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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

Recent jet measurements at the Tevatron

Sofia Vallecorsa

University of Geneva

  • 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

the tevatron
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

cdf and d0
CDF and D0

Both detectors:

  • Silicon Microvertex Tracker
  • Calorimeter
  • Muon Chambers
  • High speed Trigger/DAQ


L2 trigger on displaced vertex

Excellent tracking resolution


Excellent  ID and acceptance

Excellent tracking acceptance

|| < 2-3

jets at tevatron
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

  • Need a common and unambiguous
  • definition for theory and experiments.
    • Jet reconstruction algorithms:
    • Jet corrections

Pavia, 19-21 Aprile

jet corrections

Hadronic 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

inclusive jet cross section
Inclusive jet cross section

Pavia, 19-21 Aprile

inclusive jet production run i

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

inclusive jet cross section1
Inclusive jet cross section


  • 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

inclusive jet cross section2
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++)

inclusive jet cross section3
Inclusive jet cross section


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

inclusive jet cross section4
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

boson jets
Boson + jets

Pavia, 19-21 Aprile

w jets production
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

w jets production1
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

z jets production
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


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)



pT spectra of nth jet distribution

heavy flavour jets
Heavy flavour jets

Pavia, 19-21 Aprile

inclusive b jet cross section run i
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

tagging b jets
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’
tagged jets production
-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)


Main systematics on data:

JES and HF fraction

Data/Pythia ~1.3

inclusive b jet cross section
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)

bb correlations
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
  • [email protected] + JIMMY

Generator for multiparton interactions (links to Herwig)

--> Better description of underlying event

 = 36  2 nb

  • 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

back up

Pavia, 19-21 Aprile

jet reconstruction algorithms
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

parton to hadron correction
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

inclusive jet production
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

forward jets k t algorithm
Forward jets (kT algorithm)




Pavia, 19-21 Aprile

w jets production2
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

b quark production in hadron collisions
b quark production in hadron collisions

Leading Order

Next to Leading Order







Flavor excitation

other radiative corrections..

Flavor creation

Gluon splitting

Experimental inputs are B-Hadrons or b-jets rather than b-quark

Proton structure



=> Another stringent test of NLO QCD

Pavia, 19-21 Aprile

nlo lo comparison
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

high p t b jet cross section cdf
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