Status Report of DOE Review
Chunhui Chen
1
Introduction
Ø  Search for NP beyond the SM using boosted W/Z/H jets
ü  Major analysis focus of ISU group in last 2 years
ü  Primary analysis focus in Run 2 by ISU group
•  Not the only analysis interests of ISU group
Ø  Finished Run 1 analysis:
ü  Boosted W/Z cross section measurements
•  SM measurement
•  Validation of a pheno proposal by ISU group
•  Also a potential model independent search for NP
•  Aim for application of NP search in Run 2
ü  Search for heavy resonance decaying to diboson final state
Ø  Planned Run 2 analysis:
ü  Updated diboson resonance search using Run 2 data
ü  Search for H->bb in associated production of W/Z
ü  Search for dark matter using boosted H->bb jets
Chunhui Chen
2
Boosted hadronically decaying particle
Ø  Most NP models predict heavy resonance (~ TeV) decay into W/Z/Top:
T 0 ! W b,
X ! W W, W Z, tt̄
ü  Boosted (high pT) jets in the final decay states
ü  The hadronic decay product are highly collimated
Ø  Traditional jet reco. relying on one-to-one jet-to-parton assignment not adequate
Ø  Solution: reconstruct multi quarks in a single jet (boosted W/Z/Top jet)
ü  2 quarks for W/Z decay, 3 quarks for top decay
ü  Better (only way) to search for certain NP models
Chunhui Chen
3
How to identify a Boosted W/Z/t jet
MC simulation
Jet mass: invariant mass of jet
Problem: QCD jet (1 non top
quark or gluon) has non-zero
mass, its production a few
orders higher !
Jet pT>300GeV
Conventional method:
Smaller jet inside big one
Solution: jet substructure
Exam the topological distribution of jet energy cluster information
Chunhui Chen
4
Our Proposal: Jet Substructure in CM Frame
Ø  Boosted object (jet): A new analysis idea/techniques
ü  Significantly improve sensitivities to search for heavy new particles
•  In the decay final states containing W, Z, Higgs or top quarks
ü  One of the hottest HEP theory and experimental researches at LHC era
•  Many new theoretical and experimental papers on the subject
•  Annual workshop devoted to boosted object since 2009 (SLAC, Oxford,
Princeton, Valencia, Arizona, London)
Ø  All existing jet substructure: energy cluster distribution in lab frame
Ø  Our idea: jet substructure using energy clusters in center-of-mass frame of jet
ü  Jet CM frame: jet 4 momentum = (0,0,0,mjet)
W/Z->jj
t->Wb->jjj
QCD
Chunhui. Chen, Physical Review D 85, 034007 (2012)
Chunhui Chen
5
Jet Substructure in CM Frame
Jets / 0.02
Ø  A new idea to study jet substructure
ü  Nearby clusters in lab frame may not be close in CM frame
ü  Using full momentum information of the energy clusters
Ø  Significantly improve sensitivities to search for heavy new particles
ü  In the decay final states containing W, Z, Higgs or top quarks
0.24
0.22
0.2
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0.5
ATLAS
QCD jets
-1
s = 7 TeV, 4.6 fb
p > 320 GeV
W/Z jets
Data 2011
T
η < 1.9
0.6
0.7
0.8
0.9
1
Thrust
C. Chen, Physical Review D 85, 034007 (2012)
C. Chen, Physical Review D 87, 074007 (2013)
C. Chen, Physical Review D 88, 074009 (2013)
ATLAS, New Journal of Physics 16, 113013 (2014)
Chunhui Chen
6
Hadronic W/Z Signal (single jet) at ATLAS
25000
1800
1600
Signal + Background fit
1400
Background fit component
20000
Signal fit component
Jets / 2 GeV
Data 2011
Data - Fit bkg
Jets / 2 GeV
pp ! Z/W + anything and W/Z ! q q̄ 0 /q q̄
1200
1000
800
Data 2011
25000
Signal + Background fit
ATLAS
s = 7 TeV, 4.6 fb -1
Background fit component
p > 320 GeV |η| < 1.9
Signal fit component
L > 0.15
T
20000
600
400
200
15000
0
-200
50 60 70 80 90 100 110 120 130 140
15000
Jet Mass [GeV]
10000
ATLAS
s = 7 TeV, 4.6 fb -1
10000
5000
p > 320 GeV |η| < 1.9
T
0
L > 0.15
5000
0
50
50 60 70 80 90 100 110 120 130 140
ATLAS, arXiv:1407.0800
100
150
200
Jet Mass [GeV]
250
Jet Mass [GeV]
N W +Z = 14200 ± 1300(stat)
Chunhui Chen
Fit χ2/ndf=41.4/38
χ2 probability: 32%
χ2 probability < 10-7 if
assuming no signal peak
7
Jets / 2 GeV
Hadronic W/Z Signal (single jet) at ATLAS
10000
Data 2011
Signal + Background fit
8000
ATLAS
s = 7 TeV, 4.6 fb -1
Background fit component
p > 420 GeV |η| < 1.9
Signal fit component
L > 0.15
T
First measurements of boosted W/Z
production rate at pp collision
W +Z
= 8.5 ± 0.8 (stat.) ± 1.5 (syst.) pb
Consistent with theory
6000
W +Z
4000
2000
ATLAS, arXiv:1407.0800
0
50 60 70 80 90 100 110 120 130 140
= 5.1 ± 0.5 pb
ü  Validation of our proposal to use jet
substructure in the jet rest frame
ü  Test of SM: excess of the production rate
could be a hint of NP effects
ATLAS, New Journal of Physics 16, 113013 (2014)
Jet Mass [GeV]
Ø  First ever boosted W/Z signal (W/Z+jet production) reconstructed in single jet
ü  Only successful signal observation among all proposals at LHC
ü  Selected as NJP highlights of 2014
ü  Select for inclusion in IOPselect: Substantial advances or significant
breakthroughs, A high degree of novelty, Significant impact on future
research
Chunhui Chen
8
Application: Search for X→WW/WZ
u
d
W/Z
X
p
e/μ
p
W
ν
Ø  For heavy graviton (>1TeV), boosted W jet
ü  Conventional/old experimental technique fails
ü  W jet: known mass and momentum
Ø  One missing neutrino can be reconstructed
ü  neutrino: 3 unknowns (momentum)
ü  Energy conservation in transverse plane: 2 constraints
ü  W mass constrain: 1 constraint
Chunhui Chen
9
Search for di-Boson heavy resonance
Ø  Benchmark models for optimization/interpretation:
ü  A spin-2 Kaluza-Klein (KK) graviton for WW final state
ü  A spin-1 gauge boson (W’) of sequential SM for WZ final state
Ø  Analysis sensitive to narrow resonances decaying to WW/WZ in any NP models
Sample
W/Z + jets
tt̄ + single top
Multijet
Diboson
Total
Data
G⇤ signal
W 0 signal
Chunhui Chen
LRR
104800 ± 1600
37700 ± 1600
13500 ± 500
5500 ± 270
161500 ± 2300
157837
7000 ± 500
6800 ± 600
HRR
415 ± 10
271 ± 13
84 ± 9
96 ± 6
870 ± 40
801
36 ± 6
318 ± 21
MR
180 ± 20
42 ± 7
29.3 ± 2.9
43 ± 7
295 ± 22
323
5.5 ± 2.3
70 ± 4
10
Final Limit
Eur. Phys. J. C (2015) 75:209
Chunhui Chen
11
Analysis Plan of Run 2
Chunhui Chen
12
Update search for di-Boson resonance
Ø  Interesting hints from search using all hadronic final state in Run 1
ü  3.5 sigma excess (local significance) ~ 2TeV
Ø  Single lepton channel has highest sensitivity
ü  Expect 5 sigma sensitivity with 5fb-1 13TeV data if the excess is real
ü  Francesco Lorenzi (ISU) is leading the Run 2 effort as contact person
ü  Now an analysis aim at early data/publication by ATLAS management
Chunhui Chen
13
Search for WH/ZH, H->bb
Ø  No evidence of H->bb from run 1 analysis (7+8 TeV)
ü  1.4 sigma significance
Ø  Analysis suffer from very large background from ttbar, W/Z+jets production
ü  Sensitivity is not very good even with sophisticated multi variable analysis
Ø  Need significant data from run2 in order to have observation
ü  Require complicated multi variable analysis
Chunhui Chen
14
Our proposal to search for VH, H->bb
Ø  Using boosted topology: H->bb reconstructed in single jet
ü  Substructure in the center of mass frame of jet
ü  Double b tagging in a single jet
Ø  May significantly improve sensitivity
ü  Ongoing Pheno work, no realistic detector MC yet, assuming 100 fb-1 data
ü  Expect more detail studies using official ATLAS MC
Chunhui Chen
15
Search for DM using boosted Mono H->bb
A natural extension of search for ZH, Z->nunubar, H->bb with similar
experimental signature
Chunhui Chen
16
Summary
Ø  One of primary analysis focus at ISU: search for NP using boosted W/Z/H
ü  We propose a new idea to explore jet substructure in jet rest frame
ü  C.Chen: PRD85,034007(2012), PRD87,074007(2013),PRD88,074009(2013)
Ø  Play leading role in 2 Run 1 analysis
ü  Boosted W/Z production at 7TeV: NJP 16, 113013 (2014)
•  Done by ISU group: F. Lorenzi is contact person, editor
ü  Diboson resonance search: Eur. Phys. J. C (2015) 75:209
•  Using single lepton channel wit 8TeV data
•  Primarily done by ISU
•  A. Martinez is contact person, editor (also C. Chen)
Ø  Plan for Run 2 analysis
ü  Updated diboson search in single lepton channel
•  F. Lorenzi is leading the effort as contact person
ü  Start search for production of W/ZH, H->bb
•  Proposed new method with improved sensitivity
ü  Extend the boosted H->bb work to look for DM
Ø  All three faculties actively involved with Run1 and planned Run 2 analysis
Chunhui Chen
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