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Top pair resonance searches with the ATLAS detector. 钟家杭 University of Oxford [email protected] Frontier Physics Working Month. Outline. Background information Top reconstruction Top pair resonance searches Boosted tops. Top quark. Spin=1/2, charge=2/3 The heaviest known quark - PowerPoint PPT Presentation
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Top pair resonance searches
with the ATLAS detector钟家杭 University of Oxford [email protected]
Frontier Physics Working Month
Outline
31 Aug 2012
Background information
Top reconstruction
Top pair resonance searches
Boosted tops
hadron68%
τ11%
μ11%
e11%
Top quark
31 Aug 2012
Spin=1/2, charge=2/3 The heaviest known quark
m(t)=173.2±0.9 GeV (Tevatron)
Lifetime ~ 5x10-25 s Decay before hadronization Almost exclusively via t -> W
+ b
Beyond the Standard Model
31 Aug 2012
Two benchmark BSM models used in experiments Z’ in a leptophobic topcolor model
Proxy to narrow resonance: Γ/m=1.2%
Kaluza-Klein gluon (KKG) in Randall-Sundrum extra dimension modelsProxy to broad resonance: Γ/m=15.3%
Generic search, applicable to other BSM models Spin-0 Lee-Wick Higgs Spin-2 KK graviton …
KKG branching ratioPhys. Rev. D 77 (2008) 015003
Leptons in ATLAS
31 Aug 2012
Only prompt leptons are considered signal Electron:
Energy cluster of high EM fraction, matching to a track Muons:
Combined tracking in both Inner Tracker and Muon Chambers
Fixed-cone isolation to suppress QCD contribution Mostly real leptons from heavy-flavor quark Both calo-based and track-based
Hadronic tau channel not included
Jets in ATLAS
31 Aug 2012
Sequential clustering algorithms : Kt, C/A, anti-Kt AntiKt as the mainstream jet algorithm
R=0.4 as the standard jet R=1.0 known as the fat jet (boosted hadronic top jet) C/A algorithm with R=1.5 used for HEPTopTagger
B-tagging For antiKt4 jets Using tracks associated with the jet
Secondary vertices Impact parameter
Multivariate algorithms, 70% efficiency
Leptonic top reconstruction
31 Aug 2012
t -> W + b -> l+v+b One Lepton High missing transverse energy (MET) High transverse mass MT between lepton and MET (due to
W mass)
One b-tagged antiKt4 jet.
Neutrino reconstruction Assuming MET fully from neutrino, solve pz(v) using W-
mass Under-constrained in di-lepton channel
)cos1(2 missT
lTT Epm
Hadronic top reconstruction
31 Aug 2012
t -> W + b -> q+q+b
Resolved: 3 antiKt4 jets 2 antiKt4 jets, if one has high
mass.
Boosted: One energetic antiKt10 jet
with substructure cuts One energetic C/A1.5 jet
using HEPTopTagger Discrimination against QCD
Boos
t
pmR /~
Hadronic top reconstruction
31 Aug 2012
Jet substructure Jet mass> 100 GeV
First splitting scale >40 GeV Re-clustering jet constitutes with Kt algorithm.
The splitting scale of the last step.=min(pTi, PTj) x ΔRij
mt/2mt
Top pair resonance search
31 Aug 2012
Select ttbar-like events Di-lepton 1 lepton + 4(3) jets (resolved) 1 lepton + 1 jet + 1 fat jet
(boosted) Fully hadronic (HEPTopTagger)
Reconstruct or equivalent
Look for peaks in spectrum
Fully hadronic46%
1-lepton(e, µ) 34%
Di-lepton6%
τ (had)14%
2 fb-1, arXiv:1207.2409
2 fb-1, EPJC72 (2012) 2083
5 fb-1, ATLAS-CONF-2012-102
Single Lepton Boosted ttbar
31 Aug 2012
Single lepton trigger Exactly one offline
lepton Electron pT > 25 GeV Muon pT > 20 GeV
ETmiss>35GeV,
MT>25GeV Solve neutrino pz with
W mass constraint Closest antiKt4 jet as
from the leptonic top pT > 30 GeV 0.4 < ΔR(lepton, jet) <1.5
One antiKt10 fat jet pT > 250 GeV m > 100 GeV > 40 GeV dR(akt4, akt10)>1.5
Signal selection efficiency
Single Lepton Boosted ttbar
31 Aug 2012
tt= l + v + akt4 + akt10 (4-vector sum)Leptonic top
mass(l + v + akt4)
Hadronic top mass
(fat jet)
Single Lepton Boosted ttbar
31 Aug 2012
W+jets background Data-driven normalization
Multijets Fully data-driven
Can be further improved by b-tagging
Single Lepton Boosted ttbar
31 Aug 2012
Search for local data excess with BumpHunter
Set 95% CL upper limits on xsec
Replace the theoretical line with your favorite model
Top pair resonance search
31 Aug 2012
Di-lepton One-lepton(Resolved)
One-lepton(Boosted)
Fully hadronic
Integrated luminosity 2 fb-1 2 fb-1 2 fb-1 4.7 fb-1
Z’ limits - 0.5 – 0.88 TeV
0.6 – 1.15 TeV 0.7 – 1.3 TeV
KKG limits 0.5 – 1.08 TeV
0.5 – 1.13 TeV 0.6 – 1.5 TeV 0.7 – 1.5 TeV
More results are coming…
Boosted Top
31 Aug 2012
New challenge: TeV frontier Top decay products are more collimated
ΔR ~ m/P
Boosted Top: Leptonic
31 Aug 2012
Lepton collinear with the b-quark
Signal acceptance suffers from the fixed-cone isolation cuts
Signal selection efficiency
Boosted Top: Leptonic
31 Aug 2012
Mini-isolation Variable-cone size ΔR=KT/pT
Parameter KT, e.g. 15 GeV Lepton pT (easier than top pT)
Sum up tracks pt within the cone Sufficient angular resolution
JHEP 1103:059 (2011)
b-jet
leptonIsolation cut
Boost, dR=mtop/Etop
Fixed-cone isolation
Mini-isolation
Boosted Top: Hadronic
31 Aug 2012
Three jets tend to overlap.
Use single jet with large radius Need rejection against QCD
=> Substructure variable
Need to get rid of soft component from underlying event and pileup=> Jet Grooming
Not limited to top decay
Boos
t
Boosted Top: Jet grooming
31 Aug 2012
Algorithms to reduce soft components from UE and PU Jet kinematics more close to the constituents of hard
scattering Better resolution/discrimination of the substructure
variables
I. Mass drop/filteringII. TrimmingIII. Pruning
Boosted Top: Jet grooming
31 Aug 2012
Mass drop/filtering Works on C/A jet More optimized for two-body hadronic decay
W/Z -> qq, H -> bb
Phys.Rev.Lett.100:242001 (2008)(J. Butterworth, A. Davidson, M. Rubin, G. Salam)
Mass drop
Filtering
Boosted Top: Jet grooming
31 Aug 2012
Trimming Use jet constituents to build Kt subjets (e.g.
R=0.2) Remove soft subjets Applicable to any jet, any physics scenario
JHEP 1002:084 (2010) (D. Krohn, J. Thaler, L. Wang)
Boosted Top: Jet grooming
31 Aug 2012
Pruning Recluster jet constituents with C/A or Kt algorithm
(no need of subjets) Veto wide angle and soft constituents during jet
formation
arXiv:0912.0033 (2009)(S. Ellis, C. Vermilion, J. Walsh)
Boosted Top: Jet grooming
31 Aug 2012
Reduce unnecessary catchment area
antiKt R=1.0 (ungroomed)
antiKt R=1.0 (trimmed)
Boosted Top: Substructure
31 Aug 2012
Jet mass are more discriminating after trimming
Boosted Top: Substructure
31 Aug 2012
Splitting scale Re-clustering jet constitutes with Kt algorithm.
The splitting scale of the last step. =min(pTi, PTj) x ΔRij
√𝑑23≈𝑚𝑊 /2√𝑑12≈𝑚𝑡𝑜𝑝/2
Boosted Top: Substructure
31 Aug 2012
N-subjettiness (τN) Re-clustering with Kt algorithm until exactly N subjets are
formed Smaller τN+1 /τN => Structure described better with
additional sujet
Boosted Top: HEPTopTagger
31 Aug 2012
A multi-step algorithm starting from a large-R C/A jet
Grooming: filter out soft component Form up subjets Impose Top and W mass constraints
JHEP 1010:078 (2010)ATLAS-CONF-2012-065
Summary
31 Aug 2012
ttbar resonance are searched in all channels at ATLAS Unfortunately, we don’t have the luck yet…
Systematics still have large impact on the sensitivity Uncertainty of performance at high pt Understanding realistic performance of new techniques Rooms to improve…
New techniques for new challenges Boosted top/object Increased luminosity
