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Jet Physics at the Tevatron
Lee Sawyer
Louisiana Tech University
On Behalf of the CDF & D0 Collaborations
July 28, 2004
"Jet Physics at the Tevatron" Lee Sawyer 2
Jet Physics at the Tevatron
• Why do QCD at the Tevatron?• The Experiments • From Detector Signals to Partons• Some Results
– From CDF:• Jet cross-sections: Cone jets and kT jets
• Jet Shapes
– From DØ:• Inclusive jet cross-section• Dijet Cross-Section• Dijet Azimuthal Decorrelations
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"Jet Physics at the Tevatron" Lee Sawyer 3
The Fermilab Tevatron
• pp Collider at √s = 1.96 TeV (increased from 1.8 TeV in Run I)
• Main Injector replaces the old “Main Ring”
• Other improvements – p source
– Recycler
– electron cooling
aimed at improving p beam lifetime, increase luminosity.
• Increasing luminosity:– Run I (1992-95) ~0.1 fb-1
– Run IIa (2001~2005) ~1 fb-1
– Run IIb (2006-2009) ~4-8 fb-1
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Tevatron Performance
0
1
2
3
4
5
6
7
8
9
10
9/29/03 9/29/04 9/30/05 10/1/06 10/2/07 10/2/08 10/3/09
Start of Fiscal Year
Inte
grat
ed L
umin
osit
y (
fb-1
)
Design Projection
Base Projection
Design Base Fiscal Year (fb-1) (fb-1) FY03 0.33 0.33 FY04 0.64 0.56 FY05 1.2 0.93 FY06 2.7 1.4 FY07 4.4 2.2 FY08 6.4 3.3 FY09 8.5 4.4
Instantaneous luminosities approaching 1032 cm-2 s-1
On 16 July, store# 3657 B0Lum 110.16 x 1030 cm-2 s-1 D0Lum 91.32 x 1030 cm-2 s-1
Integrated luminosity around 400 pb-1 per experiment.
Should reach 1fb-1 by 2005 shutdown.
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The CDF Detector
• From Run I– Central Calorimeter – Solenoid– Muon System (with
Upgrades)• For Run II
– Plug and MiniPlug Calorimeters
– TOF and central Drift Chamber
– Silicon Microstrip Tracker– Forward Muon Detectors
=> Calorimeter coverage extended (||<3.6) while maintaining excellent tracking and vertex resolution.
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CDF Data Taking Performance
Efficiency > 80%Around 450 pb-1 on tape
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The DØ Detector
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DØ Data Taking Performance
Around 400 pb-1 recorded
Efficiency regularly above 85%.
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QCD at the Tevatron
– Higher √s means higher cross-sections
– Probe proton structure at large x– Test pQCD with increased
statistics– Search for high mass states (Z’,
W’, compsiteness, etc.)– QCD signals form the primary
background to most of the other measurements at the Tevatron
Inclusive jet spectrum
x2
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"Jet Physics at the Tevatron" Lee Sawyer 10
What Is a Jet?
• (For details on jet algorithms, see talk by Bernard Andrieu.)
• DØ Run II cone algorthim w/ Rcone=0.7, pT
min=8 GeV/c, f=50%
– Any “particle” (MC, cal tower, etc) used as a seed.
– Make a cone in R=√()2+()2 < Rcone around seed
– Add particle 4-vectors in cone => “proto-jet”
– Draw new cone around proto-jet, iterate until stable solution found => cone axis = jet axis
– Remove proto-jets w/ pT,<pTmin.
– Merge jets if more than overlap fraction f of pT
jet is contained in the overlap region; otherwise split jets.
– Use midpoints between pairs of jets as seeds.
• CDF JETCLU algorthim w/ Rcone=0.7– Adds ET’s of cluster’s in cone
(“Snowmass”)– Does not use midpoints between pairs of
jets as seeds.
• kT algorthim– Not a “fixed cone” algorithm– Use relative momenta of
particles, merge by pairs.– Dmin =
min(pT2(i),pT
2(j)) Ri,j/D– D = Jet Size parameter.
Cone jetkT jet
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"Jet Physics at the Tevatron" Lee Sawyer 11
From Detector Signals to Partons
• We do not see quarks and gluons– We do not really see ,K,p,,etc
• How do we go from calorimeter ADC counts to p of the outgoing partons? => Jet Energy Scale
• Factors impacting the JES include– Energy Offset (i.e. energy not from the hard
scattering process)– Detector Response
• For DØ, EM energy scale determined from Z→ee. Use pT balance in +jets, measured linearity corrections from calorimeter calibration. Extrapolate for very high pT
– Out-of-Cone showering – Resolution => Unsmearing
• Energy Scale uncertainties typically are the largest systematic errors in jet measurements.
q
Tim
e
p p
q g
K
“par
ton
jet”“
part
icle
jet”“
calo
rim
eter
jet”
hadrons
CH
FH
EM
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Inclusive Jet Cross-Section
Extends the Run I CDF measurement by approx. 150 GeV
Run I/Run II comparison plot includes 3% energy scale uncertainty band. Luminosity uncertainites not included.
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Inclusive Jet Cross-Section as a Function of Rapidity
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Inclusive Jet Cross-Section as a Function of Rapidity
Increased uncertainty in PDFs in forward region
Good agreement between theory and data at all rapidities
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KT Jet Cross-Section
• CDF measures inclusive jet production using the KT algorithm
• Jets in the region 0.1 < |Y| < 0.7 and
• PT > 72 GeV/c.
• Results based on 145 pb-1
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KT Cross-Section vs D
Data diverges from NLO prediction as D gets large, due to soft contributions
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Dijet Cross-Section
• DØ measures the cross-section for dijet production in three rapidity bins – 0<Y<0.5
– 1.5<Y<2.0
– 2.0<Y<2.4
• d/dMjj measured for central rapidities
• Good agreement between data and NLO pQCD
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Dijet Cross-Section
Data/Theory comparison for central rapidities
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Dijet Mass Spectrum
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Nifty Pictures I: Highest Mass Dijet Event From DØ
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"Jet Physics at the Tevatron" Lee Sawyer 21
Nifty Pictures II: Highest Mass Dijet Event From CDF
ET = 666 GeV
= 0.43
ET = 633 GeV
= -0.19
Dijet Mass = 1364 GeV(probing distance ~10-19 m)
CDF (-r view)
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Dijet Azimuthal Decorrelations
• Jet separation in is sensitive to final state radiation.
• At LO, • At higher order, a hard third
jet (k┴>0) leads to .– Measuring the dijet
spectrum tests O(s3)
predictions– No need to explicitly measure
third or greater jet
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Dijet Azimuthal Decorrelations
• Use central inclusive dijet sample– Data binned in pT of the
leading jet
• Normalize cross-section for each pT-bin to inclusive cross-section
• Only look at to avoid overlap region between jets
• Hard leading jets have pT spectra more sharply peaked near .
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Dijet Comparisons
• Comparison to fixed-order pQCD predictions
• Leading order (dashed blue curve) – Divergence at ΔΦ = (need soft processes)– No phase-space at ΔΦ<2/3 (only three partons)
• Next-to-leading order (red curve)– Good description over the
whole range, except in extreme ΔΦ regions
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Dijet Comparisons
• Comparison to Monte Carlo predictions
• Herwig 6.505 (default)– Good overall description!
– Slightly too high in mid-range
• Pythia 6.223 (dash line=default)– Very different shape
– Too steep dependence
– Underestimates low ΔΦ
– Vary PARP(67) = 1 → 4• Varies ISR• Radiation starts for Q*PARP(67)
– With more ISR, closer agreement to data.
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Jet Shapes
• CDF looks at the fraction of a jet ET within a subcone– Define = ET(r)/ET(R)
– Energy flow variable
• Sensitive to multiple gluon emission from the primary parton
• Also sensitive to underlying event.
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Jet Shapes
Study uses Midpoint Algorithm w/ R=0.7. is corrected to the hadron level.
Central jets: Low pT High pT
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Jet Shape vs pT
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Lagniappe
• In addition to the study of high pT QCD, there is a rich program of diffractive studies and elastic scattering measurements at both experiments– About 40% of pp total cross-section is elastic or diffractive.– Portions of upgrades designed to enhance this capability
• See talk by Mary Convery on Saturday for “Diffractive Results from CDF”
• Want to mention a few low pT results from DØ
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DØ Detector Details
• In addition to Calorimeter, can tag interaction with luminosity monitors near beampipe.
• Forward Proton Detector added to elastic scattering measurements
– Series of 18 Roman Pots arranged in 9 spectrometers
– Now fully commissioned and part of the DØ readout.
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Diffractive Z Production
• Define a “rapidity gap” between calorimeter and luminosity monitors
• In Run I, identified a handful of events consistent with W→e and Z→ee with associated rapidity gap
• In Run II, have looked for Z→ plus forward rapidity gap.
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Diffractive Z Production
“North” = Negative Rapidities
“South” = Positive Rapidities
Evidence for diffractive Z production, mass consistent.
More work to be done (backgrounds, efficiencies,..)
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Elastic Scattering
• First results from Forward Proton Detector
• Measurement of ξ = Fraction of proton longitudinal momentum lost in the scattering
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Conclusions
• Rich range of QCD topics to be pursued in Run II• First results from both experiments show generally
good agreement with theory for cross-sections• More detailed comparisons to theory needed for
details of event and jet shapes.• First DØ being produced for diffractive and elastic
physics• Both experiments will be able to explore high pT and
Mjj regions over a wide range of rapidities– Test high-x gluon contributions– Look for evidence of new physics