2. Outline
• Introduction
• LHC and ATLAS Detector
• Jets
• Boosted Objects
• Reclustering
• VariableR vs Fixed
• Top Quark Reconstruction
• W Boson Reconstruction
• Comparison
2
3. Absract
VariableR jet reclustering is an innovative technique that allows for
the reconstruction of boosted object over a wide range of
kinematic regimes. Such capability enables the efficient
identification of events with multiple boosted top quarks which is
a typical signature for new physics processes such as the
production of the supersymmetric partner of the gluon. In order to
evaluate the performance of the algorithm, the VariableR
reclustered jets are compared with fixed radius reclustered jets.
The flexibility of the algorithm is tested by reconstructing both
boosted top quarks and boosted W bosons. The VariableR
reclustering method is found to be more efficient than the fixed
radius algorithm at identifying top quarks and W bosons in events
with four top quarks, therefore enhancing the sensitivity for gluino
searches.
3
4. Introduction
• Looking at multiple top quark events in order to study new physics such as super
symmetry
• In particular the top quarks decay into a W boson and bottom quark
• Gluinos can be very massive so their decay produces can be very energetic
• Key for signal discrimination is efficient reconstruction of top (or W)
multiplicity.
W+
t
b
4
6. 6
Jets
• What are jets?
• Jets are a cone of hadrons which are a produced by a quark or
gluon hadronizing multiple times before it reaches the detector
6
7. Jets
• What are jets?
• Jets are a cone of hadrons which are a produced by a quark or
gluon hadronizing multiple times before it reaches the detector
7
Incoming quark
hadronization
Jet
8. 88
Jets
• What are jets?
• Jets are a cone of hadrons which are a produced by a quark or
gluon decaying multiple times before it reaches the detector
8
9. 9
Jets
• What are jets?
• Jets are a cone of hadrons which are a produced by a quark or
gluon decaying multiple times before it reaches the detector
9
Jets leave energy deposits on the ATLAS calorimeters
They are reclustered with 0.4 radii in the η- ϕ space
10. Boosted Objects
10
• When a top quark is highly boosted its decay products will be
collimated
• The radius of the cone produced (jet) depends on the the PT
of the top quark (R = 2M/PT)
11. Reclustering
Jet reclustering allows jets to be calibrated at a small radius
r, and then used as inputs for reconstructing larger jets of
radius R
11
http://arxiv.org/pdf/1407.2922v2.pdf
Rapidity
-2 0 2
=1.5 TeVZ'
, mtt→8 TeV PYTHIA Z'
Stable Truth Particles
R=1.0 Jetst
nti-k
Rapidity
-2 0 2
AzimuthalAngle[rad]
-2
0
2
Inside R=0.3 Jets
Stable Truth Particles
R=1.0, r=0.3 Jetst
anti-k
example event which has been clustered using the anti-kt R = 1.0 (left) and with
re-clustered r = 0.3 anti-kt jets (right). The shaded regions show the jet area
Energy Deposit
Jets
Reclustered Jets
12. VariableR vs Fixed
Fixed Reclustering: uses jets of r = 0.4 to reconstruct
jets of R = 1.0
Variable Reclustering: uses a range from Rmin = 0.4 to
Rmax = 1.5 to reconstruct jets
12
• VariableR Allows us to reconstruct
jets of various kinematic regimes
• R=2M/PT define typical scale for
the jets which is given by:
• ρT = 2*173.5 ρW = 2* 80.4
http://arxiv.org/abs/0903.0392
13. 13
VariableR vs Fixed
Fixed Reclustering: uses jets of r = 0.4 to reconstruct
jets of R = 1.0
Variable Reclustering: uses a range from Rmin = 0.4 to
Rmax = 1.5 to reconstruct jets
13
• VariableR Allows us to reconstruct
jets of various kinematic regimes
• R=2M/PT define typical scale for
the jets which is given by:
• ρT = 2*173.5 ρW = 2* 80.4
http://arxiv.org/abs/0903.0392
14. Boosted Top Reconstruction
14
Eta
-4 -3 -2 -1 0 1 2 3 4
Phi
0
1
2
3
4
5
6
148.967
160.003
198.717
192.775
148.967
160.003
198.717
192.775
148.967
160.003
198.717
192.775
148.967
160.003
198.717
192.775
148.967
160.003
429.638
148.967
160.003
429.638
148.967
160.003
429.638
Plot of Jets
Mass (GeV)
0 50 100 150 200 250 300 350 400 450 500
#ofEvents
0
200
400
600
800
1000
> 300 GeV
T
); P
T
=MρVR(
> 300 GeVT
R = 1.0; P
Mass (GeV)
0 50 100150 200250300350400 450500
#ofEvents
0
50
100
150
200
250
300
> 200 GeV
T
); P
T
=MρVR(
> 200 GeVT
R = 1.0; P
Mass distribution for
highest PT top quark
candidate
Mass distribution for
fourth highest PT top
quark candidate
VR constituent
Fixed constituent
VR
Fixed
15. Significance
Mass (GeV)
0 50 100150200250300350400450500
NumberofReclusteredJets
0
200
400
600
800
1000
Variable
Fixed
Mass (GeV)
0 50 100150200250300350400450500
NumberofReclusteredJets
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Variable
Fixed
Variable
Fixed
Variable
Fixed
Variable
Fixed
Variable
Fixed
Variable
Fixed
Variable
Fixed
L= 5 fb-1
15
The VariableR algorithm is more efficient at
reconstructing multiple jets and so allows us to
discriminate against low top quark candidate
multiplicity
Mimimum Number of Reclustered Jets
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Significance
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
# of T for VR
# of T for R = 1.0
Mass distribution
for highest PT top
quark candidate
Mass distribution
for fourth highest
PT top quark
candidate
16. Boosted W Reconstruction
Mass (GeV)
0 50 100150200250300350400450500
NumberofReclusteredJets
0
1000
2000
3000
4000
5000
6000
Variable
Fixed
16
Eta
-4 -3 -2 -1 0 1 2 3 4
Phi
0
1
2
3
4
5
6
90.8888
70.772
52.7264
90.8888
70.772
52.7264
90.8888
70.772
52.7264
179.594
70.772
52.7264
179.594
70.772
52.7264
179.594
70.772
52.7264
Plot of Jets
VariableR can
reconstruct W bosons
in various regimes
VR constituent
Fixed constituent
VR
Fixed
Mass distribution for highest PT
top quark candidate
17. Significance Comparison
17
Mimimum Number of Reclustered Jets
-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Significance
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
# of T for VR
# of W for VR
# of T for R = 1.0
# of W for R = 1.0
18. Conclusions and Next Steps
• I learned about reconstructing boosted top quarks and W bosons in the hadronic final
state
• Studied the application of a new algorithm (VariableR reclustering) for identification
of new physic processes
• This technique will be implemented in the super-symmetry search for gluino mediated
stop pair production
18
