Jet Physics at CDF Sally Seidel University of New Mexico APS’99 24 March 1999.
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Transcript of Jet Physics at CDF Sally Seidel University of New Mexico APS’99 24 March 1999.
![Page 1: Jet Physics at CDF Sally Seidel University of New Mexico APS’99 24 March 1999.](https://reader035.fdocuments.net/reader035/viewer/2022070414/5697bffc1a28abf838cc1ad5/html5/thumbnails/1.jpg)
Jet Physics at CDF
Sally SeidelUniversity of New Mexico
APS’99
24 March 1999
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1. Jets at CDF
2. The Inclusive Jet Cross Section
3. The Dijet Mass Cross Section
4. The Differential Dijet Cross Section
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CDF: A multi-purpose detector for studying hadronic collisions
at the Fermilab Tevatron:
TeV 8.1at spp
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The motivation:
Jet distributions at colliders can:
• signal new particles
• test QCD predictions
• check parton distribution functions
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The data:
CDF reconstructs jets using an iterative cone algorithm with cone radius
7.0)()( 22 R
Jet energies are corrected for
• calorimeter non-linearity
• uninstrumented regions
• contributions from spectator partons
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The iterative cone algorithm:
•Examine all calorimeter towers with ET > 1 GeV.
•Form preclusters from continuous groups of towers with monotonically decreasing ET.
•If a tower is outside a window of 7 x 7 towers from the seed of its cluster, start a new precluster with it.
•For each precluster, find the ET-weighted centroid with R = 0.7.
•Define the centroid to be the new cluster axis.
•Save all towers with ET > 100 MeV within R = 0.7 about the new axis.
•Iterate until the tower list is stable.
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The Inclusive Jet Cross Section
• For jet transverse energies in the range 40 < ET < 440 GeV: this probes distances down to 10-17 cm.
• The analysis:– For luminosity (88.8 ± 4.1) pb-1 – Trigger on jet-like events: accept 4
triggers with uncorrected ET thresholds at 20, 50, 70, and 100 GeV; correct for pre-scaling
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– Apply data quality requirements:zvertex< 60 cm to maintain projective
geometry of calorimeter towers
• 0.1 < |detector| < 0.7 for full containment of energy in central barrel
• Etotal < 1800 GeV to reject accelerator loss events
• Define ET = Esin and = missing ET. Require
to reject cosmic rays
– Correct (“unsmear”) observed ET for energy degradation and calorimeter resolution
6
all
T
T
E
ETE
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• Calculate the cross section:
TT E
N
LddE
dd
111
where
N = number of events
L = luminosity
range is 1.2
and ET bins have width 5 - 80 GeV
• Compare to EKS (Ellis, Kunszt, Soper) NLO calculation with CTEQ4M pdf and renormalization/factorization scale = ET
jet/2
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Systematic uncertainties (all uncorrelated)
on the inclusive jet cross section:
i. Calorimeter response to high-pT charged hadrons
ii. Calorimeter response to low-pT charged hadrons
iii. Energy scale stability (1%)
iv. Jet fragmentation model used in the simulation
v. Energy of the underlying event in the jet cone (30%)
vi. Calorimeter response to electrons + photons
vii. Modelling of the jet energy resolution function
viii. Luminosity (4.1%)
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The Dijet Mass Cross Section
•Many classes of new particles have a larger branching fraction to just 2 partons than to modes containing a lepton or a W/Z…so this can be a powerful way to search for new particles.
•The analysis:
•For luminosity (85.9 ± 4.1) pb-1
•Trigger on jet-like events
•Select events with 2 jets, both with |event| < 2.
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•Define * (1-2)/2, then require e2|*| < 5. This is the same as |cos*| = |tanh *| < 2/3 where * is the Rutherford scattering angle:
•Apply the data quality cuts.
•Correct for trigger efficiency, |zvertex| cut
efficiency, resolution, and calorimeter effects.
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•Define the dijet mass:
•Calculate the cross section:
where:
N = number of events, corrected for prescaling
L = luminosity
Mjj = 10% mass bins (consistent with detector resolution)
•Compare to JETRAD (Giele, Glover, Kosower) NLO calculation with CTEQ4M + = ET
max/2. Two partons are merged if they are within Rsep = 1.3 R.
jjjj M
N
LdM
d
1
)]cos()[cosh(2
)()(
21
221
221
TT
jj
EE
ppEEM
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The dijet mass cross section compared to JETRAD with
CTEQ4M:
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Compare results to data + JETRAD with other pdf’s:
Changing from 0.5 ETmax to 0.25 ET
max changes the normalization by 25%.
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Compare CDF and D0 results for CTEQ4M
(D0 examines || < 1 with no requirement on cos*)
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Systematic uncertainties on the dijet mass cross section (17-34%, asymmetric + ET-dependent):
• Absolute energy scale (14-31%):
•Calorimeter calibration: 1.3-1.8% over the ET range
• Jet fragmentation model: 1.2-1.7% over the ET range
•Calorimeter stability: 1% of E
•Energy of the underlying event: 1 GeV
• Unsmearing:
•Parameterization of the resolution function: 1-9% depending on Mjj
•Variation between analytic and MC procedure: ±4%
•Detector simulator energy scale: 2-8%
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•Relative jet energy scale (5-9% depending on Mjj and considering all instrumented regions):
•Other uncertainties:
•luminosity: 4.1%
•prescale factors: 1.7-3.5% depending on trigger used.
•|zvertex| cut efficiency: 1%
•trigger efficiency: < 1% depending on the statistics of the turn-on region of the trigger.
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The Dijet Differential Cross Section
•The rapidity dependence of the cross section probes the parton momentum fractions.
•The analysis:
•For luminosity (86.0 ± 4.1) pb-1
•Trigger on jet-like events; select events with 2 jets
•Apply data quality cuts
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•Order the jets by ET. Define:
•The “leading jet”: with highest ET. Require that it has 0.1 < |1| < 0.7 and ET1 > 40 GeV.
•The “probe jet”: with second highest ET. Require that it has ET2 > 10 GeV.
•Correct jet energies for calorimeter effects; require ET1 > 35 GeV.
•Classify events according to probe jet , 2:
0.1 < |2| < 0.7
0.7 < |2| < 1.4
1.4 < |2| < 2.1
2.1 < |2| < 3.0
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•Correct (“unsmear”) measured
•Correct for trigger efficiency, prescale, and vertex-finding efficiency
•For events in each of the 4 2 classes, calculate the cross section:
N = number of events, corrected for prescale
L = luminosity
ET1 bins are consistent with detector resolution
•Compare to JETRAD for 3 pdf’s + = ET
max/2
1
1
TT E
N
LdE
d
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Sources of systematic errors on the dijet differential cross section:
Same as for inclusive cross section + resolution
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Probing the high-x, high-Q2 regime:
Notice that for a two-body process,
and
so these data examine a range in (x,Q2)
including that where an excess was observed at HERA:
)( 21 ees
Ex T
)tanh1(cosh2
)cos1(2
ˆ
ˆ
**22
*
2
TE
s
tQ
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