Details of Jet Reconstruction and Their Impact on Jet Quenching ...

<strong>Details</strong> <strong>of</strong> <strong>Jet</strong> <strong>Reconstruction</strong>
<strong>and</strong> <strong>Their</strong> <strong>Impact</strong> on <strong>Jet</strong>
Quenching Measurements
ATL-PHYS-SLIDE-2009-064
21 April 2009
Nathan Grau
On Behalf <strong>of</strong> the ATLAS
Collaboration
High-pT at LHC '09
02/06/2009
1

<strong>Jet</strong> Results Are Here
J. Putschke (this conference)
High-pT at LHC '09
02/06/2009
Y.S. Lei (DNP’08)
PHENIX
2

ATLAS
NG (QM’08)
ALICE
J. Harris
(WWND
‘07)
With More To Come…
High-pT at LHC '09
02/06/2009
CMS
C. Rol<strong>and</strong>
(High pT at LHC ‘06)
3

Outline
• Nuts <strong>and</strong> bolts talk about what I’ve learned in
preparation for ATLAS HI jet reconstruction
– Biggest lesson: Cannot just run “st<strong>and</strong>ard” algorithms
“out <strong>of</strong> the box”
• Two specific issues will be discussed
– Fake jets from cone jet reconstruction
• Summary <strong>and</strong> extension <strong>of</strong> arXiv:0810.1219 [nucl-ex]
accepted for publication in EPJC.
– Losing peripheral jet energy in HI k T jet reconstruction
High-pT at LHC '09
02/06/2009
4

Cone <strong>Jet</strong>s in ATLAS
• Cone jet reconstruction requires subtraction <strong>of</strong> the
underlying event before jet finding
NG (QM’08)
Calorimeter energy
in 0.1x0.1 towers
High-pT at LHC '09
02/06/2009
-0.5

Cone <strong>Jet</strong>s in ATLAS
• Subtract at the cell level
– but there are upward fluctuations that exist
NG (QM’08)
Calorimeter energy
in 0.1x0.1 towers
High-pT at LHC '09
02/06/2009
-0.5

Cone <strong>Jet</strong>s in ATLAS
• Embedded jets are found
– but so are other cone jet
NG (QM’08)
Calorimeter energy
in 0.1x0.1 towers
High-pT at LHC '09
02/06/2009
-0.5

Cone <strong>Jet</strong>s in ATLAS
• Embedded jets are found
– but so are other cone jet
NG (QM’08)
Calorimeter energy
in 0.1x0.1 towers
High-pT at LHC '09
02/06/2009
-0.5

Evaluation <strong>of</strong> the Fake Rate
• Run the jet reconstruction without embedding jets
• Suppress jet production in HIJING
– Do not sample Q 2 >100 GeV 2 hard scattering
– Suppress longitudinal string fragmentation
• produces 10 -2 jets per event with E T>50 GeV
• Any reconstructed jets with E T >> 10 GeV in this
sample are FAKE.
High-pT at LHC '09
02/06/2009
9

The Fake Rate Is Not Small!
• The fake rate is
large
– Fake rate =
expected rate
at 30 GeV
– Fake rate = 1/2
expected rate
at 50 GeV
• Need to reject
(>10) to
improve purity
<strong>of</strong> jet sample
arXiv:0810.1219 [nucl-ex]
High-pT at LHC '09
02/06/2009
Expected <strong>Jet</strong> Rate
10

The Character <strong>of</strong> Fake <strong>Jet</strong>s
• Fake jets tend to be a
single tower above
threshold (5 GeV) at
large angles w.r.t. the
jet axis
• Need a discrimination
variable which
emphasizes large
angle energy
arXiv:0810.1219 [nucl-ex]
High-pT at LHC '09
02/06/2009
Fake HIJING <strong>Jet</strong>
57 GeV
11

• Define
A Rejection Variable: Σj T
• Has a trivial jet E T
dependence
• where
High-pT at LHC '09
02/06/2009
arXiv:0810.1219 [nucl-ex]
Fake jets have a much wider
energy pr<strong>of</strong>ile (factor <strong>of</strong> ~2 at 50
GeV) than real jets
12

• Recast to an
E T-dependent
# <strong>of</strong> fake jet
sigma from
the fake jet
mean:
Modified Variable: σ ΣjT
arXiv:0810.1219 [nucl-ex]
Real jet mean < Fake jet mean
High-pT at LHC '09
02/06/2009
13

• Recast to an
E T-dependent
# <strong>of</strong> fake jet
sigma from
the fake jet
mean:
Modified Variable: σ ΣjT
arXiv:0810.1219 [nucl-ex]
Rejection means efficiency loss
High-pT at LHC '09
02/06/2009
Reject
14

Trade-<strong>of</strong>f:
Efficiency vs. Rejection
arXiv:0810.1219 [nucl-ex]
High-pT at LHC '09
02/06/2009
• Strong anticorrelation
between jet
reconstruction
efficiency <strong>and</strong>
fake jet
rejection
• Need rejection
<strong>of</strong> >10
– 50%
efficiency at
50 GeV
15

Trade-<strong>of</strong>f:
Efficiency vs. Rejection
arXiv:0810.1219 [nucl-ex]
High-pT at LHC '09
02/06/2009
• Strong anticorrelation
between jet
reconstruction
efficiency <strong>and</strong>
fake jet
rejection
• Need rejection
<strong>of</strong> >10
– 50%
efficiency at
50 GeV
16

arXiv:0810.1219 [nucl-ex]
The Final Spectrum
High-pT at LHC '09
02/06/2009
• With fake
rejection,
good S/B for
reconstructed
jet down to
10 GeV.
17

The Outlook:
Data-Driven Rejection
All Pairs
High-pT at LHC '09
02/06/2009
• Azimuthal jet
correlations
are sensitive to
fake jets
• Fake jets are
r<strong>and</strong>omly
correlated
with real jets
18

• Correlate
trigger jets
(A) with the
next highest-
E T jet (B)
• Only jet
pairs with
|Δφ|>2π/3
are real
Highest E T Di-jet Pairs:
Pure Fake <strong>Jet</strong> Sample
High-pT at LHC '09
02/06/2009
19

Associated <strong>Jet</strong> E T
High-pT at LHC '09
02/06/2009
• Fake jets
have a high
concentrati
on at low
E T
20

• Clearly the
real <strong>and</strong>
fake jets
are well
separated
in this
variable.
Associated <strong>Jet</strong> σ ΣjT
High-pT at LHC '09
02/06/2009
21

• Clearly the
real <strong>and</strong>
fake jets
are well
separated
in this
variable.
Associated <strong>Jet</strong> σ ΣjT
High-pT at LHC '09
02/06/2009
Reject
22

The Outlook: Quenched <strong>Jet</strong>s
arXiv:0810.1219 [nucl-ex]
High-pT at LHC '09
02/06/2009
• vs. E T
(see slide 10)
• 40% difference
between
HIJING <strong>and</strong>
PYTHIA
• No statistically
significant
difference
between
PYQUEN <strong>and</strong>
PYTHIA
23

k T <strong>Jet</strong> <strong>Reconstruction</strong>
• Apply Fast-kT directly on the calorimeter towers
i.e. no subtraction first
Calorimeter energy
in 0.1x0.1 towers
High-pT at LHC '09
02/06/2009
A Angerami,
W. Holzmann (QM’08)
24

k T <strong>Jet</strong> <strong>Reconstruction</strong>
• Apply Fast-kT directly on the calorimeter towers
i.e. no subtraction first
All towers are included
in a jet.
Most jets not “real”
Embedded jets are circled
Calorimeter energy
in 0.1x0.1 towers
High-pT at LHC '09
02/06/2009
A Angerami,
W. Holzmann (QM’08)
25

k T <strong>Jet</strong> <strong>Reconstruction</strong>
• Apply Fast-k T directly on the calorimeter towers
i.e. no subtraction first
Discriminate between real
<strong>and</strong> background jets
Embedded jets are found in
this variable
A Angerami,
W. Holzmann (QM’08)
High-pT at LHC '09
02/06/2009
26

A Wrinkle: <strong>Jet</strong> Periphery
A Angerami, W. Holzmann
Single jet 0.1x0.1 Calo Towers
PYTHIA ONLY
• Many more towers in the embedded PYTHIA jet
High-pT at LHC '09
02/06/2009
MERGED ONLY
BOTH
27

A Wrinkle: <strong>Jet</strong> Periphery
A Angerami, W. Holzmann
High-pT at LHC '09
02/06/2009
• PYTHIA only
towers primarily
in the jet
periphery
• Really effects
the energy scale
• Possible
solution: modify
the energy scale
correction
28

Summary
• There are subtleties in the jet reconstruction algorithm in
heavy ion collisions
– Fake rate from correlate background fluctuations
– Modification <strong>of</strong> k T jet shape
• These issues can <strong>and</strong> need to be overcome by careful study
from data for jet reconstruction to become st<strong>and</strong>ard in heavy
ion collisions.
• Underst<strong>and</strong>ing <strong>of</strong> the underlying event is crucial to
underst<strong>and</strong>ing <strong>and</strong> performing jet reconstruction
measurements in heavy ion collisions.
– Can we measure, non-jet, correlated background fluctuations?
High-pT at LHC '09
02/06/2009
29

Backup Slides
High-pT at LHC '09
02/06/2009
30

Example <strong>of</strong> Fake <strong>Jet</strong>s
Real PYTHIA <strong>Jet</strong> Fake HIJING <strong>Jet</strong>
57 GeV 57 GeV
High-pT at LHC '09
02/06/2009
31

• Separation
increases with
increasing jet
E T
• Fake rate drops
faster than
expected jet
rate
Recast σ ΣjT
Real jet mean < Fake jet mean
High-pT at LHC '09
02/06/2009
32