured cross sections ME + PS: with MADGRAPH tree level process up to 3 partons  reproduce shape of Njet distributions (Pythia used for PS) Z+2j Z+3j pT spectra of nth jet distribution

  17. Heavy flavour jets Pavia, 19-21 Aprile

  18. Inclusive b jet cross section: Run I • In Run I, a factor 3 discrepancy was reported between theory predictions and experimental data by both CDF and DØ in b-hadron cross sections • Recent theory development: • FONLL (Cacciari et. al.) – NLO resummed • • Very good agreement with more exclusive • B-hadron production • • check for more inclusive observable - bjet production – comparison with NLO only Pavia, 19-21 Aprile

  19. Tagging b jets • B hadrons are massive • decay into lighter flavors • use decay products to tag B • ‘Soft Lepton Tag’ • B hadrons are long lived • c ~ 460 m • give rise to secondary vertices • tracks from secondary vertex have non-vanishing impact parameter d0 at primary vertex • ‘Secondary Vertex Tag’ & ‘Jet probability’

  20. -tagged jets production • Midpoint cone R=0.5 jets • Central region |Y|<0.5 • Require pt>5 GeV/c  inside cone R=0.5 • Heavy flavour fraction 70-45 % from MC L=300 pb-1 NLOJET++(CTEQ6M, =pt/2) b - fraction (from Pythia) NLOJET++ Main systematics on data: JES and HF fraction Data/Pythia ~1.3

  21. Inclusive b-jet cross section L = 300 pb-1 • MidPoint jets: R = 0.7, |y jet|< 0.7 • Reconstruct secondary vertex from B hadron decays (b-tagging) • Shape of secondary vertex mass used to extract b-fraction from data • More than 6 orders of magnitude covered • Data systematic uncertainties dominated by Jet Energy Scale and b-fraction uncertainties • Main uncertainties on NLO due R/F scales Agreement with pQCD NLO within systematic uncertainties  Sensitive to high order effect (NNLO)

  22. bb correlations Total cross section • JetClu (RunI cone) jets R=0.7 • ||<1.2, Et>20 GeV,30 GeV • Main systematics: • Jet energy scale (~20%) • b tag efficiency (~8%) • UE Herwig description • MC@NLO + JIMMY Generator for multiparton interactions (links to Herwig) --> Better description of underlying event  = 36  2 nb

  23. Conclusions • In 2005, Tevatron achieved the 1 fb-1 goal • Delivered total luminosity 1.6 fb-1 • 1.2 fb-1 on tape available to analyses • Very rich QCD physics program ongoing at CDF and D0 • Explore different jet algorithms • W/Z + jets production provides good feedback for MC tools (Matrix element and Parton showering) • Precision measurements to test pQCD and constrain PDF Pavia, 19-21 Aprile

  24. Back-up Pavia, 19-21 Aprile

  25. Cone algorithms: Seed towers Only iterate over towers above certain threshold JETCLU:Snowmass (ET) - scheme MIDPOINT: E - scheme MidPoint adds extra seed in centre of each pair of seeds  Infrared and collinear safe Ratcheting (JetClu only) All towers initially inside a cone must stay in a cone Jet merging/splitting is an issue: Need to define a Fmerge parameter Jet reconstruction algorithms KT algorithm: • Preferred by theory • Partons are separated into jets according to their transverse momentum • Compute for each pair (i,j) and for each particle (i) the quantities • Iteration until find stable jets • Use E-scheme • Infrared and collinear safe • No merging/splitting parameter needed • successfully used at LEP and HERA  relatively new in hadron colliders • More sensitive to Underlying event and multiple interactions Pavia, 19-21 Aprile

  26. Parton to hadron correction • NLO calculation for inclusive jet production only has 2,3 partons in the final state • -> prediction at parton level • -> Need to correct to hadron level to compare to data • UE : additional energy is added in the jet cone (Multiple Particle Interactions + Beam Remnants) -> Need to add to theory prediction • Hadronization effects: some energy is loss from the jet cone -> Need to subtract to theory prediction • Use Pythia tune A which include tuned parameters for UE Pavia, 19-21 Aprile

  27. Inclusive Jet Production • Probes physics at small distances ≈10-19m • Higher reach in pT due to increased √s • Test pQCD over more than 9 decades in  • Sensitive to PDF (gluon @ high-x) Uncertainty on gluon PDF (from CTEQ6) Pavia, 19-21 Aprile

  28. Forward jets (kT algorithm) 1.1<|Y|<1.6 1.6<|Y|<2.1 0.7<|Y|<1.1 Pavia, 19-21 Aprile

  29. W+jets production Integrated cross section w.r.t. jet ET in each of the 4 W+n jet inclusive samples Pavia, 19-21 Aprile

  30. b quark production in hadron collisions Leading Order Next to Leading Order Q g Q g g g Flavor excitation other radiative corrections.. Flavor creation Gluon splitting Experimental inputs are B-Hadrons or b-jets rather than b-quark Proton structure Fragmentation NLO QCD => Another stringent test of NLO QCD Pavia, 19-21 Aprile

  31. NLO/LO comparison As example: bjet cross section calculated in the past (2003) R cone jet=0.4, |bjet|<0.6 as function of ET not meant to be direclty compared to the measured one Initial states As function of b inside jets NLO/LO increase at high ET  Gluon splitting dominant Contribution more suppressed in a LO MC Pavia, 19-21 Aprile

  32. High PT b-jet cross section (CDF) Displaced tracks inside jet used to reconstruct secondary vertex from B hadron decays (b-tagging) 82 < pTjet < 90 GeV/c Extract fraction of b-tagged jets from data:  use shape of secondary vertex mass Pavia, 19-21 Aprile

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