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Physics at the Tevatron (high P T ) . Rocío Vilar Cortabitarte Universidad de Cantabria/IFCA. Cosas. I want to thank to people that I took their material to prepare mine : B.Heineman, F.Canelli, O.Gonzalez, B. Klima, G.Bernardis, K.Pitts, S. Seidel, M. Wobisch etc …

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physics at the tevatron high p t

Physics at the Tevatron (high PT)

Rocío Vilar Cortabitarte

Universidad de Cantabria/IFCA

  • I want to thank to people that I took their material to prepare mine :
    • B.Heineman, F.Canelli, O.Gonzalez, B. Klima, G.Bernardis, K.Pitts, S. Seidel, M. Wobisch etc…
  • There are many topics that I am not covering in these lectures. I select some topics and analysis as an example of physics at Tevatron (actually, a tiny bit.. )
    • More on: and
  • I will talk on high Pt physics, no B-Physics is covered
  • This is a experimentalist point of view talk !!!

Physics at Tevatron

  • Accelerators and Detectors (Lecture I):
    • Chain of Particle Accelerator
    • Tevatron performance
    • CDF
    • D0
  • Physics
    • QCD (Lecture I)
    • Top Pair and ElectroWeak (Lecture 2)
    • Single Top, Higgs and New Physics beyond the Standard Model (Lecture 3)

Physics at Tevatron

w ha t is fermilab
What is Fermilab?

Batavia, IL


Physics at Tevatron

w ha t is fermilab1
What is Fermilab?
  • In 1989, Fermilab was designated a National Environmental Research Park

Physics at Tevatron

w ha t is fermilab2
What is Fermilab?
  • Fermilab is a national laboratory located in Batavia, Illinois USA, founded by the Department of Energy.
  • Fermilab is home to the world’s most powerful particle accelerator, the Tevatron, four miles in circumference.
  • 1,900 employees include about 900 physicists, engineers and computer professionals. Another 2,300 scientists and students, from across the United States and around the world.
  • It is open to the sorroundings communities with educational and recreation activities
  • 6,800-acre Fermilab site contains wetlands, woodlands, grasslands and more than 1,100 acres of reconstructed tall-grass prairie

Physics at Tevatron

chain of accelerators
Chain of Accelerators
  • pp collider:
    • 6.5 km circumference
    • Beam energy: 980 GeV
      • √s=1.96 TeV
    • 36 bunches:
      • Time between bunches: Dt=396 ns
  • Main challenges:
    • Anti-proton production and storage:
      • Stochastic and electron cooling
    • Irregular failures:
      • Kicker prefires, Quenches
  • CDF and DØ experiments:
    • 700 physicists/experiment







P source

Main injector &



tevatron performance
Tevatron Performance

∫ Ldt= 3.1 fb-1


  • Integrated luminosity more than 5 fb-1 by now
    • First years were difficult
      • March’01-March’02 used for commissioning of detectors
      • Physics started in March’02
    • Now is performing really well
      • Typical instantaneous luminosity(peak) ≈ 3.3(3.5) · 10 32cm−2 s −1
      • Integrated luinosity per week (month) ≈ 75pb− 1 /(260pb−1 )
  • Just coming out of 2-month shutdown
  • Core detector operates since 1985:
    • Central Calorimeters
    • Central muon chambers
  • Major upgrades for Run II:
    • Drift chamber: COT
    • Silicon: SVX, ISL, L00
      • 8 layers
      • 700k readout channels
      • 6 m2
      • material:15% X0
    • Forward calorimeters
    • Forward muon system
      • Improved central too
    • Time-of-flight
    • Preshower detector
    • Timing in EM calorimeter
    • Trigger and DAQ


h = 1.0





h = 2.0

h = 3.0

Silicon Vertex Detector





Physics at Tevatron

d detector
DØ Detector
  • Retained from Run I
    • Excellent muon coverage
    • Compact high granularity LAr calorimeter
  • New for run 2:
    • 2 Tesla magnet !
    • Silicon detector
    • Fiber tracker
    • Trigger
    • Readout
    • Forward roman pots
d detector1
DØ Detector

Physics at Tevatron

detector operation
Detector Operation
  • Data taking efficiency about 85%
  • All components working very well:
    • 93% of Silicon detector operates, 82-96% working well
    • Expected to last up to 8 fb-1
detector performances
Good resolution for

track momenta

calorimeter energies


Detector Performances

Physics at Tevatron

particle detected
Particle detected

There are only few particles that we are able to detect at the end of the day





Some light mesons (kaons, pions)

Experimental Techniques in High Energy

processes and cross sections
Processes and Cross Sections


  • Cross section:
    • Total inelastic cross section is huge
      • Used to measure luminosity
  • Rates at e.g. L=1x1032 cm-2s-1:
    • Total inelastic: 70 MHz
    • bb: 42 kHz
    • Jets with ET>40 GeV: 300 Hz
    • W: 3 Hz
    • Top: 25/hour
  • Trigger needs to select the interesting events
    • Mostly fighting generic jets!


Jet ET>20 GeV



Jet ET>40 GeV







triggering at hadron colliders
Triggering at hadron colliders

The trigger is the key at hadron colliders

CDF Detector

Hardware tracking for pT1.5 GeV

1.7 MHz crossing rate

Muon-track matching

42 L1




L1 trigger

Electron-track matching

Missing ET, sum-ET

25 kHz L1 accept

Silicon tracking

Hardware +

Linux PC's

4 L2


L2 trigger

Jet finding, improved Missing ET

Refined electron/photon finding

800 Hz L2 accept

DØ trigger:

L1: 1.6 kHz

L2: 800 Hz

L3: 50 Hz

Linux farm (200)

L3 farm

Full event reconstruction

200 Hz L3 accept


Physics at Tevatron

typical triggers and their usage
Unprescaled triggers for primary physics goals


Inclusive electrons, muons pT>20 GeV:

W, Z, top, WH, single top, SUSY, Z’,Z’

Dileptons, pT>4 GeV:


Lepton+tau, pT>8 GeV:


Also have tau+MET: W->taunu

Jets, ET>100 GeV

Jet cross section, Monojet search

Lepton and b-jet fake rates

Photons, ET>25 GeV:

Photon cross sections, Jet energy scale

Searches (GMSB SUSY)

Missing ET>45 GeV



Prescale triggers because:

Not possible to keep at highest luminosity

Needed for monitoring

Prescales depend often on Lumi


Jets at ET>20, 50, 70 GeV

Inclusive leptons >8 GeV

B-physics triggers

Backup triggers for any threshold, e.g. Met, jet ET, etc…

At all trigger levels

Typical Triggers and their Usage

single electron trigger


Rate= 6 Hz at L=100x1030 cm-2s-1

Physics at Tevatron

trigger operation
Trigger Operation
  • Aim to maximize physics at trigger level:
    • Trigger cross section:
      • Nevent/nb-1
      • Independent of Luminosity
    • Trigger Rate:
      • Cross Section x Luminosity
  • Luminosity falls within store
    • Rate also falls within store
    • 75% of data are taken at <2/3 of peak luminosity
  • Use sophisticated prescale system to optimize bandwidth usage
    • Trigger more physics!
Accelerators and Detectors (Lecture I):

Chain of Particle Accelerator

Tevatron performance




QCD (Lecture I)

Top Pair and ElectroWeak (Lecture 2)

Single Top, Higgs and New Physics beyond the Standard Model (Lecture 3)

Physics at Tevatron

the proton
The Proton
  • It’s complicated:
    • Valence quarks
    • Gluons
    • Sea quarks
  • Exact mixture depends on:
    • Q2: ~(M2+pT2)
    • xBj: fractional momentum carried by parton
  • Hard scatter process:






parton kinematics
Parton Kinematics

pdf’s measured in deep-inelastic scattering

  • Examples:
    • Higgs: M~100 GeV
      • LHC: <xp>=100/14000≈0.007
      • TeV: <xp>=100/2000≈0.05
    • Gluino: M~1000 GeV
      • LHC: <xp>=1000/14000≈0.07
      • TeV: <xp>=1000/2000≈0.5
  • Parton densities rise dramatically towards low x
    • Results in larger cross sections for LHC, e.g.
      • factor ~1000 for gluinos
      • factor ~40 for Higgs
      • factor ~10 for W’s
tevatron vs lhc
Tevatron vs LHC
  • Compare to LHC
    • Cross sections of heavy objects rise much faster, e.g.
      • top cross section
      • Jet cross section ET>100 GeV
  • Relative importance of processes changes
    • Jet background to W’s and Z’s
    • W background to top
    • backgrounds to Higgs
hard qcd processes
Hard QCD Processes

CTEQ6.1 gluon uncertainty

high pT  hard partonic scattering

kinematic plane

  • Sensitive to:
  • strong coupling constant
  • proton’s parton content  unique sensitivity to high-x gluon
  • dynamics of interaction- validity of approximations (NLO, LLA, …)- QCD vs. new physical phenomena
kinematic constraints and variables
Kinematic Constraints and Variables
  • Transverse momentum, pT
    • Particles that escape detection (<3o) have pT≈0
    • Visible transverse momentum conserved ∑I pTi≈0
      • Very useful variable!
  • Longitudinal momentum and energy, pz and E
    • Particles that escape detection have large pz
    • Visible pz is not conserved
      • Not so useful variable
  • Angle:
    • Polar angle  is not Lorentz invariant
    • Rapidity: y
    • Pseudorapidity: 

For M=0

physics objects
Physics Objects


(all flavors)

Heavy Flavor

physics objects1
Physics Objects


(all flavors)


W/Z Bosons

Heavy Flavor

physics objects2
Physics Objects


(all flavors)


W/Z Bosons

Heavy Flavor

Multi-Parton Interactions / Underlying Event

qcd processes and cross section
QCD Processes and Cross section
  • Jet Production
  • Photon Production (+ Jet)
  • Vector Boson + Jet(s)
  • Double Parton Interactions
  • Underlying Event
hadron hadron collisions are messy
Hadron-hadron collisions are messy
  • Energy flow:

project the energy flow

on to the (,) plane

“Lego plot”



Physics at Tevatron

but become simple at high energies
But become simple at high energies

largest high pT cross sectionat a hadron collider

 highest energy reach

  • Jets are unmistakable:

Unique sensitivity to new physics:

- new particles decaying to jets,

- quark compositeness,

- extra dimensions,

- …(?)…




In absence of New Physics

theory @NLO is reliable (±10%)

 Precision phenomenology

- sensitivity to PDFs  high-x gluon- sensitive to

Physics at Tevatron


what are jets
What are jets?





colorless states - hadrons

Fragmentation process


outgoing parton

Hard scatter


  • The hadrons in a jet have small transverse momentum relative to the parent parton’s direction and the sum of their longitudinal momenta is roughly the parent parton momentum
  • Jets are the experimental signatures of quarks and gluons and manifest themselves as localized clusters of energy

Physics at Tevatron

jet triggering
Jet Triggering
  • Unlike most physics at hadron colliders, the principal background for jets is other jets
    • because the cross section falls steeply with ET, lower energy jets mismeasured in ET often have a much higher rate than true high ET jets (“smearing”)
  • Multi-level trigger system with increasingly refined estimates of jet ET
  • Large dynamic range of crosssection demands that many trigger thresholds be used e.g.
    • 15 GeV prescaled 1/1000
    • 30 GeV prescaled 1/100
    • 60 GeV prescaled 1/10
    • 100 GeV no prescale



Factor of ~ 30

rate reduction

Physics at Tevatron

jets from parton to detector level
Jets: from parton to detector level

Quark and gluon jets (identified to partons) can be compared to detector jets, if jet algorithms respect collinear and infrared safety(Sterman&Weinberg, 1977)


  • Jets at the particle (hadron) level
  • Jets at the “detector level”


fragmentation process

outgoing parton

Hard scatter

Particle Shower



The idea is to come up with a jet algorithm which minimizes the non-perturbative hadronization effects

fragmentation process

outgoing parton

Hard scatter

Physics at Tevatron

jet algorithms
Jet Algorithms
  • The goal is to be able to apply the “same” jet clustering algorithm to data and theoretical calculations without ambiguities.
  • Jets at the “Parton Level”
    • i.e., before hadronization
    • Fixed order QCD or (Next-to-) leading logarithmic summations to all orders
  • Traditional Choice at hadron colliders: cone algorithms (jetclu, siscone)
    • Jet = sum of energy within R2 = 2 + 2
  • Traditional choice in e+e–: successive recombination algorithms
    • Jet = sum of particles or cells close in relative kT

Physics at Tevatron

jet energy calibration
Jet Energy Calibration

1. Establish calorimeter stability and uniformity

  • pulsers, light sources
  • azimuthal symmetry of energy flow in collisions
  • muons

2. Establish the overall energy scale of the calorimeter

  • Testbeam data
  • Set E/p = 1 for isolated tracks
    • momentum measured using central tracker
  • EM resonances (0 , J/,  and Z  e+e–)
    • adjust calibration to obtain the known mass

3. Relate EM energy scale to jet energy scale

  • Monte Carlo modelling of jet fragmentation + testbeam hadrons
    • CDF
  • ET balance in jet + photon events

Physics at Tevatron

why measure cross sections
Why Measure Cross Sections?
  • They test QCD calculations
    • They help us to find out content of proton:
      • Gluons, light quarks, c- and b-quarks
    • A cross section that disagrees with theoretical prediction could be first sign of new physics:
      • E.g. quark substructure (highest jet ET)

Physics at Tevatron

why measure cross sections1
Why Measure Cross Sections?
  • They test QCD calculations
    • They help us to find out content of proton:
      • Gluons, light quarks, c- and b-quarks
    • A cross section that disagrees with theoretical prediction could be first sign of new physics:
      • E.g. quark substructure (highest jet ET)
  • They force us to understand the detector

Physics at Tevatron

why measure cross sections2
Why Measure Cross Sections?
  • They test QCD calculations
    • They help us to find out content of proton:
      • Gluons, light quarks, c- and b-quarks
    • A cross section that disagrees with theoretical prediction could be first sign of new physics:
      • E.g. quark substructure (highest jet ET)
  • They force us to understand the detector
  • No one believes us anything without us showing we can measure cross sections

Physics at Tevatron

luminosity measurement
Luminosity Measurement
  • Measure events with 0 interactions
    • Related to Rpp
  • Normalize to measured inelastic pp cross section
    • Measured by CDF and E710/E811
    • Differ by 2.6 sigma
    • For luminosity normalization we use the error weighted average
    • CDF and DØ use the same
    • Unlike in Run 1…


pp (mb)


inclusive jets
Inclusive Jets

pT (GeV)

pT (GeV)

Phys. Rev. Lett. 101, 062001 (2008)

Phys. Rev. D 78, 052006 (2008)

  • benefit from:
  • high luminosity in Run II
  • increased Run II cm energy  high pT
  • hard work on jet energy calibration

steeply falling pT spectrum:

1% error in jet energy calibration

 5—10% (10—25%)

central (forward) x-section

inclusive jets1
Inclusive Jets
  • high precision results
  • consistency between CDF/D0
  • well-described by NLO pQCD
  • experimental uncertainties: smaller than PDF uncertainties!!
  •  sensitive to distinguish between PDFs


  • data are used in PDF fits:
  • included in MSTW2008 PDFs
  • at work: forthcoming CTEQ PDFs

pT (GeV)

strong coupling constant
Strong Coupling Constant



inclusive jet cross section is sensitive to

previous CDF result from Run I: PRL88, 042001 (2002)

strong coupling constant1
Strong Coupling Constant
  • From 22 (out of 110) inclusive jet cross section data points at 50<pT<145 GeV
  •  Exclude data points with
  • - NLO + 2-loop threshold corrections
  • - MSTW2008NNLO PDFs
  • - Extend results from HERA to high pT
dijet mass distribution
Dijet Mass Distribution
  • central dijet production |y|<1
  • test pQCD predictions
  • sensitive to new particles decaying into dijets: excited quarks, Z’, W’, Randall-Sundrum gravitons, color-octet, techni-rho, axigluons, colorons

Many extensions of the Standard Model (motivated by the generational structure and mass hierarchy) predict resonances in the dijet mass spectrum.



Search for new particles decaying to dijets, continued

Results: the most stringent lower mass limits available on excited quark1, axigluon2, flavor-universal coloron3, E6 diquark4, and color-octet techni-ρ5.

Excluded mass limits (GeV):

q* 260-870 axigluon, coloron 260-1250 E6 diquark 290-630 ρT8 260-1100 W'6 280-840 Z'6 320-740

1 PRD 42, 815 (1990). 2 Phys. Lett. B 190, 157 (1987); PRD 37, 1188 (1988). 3 Phys. Lett. B 380, 92 (1996); PRD 55, 1678 (1997). 4 Phys. Rept. 183, 193 (1989). 5 PRD 44, 2678 (1991); PRD 67, 115011 (2003). 6Rev. Mod. Phys. 56, 579 (1984); Rev. Mod. Phys. 58, 1065 (1986).

dijet mass distribution1
Dijet Mass Distribution

Like the inclusive jet cross section, the dijetmass cross section is sensitive to new physics and can constrain the PDF's.

  • CTEQ6.6 prediction too high
  • MSTW2008 consistent w/ data(but correlation of experimental and PDF uncertainties!)
  • measurement systematic ~PDF uncertainty: constraints on future predictions
jets bosons w z

Physics at Tevatron

direct photon production
Direct Photon Production

(all quark/anti-quark


direct photons emerge unaltered from the hard subprocess

 direct probe of the hard scattering dynamics

 sensitivity to PDFs (gluon!) …but only if theory works

also fragmentation contributions:

suppress by isolation criterion

 observable: isolated photons

photon identification
Photon Identification
  • Essentially every jet contains one or more 0 mesons which decay to photons
    • therefore the truly inclusive photon cross section would be huge
    • we are really interested in direct (prompt) photons (from the hard scattering)
    • but what we usually have to settle for is isolated photons (a reasonable approximation)
      • isolation: require less than e.g. 2 GeV within e.g. R = 0.4 cone
  • This rejects most of the jet background, but leaves those (very rare) cases where a single 0 or  meson carries most of the jet’s energy. This happens perhaps 10–3 of the time, but since the jet cross section is 103 times larger than the isolated photon cross section, we are still left with a signal to background of order 1:1.
  • Photon candidates: isolated electromagnetic showers in the calorimeter, with no charged tracks pointed at them
    • what fraction of these are true photons?


  • Experimental techniques
  • DØ measures longitudinal shower development at start of shower
  • CDF measures transverse profile at start of shower (preshower detector) and at shower maximum





Shower maximum


incl isolated photons
Incl. Isolated Photons

Phys. Lett. B 639, 151 (2006)

pTg (GeV)

pTg (GeV)

  • CDF and D0 measurements: 20< pT <400GeV  agreement
  • data/theory: difference in low pT shape
  • experimental and theory uncertainties > PDF uncertainty no PDF sensitivity yet
  • first: need to understand discrepancies in shape
isolated photon jet
Isolated Photon + Jet

L = 1 fb-1

  • tag photon and jet  reconstruct full event kinematics
  • measure in 4 regions of yg / yjet- photon: central - jet: central / forward - same side / opposite side

Phys. Lett. B 666, 2435 (2008)

  • discrepancies in data/theory
  • investigate source for disagreement
  • measure more differential cross section
  • reduces uncertainties by cancellations
  • , but disagreement persists
  •  figure out what is missing…
  • higher orders?
  • resummation?
  • …???

pTg (GeV)

This probes the gluon distribution, and generally the dynamics of hard QCD interactions, over a range of x and Q2 through qg→qγ and qq→gγ

Explores 0.007 ≤x ≤0.8 and 900 ≤ Q2, i.e. (pTγ)2 ≤ 1.6 x 105 GeV2

isolated photon hf jet
Isolated Photon + HF Jet

Phys. Rev. Lett. 102, 192002 (2009)

Photon + (b/c) jet + X

Photon pT : 30-150 GeV

0.01<x<0.3  b, c, gluon PDF

 test gluon splitting contribution

tag photon and jet


 triple differential

vector boson jet
Vector Boson + Jet





  • relevant to other high-multiplicity processes
  • background to Higgs
  • test “matched” predictions  critical to
  • Tevatron / LHC physics
  • Provide detailed measurements of pT and angular distributions of vector boson and V+jets
  • test perturbative QCD calculations
  • testing and tuning of phenomenological models
  • Final state for many important signatures:
    • Top, Higgs, WH, Leptquarks, Z´..
  • Low PT (Z)
    • Test gluon resumation
    • Good calibration sample

MCFM: NLO pQCD V + <=2 partons

Current event generators: Tree level matrix element + parton shower matching schemes to avoid double counting jets from ME and PS

MLM matching (Alpgen, MadEvent, Helac)

CKKW scheme (Sherpa)

Dipole Cascade (Ariadne)

Contain (~untuned) internal parameters ( arXiv:0706.2569v1 hep-ph )

Alpgen is the main generator at CDF/DØ Pythia or Herwig used for showering

w and z bosons
W and Z Bosons
  • Focus on leptonic decays:
    • Hadronic decays ~impossible due to enormous QCD dijet background
  • Selection:
    • Z:
      • Two leptons ET>20 GeV
        • Electron, muon, tau
    • W:
      • One lepton ET>20 GeV
      • Large imbalance in transverse momentum
        • Missing ET>20 GeV
        • Signature of undetected particle (neutrino)
  • Excellent calibration signal for many purposes:
    • Electron energy scale
    • Track momentum scale
    • Lepton ID and trigger efficiencies
    • Missing ET resolution
    • Luminosity …
lepton identification
Lepton Identification
  • Electrons:
    • compact electromagnetic cluster in calorimeter
    • Matched to track
  • Muons:
    • Track in the muon chambers
    • Matched to track
  • Taus:
    • Narrow jet
    • Matched to one or three tracks
  • Neutrinos:
    • Imbalance in transverse momentum
    • Inferred from total transverse energy measured in detector
electrons and jets
Electrons and Jets

Hadronic Calorimeter Energy

  • Jets can look like electrons, e.g.:
    • photon conversions from 0’s: ~13% of photons convert (in CDF)
    • early showering charged pions
  • And there are lots of jets!!!

Electromagnetic Calorimeter Energy

the isolation cut
Isolation is very powerful for isolated leptons

E.g. from W’s, Z’s

Rejects background from leptons inside jets due to:


photon conversions

pions/kaons that punch through or decay in flight

pions that shower only in EM calorimeter

This is a physics cut!

Efficiency depends on physics process

The more jet activity the less efficient

Depends on luminosity

Extra interactions due to pileup

Isolation cut:

Draw cone of size 0.4 around object

Sum up PT of objects inside cone

Use calorimeter or tracks

Typical cuts:

<10% x ET

<2-4 GeV

The Isolation Cut

 candidates

Non-isolated isolated

Physics at Tevatron

electron identification
Electron Identification
  • Desire:
    • High efficiency for isolated electrons
    • Low misidentification of jets
  • Cuts:
    • Shower shape
    • Low hadronic energy
    • Track requirement
    • Isolation
  • Performance:
    • Efficiency:
      • Loose cuts: 86%
      • Tight cuts: 60-80%
      • Measured using Z’s
    • Fall-off in forward region due to limited tracking efficiency
muon identification
Muon Identification
  • Desire:
    • High efficiency for isolated muons
    • Rejection of background due to punch-through etc.
  • Typical requirements:
    • Signal in muon chamber
    • Isolation
    • Low hadronic and electromagnetic energy
      • Consistent with MIP signal
  • Efficiency 80-90%
  • Coverage:
    • DØ: Up to ||=2
    • CDF: up to ||=1.5
jets faking electrons
Jets faking Electrons
  • Jets can pass electron ID cuts,
    • Mostly due to
      • early showering charged pions
      • Conversions:0ee+X
      • Semileptonic b-decays
    • Difficult to model in MC
      • Hard fragmentation
      • Detailed simulation of calorimeter and tracking volume
  • Measured in inclusive jet data at various ET thresholds
    • Prompt electron content negligible:
      • Njet~10 billion at 50 GeV!
    • Fake rate per jet:
      • Loose cuts: 5/10000
      • Tight cuts: 1/10000
    • Typical uncertainties 50%

Jets faking “loose” electrons

Fake Rate (%)

z 1 2 or 3 jets p t jet
Z + (1, 2 or 3) jets pT -jet
  • Measurement of 1st, 2nd and 3rd jet pT in Z events:
  • normalize to inclusive Z production (cancel some uncertainties)
  • compare to pQCD @ LO / NLO

Inclusive jet pT spectrum

  • Events are binned in the pT of the Nth jet. Data agree well with NLO-MCFM but diverge from PYTHIA, HERWIG increasingly with pTjet and #jets.
  • pT-ordered PYTHIA describes leading jet well.
z b jet e t jet p t z
Z + b jet  ET -jet / pT -Z

W+b jet

  • Important background for the higgs search, cross section have information about the b content of the proton
  • Rare processes:
  • now we have statistics for differential distributions!
  • with 12 fb-1
  • will provide constraints on models

σ b-jets (W+b-jets) ⋅ BR(W → l v) =

2.74 ± 0.27 (stat) ± 0.42(syst) pb

underlying event studies
Underlying Event Studies

goal: improve understanding and modeling of high energy collider events

define 3 regions in an event,

based on the leading jet

  • “toward”
  • “away”
  • “transverse”
  • “transverse” region  very sensitive to underlying event
  • study (in all regions)
  • charged particle density
  • pT sum density
  • ET sum density
underlying event in drell yan and jet production
Underlying Event in Drell-Yanand Jet Production

 charged pT sum density

comparison of three regions in DY:

  • “away” region: pT density increases with lepton pair pT
  • “transverse”, “toward” regions: pT density flat with lepton pair pT
  • comparison of “transverse” regionbetween jets and DY
  • similar trend in both
  • tuned PYTHIA describes data
underlying event in drell yan and jet production1
Underlying Event in Drell-Yanand Jet Production

 charged pT sum density

comparison of three regions in DY:

  • “away” region: pT density increases with lepton pair pT
  • “transverse”, “toward” regions:pT density flat with lepton pair pT

… plus corrected results for many other distributions are provided

 Important sources for MC tuning!

  • comparison of “transverse” regionbetween jets and DY
  • similar trend in both
  • tuned PYTHIA describes data
inclusive jets tevatron vs lhc
Inclusive Jets: Tevatron vs. LHC

PDF sensitivity:

  • compare jet cross section at fixed xT = 2 pT / sqrt(s)

Tevatron (ppbar)

>100x higher cross section @ all xT

>200x higher cross section @ xT >0.5

LHC (pp)

  • need more than 2400 fb-1 luminosityto improve Tevatron@12 fb-1
  • more high-x gluon contributions
  • but more steeply falling cross highest pT (=larger uncertainties)

 Tevatron results will dominate high-x gluon for some years

  • Tevatron is world’s highest energy collider today
    • Large datasets with L=5 fb-1 now available in Run II
    • CDF and DØ detectors operate well
      • Powerful tracking
      • Good calorimeter coverage
      • Good lepton identification
  • A hadron machine provides a challenging environment
  • underlying event / multiple parton interactions
    • strong constraints: tune/improve phenomenological models
  • Z/W + jet production (pT spectra :: angular distributions)
    • many distributions for pQCD tests and for model tuning
  • photon production (inclusive :: plus jet :: plus HF jet)
    • need to find missing pieces in theory
  • jet production (inclusive pT :: dijet mass :: dijet angle)
    • first look into physics in the TeV regime
    • strongest constraints on high-x gluon – for some time

Physics at Tevatron

jets faking muons
Jets faking Muons

Tracks faking muons

  • Jets can pass muon ID cuts,
    • Mostly due to
      • Pions punching through
      • Pions or kaons decaying in flight:
        • K±±, ± ± 
      • Semileptonic b-decays
    • Difficult to model in MC
      • Hard fragmentation
      • Detailed simulation of calorimeter and tracking volume
  • Measured in inclusive jet data at various ET thresholds
    • Prompt muon content negligible
    • Fake rate per loosely isolated track:
      • Cannot measure per jet since isolated muon is usually not a jet!
      • 2/1000
    • Typical uncertainties 50%

Fake Rate (%)

a few comments on monte carlo
A Few Comments on Monte Carlo
  • Critical for understanding theacceptance and the backgrounds
    • Speed: CDF ~10s per event, DØ ~ 3m per event
  • Two important pieces:
    • Physics process simulation:
        • Working horses but limitations at high jet multiplicity
      • “ME generators”: ALPGEN, MADGRAPH, SHERPA, COMPHEP,…
        • Better modeling at high number of jets
        • Some processes only available properly in dedicated MC programs
          • e.g. W or single top
      • NLO generators (MC@NLO)
        • Not widely used yet but often used for cross-checks
    • Detector simulation:
      • GEANT, fast parameterizations (e.g. GFLASH)
  • Neither physics nor detector simulation can generally be trusted!
      • Most experimental work goes into checking Monte Carlo is right

Physics at Tevatron

diphoton cross section
Diphoton Cross Section
  • Select 2 photons with ET>13 (14) GeV
  • Statistical subtraction of background
    • mostly p0
  • Data agree well with NLO
  • PYTHIA describes shape
    • normalization off by factor two
parton kinematics1
Parton Kinematics
  • For M=100 GeV: probe 10 times higher x at LHC than Tevatron
theoretical requirements
Theoretical requirements
  • Infrared safety
    • insensitive to “soft” radiation
  • Collinear safety
  • Low sensitivity to hadronization
  • Invariance under boosts
    • Same jets solutions independent of boost
  • Boundary stability
    • maximum ET = s/2
  • Order independence
    • Same jets at parton/particle/detector levels
  • Straightforward implementation

Physics at Tevatron

experimental requirements
Experimental requirements

Effect of pileup on Thrust

kT algorithm jets, ET > 30 GeV


  • Detector independence
    • can everybody implement this?
  • Best resolution and smallest biases in jet energy and direction
  • Stability
    • as luminosity increases
    • insensitive to noise, pileup and small negative energies
  • Computational efficiency
  • Maximal reconstruction efficiency
  • Ease of calibration
  • ...

Physics at Tevatron

dijet angular distribution
Dijet Angular Distribution
  • variable:
  • at LO, related to CM scattering angle
  • flat for Rutherford scattering
  • slightly shaped in QCD
  • new physics, like - quark compositeness - extra spatial dimensions enhancements at low

large y

small y

dijet angular distribution1
Dijet Angular Distribution
  •  normalized distribution
  • reduced experimental
  • and theoretical uncertainties

Measurement for dijet masses

from 0.25 TeV to >1.1 TeV

dijet angular distribution new physics limits
Dijet Angular DistributionNew Physics Limits
  • At highest possible energy:
  • Probing quark substructure
  • Sensitive to extra spatial dimensions- virtual exchange of KK excitation of graviton (ADD LED) - virtual KK excitation of gluon (TeV-1 ED)

CDF: D0:

  • detector-level comparison of data and PYTHIA: study ratio
  • R = N( <15)/N( >15)
  • for 550 < Mjj < 950 GeV
  • From pseudo experiments: Feldman Cousins limits @95%CL
  • Quark Compositeness Λ > 2.4 TeV
  • Use full shape
  • of corrected data
  • Bayesian and methods @95%CL
  • Quark Compositeness Λ > 2.9TeV
  • ADD LED (GRW) Ms > 1.6 TeV
  • TeV-1 ED Mc > 1.6 TeV

all: most stringent limits!

z 1 2 3 jets p t jet
Z + (1, 2, 3) jets  pT -jet

Ratios of data and different MC generators

 favor ALPGEN w/ low scale

Leading jet in Z + jet + X

Second jet in Z + 2jet + X

Third jet in Z + 3jet + X



Measure the particle energy. High density and often they are stopped here

Generic detector

Vertex detector

Measure the position of the charge particle near the interaction point


Solenoid (Magnets


Muon chambers

(Curve particle inside the charge field, we can measure the momentum of the particle)


Hadronic calorimeter



Electromagnetic calorimeter


Tracking detector

Tracking detectors

Measure the position of the charge particle and measure the particle momentum


Interaction of different particles with the same high energy (here 300 GeV) in a big block of iron:


The energetic electron radiates photons

which convert to electron-positron pairs

which again radiate photons

which ... This is the electromagnetic shower.


The energetic muon causes mostly just the

ionization ...


pion (or another hadron)

Electrons and pions with their “children” are almost completely absorbed in the sufficiently large iron block.

The strongly interacting pion collides with an iron nucleus,

creates several new particles which interact again with iron nuclei,

create some new particles ... This is the hadronic shower.

You can also see some muons from hadronic decays.

Experimental Techniques in High Energy