search for new physics in tt final state in boosted regime at cms
DESCRIPTION
Search for New Physics in tt Final State in Boosted Regime at CMS. Motivation Analysis method Jet topologies Background estimation Results Summary. Sam Meehan o n behalf of the Group E Collaboration. Example Boosted Top Event Display from CMS. Motivation. - PowerPoint PPT PresentationTRANSCRIPT
1Group E - tt Resonances Search
o Motivationo Analysis methodo Jet topologieso Background estimationo Resultso Summary
17 June 2012
Search for New Physics in tt Final State in Boosted
Regimeat CMS
Example Boosted Top Event Display from CMS
Sam Meehanon behalf of the
Group E Collaboration
Group E - tt Resonances Search 2
Motivation• What are they searching for and why?
o CERN has a top factory in the LHCo New physics will likely couple to the top (oddly heavy)o Benchmark models : RS-KK gluons , Z’ resonances
• How are they doing this?o Using 5 fb-1 of 7 TeV pp data collected by CMS during 2011o Reconstruct fully hadronic final state gain in BR for top decayo Examine tt invariant mass spectrum for excess in data
17 June 2012
Z’, KK-gluon, Spaghetti
monster … ????
t
t
p
p
Group E - tt Resonances Search 3
• Massive resonances (>1 TeV) boosted topso ΔR ~ 2M/pT R = 0.8 jets contain boosted top with few 100 GeV of pT
• Derive data-driven estimate of dominant QCD background• Examine invariant mass of tt system for excesses• Set CLs limits on σ✕BR
Analysis Overview
17 June 2012
1+1 – Dijet Channel :
Require both jets to betype I top-tagged jets
1+2 – Trijet Channel :Type I top-tag jet
+Type II top-tag from
W-tag plus jet
Type-1 top candidate Type-2 top
candidate
Wb W (W-tagged)
b
Group E - tt Resonances Search 417 June 2012
Top-tagging• Decompose C/A jet building algorithm from the
final recombination to find subjets and then ask about their kinematics C/A Jets : R=0.8
dij = ΔR(i,j)/RdiBeam = 1
Group E - tt Resonances Search 517 June 2012
C/A Jets : R=0.8dij = ΔR(i,j)/R
diBeam = 1
Top-tagging• Decompose C/A jet building algorithm from the
final recombination to find subjets and then ask about their kinematics
Group E - tt Resonances Search 6t
This is just out workinghypothesis to get a pictureof how a boosted top maybe formed
17 June 2012
C/A Jets : R=0.8dij = ΔR(i,j)/R
diBeam = 1
Top-tagging• Decompose C/A jet building algorithm from the
final recombination to find subjets and then ask about their kinematics
Group E - tt Resonances Search 7t
t FailsPasses
17 June 2012
Subjet Decomposition :Check pTi/pTjet > δP=0.05 and separation splitting
Both pass
Consider bothas “subjets”
Try to split both
One Pass
Softer protojetfrom radiation
Discard softer
Both Fail
Irreducible
Final subjet identified
C/A Jets : R=0.8dij = ΔR(i,j)/R
diBeam = 1
Top-tagging• Decompose C/A jet building algorithm from the
final recombination to find subjets and then ask about their kinematics
Group E - tt Resonances Search 8t
t
Both pass subjets
17 June 2012
W b
Subjet Decomposition :Check pTi/pTjet > δP=0.05 and separation splitting
Both pass
Consider bothas “subjets”
Try to split both
One Pass
Softer protojetfrom radiation
Discard softer
Both Fail
Irreducible
Final subjet identified
C/A Jets : R=0.8dij = ΔR(i,j)/R
diBeam = 1
Top-tagging• Decompose C/A jet building algorithm from the
final recombination to find subjets and then ask about their kinematics
Group E - tt Resonances Search 9t
t
Both fail irreducible
17 June 2012
W b
Subjet Decomposition :Check pTi/pTjet > δP=0.05 and separation splitting
Both pass
Consider bothas “subjets”
Try to split both
One Pass
Softer protojetfrom radiation
Discard softer
Both Fail
Irreducible
Final subjet identified
C/A Jets : R=0.8dij = ΔR(i,j)/R
diBeam = 1
Top-tagging• Decompose C/A jet building algorithm from the
final recombination to find subjets and then ask about their kinematics
Group E - tt Resonances Search 10t
t
Both pass subjets
17 June 2012
W b
qq’
Subjet Decomposition :Check pTi/pTjet > δP=0.05 and separation splitting
Both pass
Consider bothas “subjets”
Try to split both
One Pass
Softer protojetfrom radiation
Discard softer
Both Fail
Irreducible
Final subjet identified
C/A Jets : R=0.8dij = ΔR(i,j)/R
diBeam = 1
Top-tagging• Decompose C/A jet building algorithm from the
final recombination to find subjets and then ask about their kinematics
Group E - tt Resonances Search 11t
t
Both fail irreducible
17 June 2012
W b
qq’
Subjet Decomposition :Check pTi/pTjet > δP=0.05 and separation splitting
Both pass
Consider bothas “subjets”
Try to split both
One Pass
Softer protojetfrom radiation
Discard softer
Both Fail
Irreducible
Final subjet identified
C/A Jets : R=0.8dij = ΔR(i,j)/R
diBeam = 1
Kinematic Top-Tagging:
If subjets satisfy Yay! Top-Tag!
M(subjets) ε [100 GeV, 250 GeV]min[M(i,j)] > 50 GeV
Top-tagging• Decompose C/A jet building algorithm from the
final recombination to find subjets and then ask about their kinematics
Group E - tt Resonances Search
Pruning Mass Drop W Tagging• Goal : better define M(jet)
by removing “bad” constituents
• Discard soft and large angle radiation by checking if– Z = min(pT1,pT2)/pT(1+2) < Zcut
– D = ΔR12 > Dcut
• Decompose into subjets • For 2 subjet jets, require:
– mj-heavy/Mjet-total < 0.4 (mass drop μ)
– 60 GeV < Mjet < 100 GeV
C/A R=0.8 Jet Subjet 1 Subjet 2
μ>0.4 μ < 0.4Mass Drop
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W-tagging• Prune jets to increase mass resolution and tag
them using mass drop technique
W- tag !
Group E - tt Resonances Search 13
Background Estimation (I)
• Main Backgroundso Standard Model tt taken from MCo Non-Top MultiJet (NTMJ) data-driven technique using control region
and estimated mistag rate for normalization• Mistag Probability
o Using 1+2 topology invert mass drop requirement on type 2 top μ>0.4 to obtain QCD fake sample with “same” kinematics
o As function of type 1 top pT calculate top-tag mistag rate
17 June 2012
50-60% efficiency plateau
4-5% mistag probability plateau
Pm(pT) = Probability of mistagging at jet pT
Group E - tt Resonances Search 14
Background Estimation (II)
• Dominant NTMJ background taken from loosened selection in each channel
• Event-by-event weight calculated as mistag probability for pT of probe jet
• Background shape extractiono Probe jet mass distribution kinematically biasedo Replace mprobe-jet in probe-jet four vector by random value drawn from
NTMJ MC in [140,250] GeVo Closure of procedure tested on MC
17 June 2012
1+1 – Dijet Channel :
Tag type I candidate Second jet is “probe
jet”
1+2 – Trijet Channel :Tag type 2 candidate using two jets in single event hemisphere
Third jet is “probe jet”
Group E - tt Resonances Search 1517 June 2012
• Combined SM tt (red) and MultiJet (yellow) backgrounds compared to data
• Dominant systematics shown in grayed bands:o Trigger, JES, Luminosity, reconstruction efficiency
• No significant excess observed
Results
1+1 Channel 1+2 Channel
Group E - tt Resonances Search 16
SM + New physics
SM only
Model Exclusion Limit
Z’ (1% width) 1.0-1.6 TeV/c2
Z’ (3.0% width) ~1.0 TeV/c2
1.3-1.5 TeV/c2
Z’ (10% width) 1.0-2.0 TeV/c2
Kaluza–Klein gluon production
With σg∙Br(g’ tt̅) < 1 pb ➝~1 TeV/c2
1.4-1.5 TeV/c2
tt̅ enhancement S < 2.6
17 June 2012
• Resonance search:o Limits set on σ✕BR as a function of resonance mass using CLs method with
likelihood ratio as test statistic• tt Continuum Enhancement
o Limits set on σ(tt) enhancement rom new physics using CLs method
Interpretation of Results
Excluded
M(Z’)>1.6 TeV
Group E - tt Resonances Search 17
Conclusions• First search constraining mtt > 1 TeV and using
developing jet substructure techniques• No indication of new physics (Z’, KKG, ???)
coupling to top pairs• Limits on σ✕BR set for benchmark models and
general broad excesses• Substructure is interesting and will allow us to
probe high pT regime in hadronic final states• Many thanks to all organizers of ESHEP, and
especially Maurizio for many wonderful discussion sessions these past two weeks!!!
17 June 2012
Group E - tt Resonances Search 18
Group E Collaboration• Boris Bulanek• Alexander Gramolin• Claudio Heller• Brett Jackson• Houry Keoshkerian• Olga Kochebina• Lukas Marti• Sam Meehan• Nicola Orlando
17 June 2012
• Maurizio Pierini• Leonid Serkin• Rosa Simoniello• Jared Sturdy• Dmitry Tsirkov• Xiaoxiao Wang• Christoph Wasicki• Adam Webber• Jonas Weichert
Group E - tt Resonances Search 19
Backup
17 June 2012
20Group E - tt Resonances Search
Systematics
• Determination of efficiency: Trigger, jet energy scale • Mistag probability: statistics (high-mass region), systematic associated to mass spectrum correction (low-mass)• Shape of the tt_bar invariant mass: Renormalisation and factorisation scales.
17 June 2012
Group E - tt Resonances Search 21
Topcolor: width = 10%
17 June 2012
Group E - tt Resonances Search 22
Randall–Sundrum Kaluza–Klein gluon production
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Group E - tt Resonances Search 23
CMS Detector
17 June 2012