Post on 03-Dec-2021
transcript
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
Peaky blinders: searches for tt̄ resonances
James Ferrando
University of Glasgow
Elementary Particle Physics Group SeminarUniversity of Birmingham
11th December 2013
James Ferrando Searches for tt̄ resonances 1/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
Outline
Why search for tt̄resonances?
Review of tt̄resonance searches
Looking towardsthe future
Will focus on the details of ATLAS searches but also show the bestresults from the competition
James Ferrando Searches for tt̄ resonances 2/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
The StandardModel
The Standard Model (SM) of particle physics:
Fermionic matter:
Three generations of quarksThree generations of leptons
Gauge Bosons:
Four Force carriers : γ(EM), W±,Z(Weak), g (strong)The Higgs Boson to give mass
”Was she pretty?” asked the bigger of the smallgirls. ”Not as pretty as any of you,” said thebachelor, ”but she was horribly good.”The storyteller - H. H. Munro (Saki)
James Ferrando Searches for tt̄ resonances 3/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
SM Problems
So what’s wrong with the Standard Model?
No Dark Matter candidates
Not enough CP violation to explain theobserved matter-antimatter imbalance
The Higgs boson has still not been observed
No gravity
Particle masses are not understood
Is there physics beyond the Standard Model?
James Ferrando Searches for tt̄ resonances 4/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
The LHC
Where to look for answers? The Large Hadron Collider at CERN
27 km circumference ring
Currently collides protons atcentre-of-mass energy 8TeV
Four detectors installed aroundthe ring
An excellent environment to testthe Standard Model and searchfor new Physics
Triviality/Unitarity constraintson some SM cross sectionsimply a Higgs Boson orsomething else at an energyscale < 800GeV
James Ferrando Searches for tt̄ resonances 5/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
LHCDetectors
What equipment to use? A Toroidal Large ApparatuS (ATLAS)
4 Detectors:2 General Purpose
ATLASCMS
Two Specialised
ALICE - Heavy ionLHCb - CPviolation
ATLAS with full solid angle coverage, excellent charged particletracking, particle ID and energy measurement is well-suited forTeV-Scale physics (and so is CMS of course)
James Ferrando Searches for tt̄ resonances 6/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
Introducing:The Top Quark
The top quark was discovered atTeVatron in 1995
Extremely heavy for afundamental particle:
Similar mass to a gold atom∼ 35 times heavier than thenext heaviest quark (thebottom quark)
Usually produced in a tt̄ pairwith its partner the anti-top
Could it provide a gateway tonew physics?
James Ferrando Searches for tt̄ resonances 7/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
Top andBSM Physics
Many BSM scenarios on the market
Large top mass (mt ≈ 173GeV) → topoften plays a special role in BSM theories
BSM physics often has consequences forthe third generation quarks
Some examples:
Add new heavy quarks: Often decay to tops or look likeheavy tops
Incorporate Gravity using Extra Dimensions: Manymodels predict new states with strong coupling to the top
Exotic Higgs Bosons: large coupling to the top
SUSY: naturalness prefers top-partners not too far from mt
James Ferrando Searches for tt̄ resonances 8/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
Hints ofNew Physics?
Extra motivation: TeVatron pp̄ data
Att̄ =N(y tt̄t > 0)− N(y tt̄t < 0)
N(y tt̄t > 0) + N(y tt̄t < 0)
Tevatron collides p and p̄producing tt̄
Att̄ a measure of how much thet prefers the p direction
p-value of such a large slope0.00646 (CDF)
“Strengthens the case that newphysics plays a role in tt̄ production”
James Ferrando Searches for tt̄ resonances 9/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
A TopFactory
Measurements of top properties at TeVatron statistically limited
The LHC tt̄ productioncross-section is much larger
In effect the LHC is atop quark factory
TeVatron: < 8× 104
(10 fb−1) top pairs perexperiment ∼10 yearsrunning
LHC: > 6× 106 top pairs perexperiment in 2011-12
At the LHC many top quark studies are possiblethat were not feasible at TeVatron
James Ferrando Searches for tt̄ resonances 10/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
TopSignatures
t
t
t
t
t
t
t
t
q
q
q
q
g
g
g
g
g
gt
In the SM, top decaysapproximately 100% t →Wb
Classified according to the Wdecays
James Ferrando Searches for tt̄ resonances 11/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
New Physicswith Tops
Broadly speaking can study new physics in tt̄ in three differentways
Look for anomalous production of tops
Look for unexpected behaviour in top quark decays
Directly search for new particles decaying to tops (andpossibly something else)
This talk focuses on the latter, searching for a peak in the mtt̄
distribution from production of new particles that decay to tt̄ pairs
James Ferrando Searches for tt̄ resonances 12/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
tt̄ resonances I
A wealth of peaky new physics signals from different scenarios:
Extra dimensions (Bulk RS):Excitations of gluon (gKK)/ graviton(GKK) preferentially decay to tt̄
Topcolor-assisted Technicolor:Strong EWSB model via a topcondensate - expect top-π (H-like)and top-ρ (Z ′-like ) the latter heavyenough to decay to tt̄
Composite Higgs scenarios: Usually require (naturalness)extra heavy-fermions, and commonly heavy “gluons” thatdecay to tR or new heavy fermions depending on the masses
BSM Higgs: New heavy pseudoscalar Higgs-like particles in,e.g. the MSSM, would also have a large tt̄ branching ratio
James Ferrando Searches for tt̄ resonances 13/ 65
IntroductionRecent Searches
The FutureSummary
OutlineThe Standard ModelWhat’s wrong with the SM?Why Top quarks?Signatures and Models
tt̄ resonances II
Searches so far have focused on two benchmark scenarios:
Topcolor-assisted technicolor (TC2)Z ′TC2 → tt̄
Spin-1Color singletNarrow width (1.2%) modelled withSSM Z ′ (3%) widthhep-ph/9911288,Eur. Phys. J. C (2012) 72 2072
RS Kaluza-Klein Gluon gKK → tt̄
Spin-1color octetwide (10-15%)BR(gKK → tt̄) ∼ 92.5%JHEP 0709 (2007) 074
q
q
t
t
0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
) [TeV]TC2
m(Z’
Bra
nch
ing
Rati
o
Branching RatiosTC2
LO Z’
=0.02
=1.0, f1
hepph/9911288 Harris et. al, Model IV, f
Including correction from Ferrando and Frandsen used in
Eur. Phys. J. C (2012) 72, 2072 Harris and Jain
uu ttdd bb
James Ferrando Searches for tt̄ resonances 14/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Selecting tt̄
First tt̄ resonance search at the LHC selected tops in a familar way(ATLAS - Eur.Phys.J. C72 (2012) 2083)
dilepton channel
Two isolated leptonsll = ee, eµ, µµ
ee or µµ: Mll outside MZ
window
eµ : Require large HT
Mll > 10 GeV
EmissT
2 or more jets
l+jets channel
Isolated electron or muon
Missing Transversemomentum (Emiss
T )
4 or more jets (inclusiveAnti-KT , R = 0.4) or, 3 jetsand one jet has mass> 60 GeV
At least 1 b-tagged jet
HT is the scalar sum of PT of all hard objects in event.
James Ferrando Searches for tt̄ resonances 15/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
tt̄ resonances
Use kinematic distributions to searchfor background
l+jets reconstruct mtt̄
Solve quadratic for EmissT to
reconstruct neutrino usingmlν = mW constraintexclude jets if∆R > 2.5− 0.0015×mj
iteratively until none fail, orthere are only three jetstake 4 (or 3) highest pTremaining jetsreconstruct the mass of the 4jet, lepton + neutrino system
dilepton: use HT
mass [GeV]tReconstructed t
0 500 1000 1500 2000E
vent
Fra
ctio
n0
0.05
0.1
0.15
0.2
=500 GeVZ'm=700 GeVZ'm=1000 GeVZ'm=1300 GeVKKm
ATLAS=7 TeVsSimulation
James Ferrando Searches for tt̄ resonances 16/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Backgrounds
Estimation of backgrounds:
top-pair: (irreducible) taken from Monte Carlo (MC)
W+jets: taken from MC and then normalised using datacontrol regions (l+jet channel)
Z+jets: taken from MC and then normalised using datacontrol regions (dilepton channel)
single-top: taken from MC
Massive di-boson: taken from MC
non-top multijet: estimated directly from data
James Ferrando Searches for tt̄ resonances 17/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
tt̄ resonances
l+jets
search for bumps in Mtt̄ :
mass [GeV]tt
0 500 1000 1500 2000 2500 3000
Eve
nts
/ GeV
-310
-210
-110
1
10
210
mass [GeV]tt
0 500 1000 1500 2000 2500 3000
Eve
nts
/ GeV
-310
-210
-110
1
10
210
data
ttW+jets
Other Backgrounds
UncertaintiesZ' (800 GeV) (1300 GeV)
KKg
ATLAS -1Ldt=2.05 fb∫ =7TeVs
dileptons
Use HT + EmissT :
[GeV]missT+ETH
200 400 600 800 1000 1200
Eve
nts
/ GeV
-310
-210
-110
1
10
210
[GeV]missT+ETH
200 400 600 800 1000 1200
Eve
nts
/ GeV
-310
-210
-110
1
10
210data
tt
Z+jets
Other Backgrounds
Uncertainties
(1100 GeV)KK
g
ATLAS
=7TeV s -1
Ldt=2.05 fb∫
[GeV]missT+ETH
200 400 600 800 1000 1200
Eve
nts
/ GeV
-310
-210
-110
1
10
210
No evidence for new physics signals
James Ferrando Searches for tt̄ resonances 18/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
tt̄ resonances
l+jets
Z' mass [GeV]
600 800 1000 1200 1400 1600 1800 2000
) [p
b]t t
→ B
R(Z
'× σ
-110
1
10
210
Lepton + jetsObs. 95% CL upper limitExp. 95% CL upper limit
uncertaintyσExp. 1 uncertaintyσExp. 2
Leptophobic Z'
Lepton + jetsObs. 95% CL upper limitExp. 95% CL upper limit
uncertaintyσExp. 1 uncertaintyσExp. 2
Leptophobic Z'
ATLAS
-1 = 2.05 fbdt L ∫
= 7 TeVs
mass [GeV]KK
g
600 800 1000 1200 1400 1600 1800
) [p
b]t t
→K
K B
R(g
× σ
-110
1
10
210
Lepton + jetsObs. 95% CL upper limitExp. 95% CL upper limit
uncertaintyσExp. 1 uncertaintyσExp. 2
Kaluza-Klein gluon
Lepton + jetsObs. 95% CL upper limitExp. 95% CL upper limit
uncertaintyσExp. 1 uncertaintyσExp. 2
Kaluza-Klein gluon
ATLAS
-1 = 2.05 fbdt L ∫
= 7 TeVs
l+jets: Limits set on narrow Z ′-like resonances: Exclude500 < MZ ′ < 880 GeV for benchmark (Topcolor-assistedtechnicolor) Z’ model.
James Ferrando Searches for tt̄ resonances 19/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
tt̄ resonances
dileptons
mass [GeV]KK
g
600 800 1000 1200 1400 1600 1800
) [p
b]t t
→K
K B
R(g
× σ
-110
1
10
210
Lepton + jetsObs. 95% CL upper limitExp. 95% CL upper limit
uncertaintyσExp. 1 uncertaintyσExp. 2
Kaluza-Klein gluon
Lepton + jetsObs. 95% CL upper limitExp. 95% CL upper limit
uncertaintyσExp. 1 uncertaintyσExp. 2
Kaluza-Klein gluon
ATLAS
-1 = 2.05 fbdt L ∫
= 7 TeVs
mass [GeV]KK
g
600 800 1000 1200 1400 1600 1800
) [p
b]t t
→ K
K B
R(g
× σ
-110
1
10
210
DileptonObs. 95% CL upper limitExp. 95% CL upper limit
uncertaintyσExp. 1 uncertaintyσExp. 2
Kaluza-Klein gluon
= 7 TeVs
-1 = 2.05 fbdt L ∫ATLAS
dileptons: Limits set on broader gKK -like resonances.Benchmark scenario: MgKK < 1025 TeV excluded. (l+jetsexcludes 500 < MgKK < 1130 for the main benchmarkscenario)
James Ferrando Searches for tt̄ resonances 19/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Going Boosted
On the previous slides, expected limits flatten at higher mtt̄
James Ferrando Searches for tt̄ resonances 20/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
tt̄
When pushing to higher energies, new factors come into play:
Low-energy tops
t → bW ,W → qq′ gives threedistinct “jets”:
High-energy tops
top decay system is highly boostedand reconstructed as only one jet:
Need new techniques to identify these boosted objects
James Ferrando Searches for tt̄ resonances 21/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Parton Merging
Merging of some description occurs for SM tt̄ production:
Effect must be taken into account for SM measurements athigher Pt
T or Mtt̄
James Ferrando Searches for tt̄ resonances 22/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Top Tagging
Hadronic top decays give the simplest case:
Use the mass of the jet and exploit features of the KT
algorithm:
Start with standard Anti-KT jets and run exclusive KT
algorithm on the constituents.KT effectively undoes the QCD showeringObjects merged at each step have smallest
dij = min(k2T ,i , k
2T ,j)
(∆R)2
RSo the last objects merged have the largest dij (e.g. come fromthe highest scale splitting)We force KT to give us n jets and ask what the last dijwas
James Ferrando Searches for tt̄ resonances 23/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
SubstructureMeasurement
0 50 100 150 200 250 300
GeV
1
d mσ
d
σ1
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02 ATLAS 1 L = 35 pb∫2010 Data,
Statistical unc.
Total unc.
Pythia
Herwig++
= 1, |y| < 2PVN
R=1.0tantik
< 500 GeVT
400 < p
Jet mass [GeV]0 50 100 150 200 250 300
MC
/Data
0.20.40.60.8
11.21.41.61.8 0 10 20 30 40 50 60 70 80 90 100
GeV
1
12
dd
σ d
σ1
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07ATLAS 1
L = 35 pb∫2010 Data,
Statistical unc.
Total unc.
Pythia
Herwig++
= 1, |y| < 2PVN
R=1.0tantik
< 500 GeVT
400 < p
[GeV]12
d0 10 20 30 40 50 60 70 80 90 100
MC
/Data
0.20.40.60.8
11.21.41.61.8
Jet substructure (including mass and splitting scales) wasmeasured in ATLAS - JHEP 1205 (2012) 128
Calibration and uncertainties of simple splitting variables andjet mass already understood at ATLAS
James Ferrando Searches for tt̄ resonances 24/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
tt̄
Put into practice in: ATLAS - JHEP 1209 (2012) 041
Same lepton selection asprevious analysis (l+jets)
No b-tag
Look for boosted t → bqq:
Large-R (1.0) anti-kT jetRequire large jet mass andfirst kT splitting scale(d12)
Reconstruct mtt̄ fromhadronic top cand +lepton,EmissT and nearest anti-kT
(R=0.4) jet
James Ferrando Searches for tt̄ resonances 25/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Candidate
fat jet mass [GeV]
100 120 140 160 180 200 220 240 260 280 300
events
/ 1
0 G
eV
0
10
20
30
40
50
60ATLAS
= 7 TeVs 1
Ldt=2.05 fb∫Background subtracted
Datatt
Uncertainty
James Ferrando Searches for tt̄ resonances 26/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
tt̄
Extract limits on narrow (<3%) or Wide (∼10%) resonance → tt̄:
comfortably outperforming the conventional analysis at high mtt̄
James Ferrando Searches for tt̄ resonances 27/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
ATLAS l+jets7 TeV
The first search to fully combine boosted and resolved approaches:Phys. Rev. D 88, 012004 (2013)
Boosted:leptonEmissT
≥ 1 large-R jet with pT > 350 GeV and large jet-mass≥ 1 b-jet
ResolvedFails boosted selectionleptonEmissT
≥ 4 jets or ≥ 3 jets and one jet has a mass > 60 GeV≥ 1 b-jet
included also several improvements compared to previous iterations
James Ferrando Searches for tt̄ resonances 28/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
We can makeit better
Building a better search:
Resolved: Improve tt̄ reconstruction
Boosted: Add b-tagging (reduce largeW+jets background),
Both: Improve isolation definition,
James Ferrando Searches for tt̄ resonances 29/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
tt̄ Reconstruction
Adopted a χ2 method for choosing jets to use in calculation of thett̄ mass:
χ2 =
[mjj −mW
σW
]2
+
[mjjb −mjj −mth−W
σth−W
]2
+
[mj`ν −mt`
σt`
]2
+
[(pT,jjb − pT,j`ν)− (pT,th − pT,t`)
σdiffpT
]2
James Ferrando Searches for tt̄ resonances 30/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Lepton Isolation
Conventional lepton isolation is also a problem for boosted tops:Standard isolation requirements:
Require lepton and nearest jetwell separated ( ∆R(l , j) > 0.4 )
require pT within a small conearound the lepton track is lessthan some value
Solution: Adopt mini-isolation (JHEP 1103 (2011) 059)
Size of isolation cone shrinks with pT , ∆R = k/plT (in thecase of ATLAS k = 10 GeV is used)
Require pT is less than some value (in the case of ATLAS< plT/20.0 )
... and relax requirement on ∆R(l , j) in the µ channel.
James Ferrando Searches for tt̄ resonances 31/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Lepton Isolation
QCD background falseidentification rate [%]
0 1 2 3 4 5 6
Sig
na
l e
ffic
ien
cy [%
]
0
20
40
60
80
100
Tp / miniI
miniI
TpPtcone30 / Ptcone30
TpEtcone20 / Etcone20
PreliminaryATLAS1 = 14.2 fbL dt∫ = 8 TeVs
QCD background falseidentification rate [%]
0 1 2 3 4 5 6
Sig
na
l e
ffic
ien
cy [%
]
0
20
40
60
80
100
Tp / miniI
miniI
TpPtcone30 / Ptcone30
TpEtcone20 / Etcone20
PreliminaryATLAS1 = 14.2 fbL dt∫ = 8 TeVs
Performance of mini-isolation is very good and stable for differentZ ′ masses (1.0 TeV (left) and 2 TeV (right))
James Ferrando Searches for tt̄ resonances 32/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Selection Efficiency
Muon channel efficiency now rises with mtt̄
Fall-off at high masses for electrons because ∆R(l , j) cutcould not be relaxed
James Ferrando Searches for tt̄ resonances 33/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Selection efficiency
Overall signal efficiency is high (this value is relative to all tt̄)
James Ferrando Searches for tt̄ resonances 34/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
mtt̄
James Ferrando Searches for tt̄ resonances 35/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
mtt̄
James Ferrando Searches for tt̄ resonances 36/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
mtt̄
benchmark models excluded up to 1.75 TeV (Z ′) and 2.1 TeV(gKK)
James Ferrando Searches for tt̄ resonances 37/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Going to 8 TeV
First ATLAS search using partial 8 TeV dataset:
Improvements:Introduced Trimming of large-R jet to mitigate pile-up
Disadvantages:large-R jet triggers not available at this time (large hit immuon channel efficiency)
James Ferrando Searches for tt̄ resonances 38/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Trimming:Concepts
Performance of Trimming discussed in detail in:ATLAS - JHEP 1309 (2013) 076
Works by:
running a small-R (0.3) kT algorithm on large-R jetconstituents to make subjets
keeping only subjets with pT greater than a certain fraction(0.05) of the large-R-jet
This “trims” away soft activity in the jet
James Ferrando Searches for tt̄ resonances 39/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Trimming:Performance
)PV
Reconstructed vertex multiplicity (N0 2 4 6 8 10 12 14
[GeV
]⟩
jet
m⟨
20
40
60
80
100
120
140
160 ATLAS = 7 TeVs, -1 Ldt = 1 fb∫Data 2011
LCW jets with R=1.0tanti-k| < 0.8η < 300 GeV, |
T
jet p≤200 No jet grooming =0.3
sub=0.01, Rcutf
=0.3sub
=0.03, Rcutf =0.3sub
=0.05, Rcutf=0.2
sub=0.01, Rcutf =0.2
sub=0.03, Rcutf
=0.2sub
=0.05, Rcutf
Jet mass [GeV]0 50 100 150 200 250 300
Arb
itrar
y un
its0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
t t→Ungroomed Z'Ungroomed Dijets
t t→Trimmed Z'Trimmed Dijets
SimulationATLAS < 800 GeVjet
T p≤ LCW jets, 600 tanti-k
Trimming makes jet substructure quantities robust against pile-up
James Ferrando Searches for tt̄ resonances 40/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
Selection Efficiency
Electron channel loss - due to trimming
Muon channel loss - trimming and trigger
Partly mitigated by some other gains in reconstruction
James Ferrando Searches for tt̄ resonances 41/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
mtt̄
James Ferrando Searches for tt̄ resonances 42/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
mtt̄
Eve
nts
/ T
eV
1
10
210
310
410
510
610Data tt
Multijets W+jets
Other Backgrounds
0 0.5 1 1.5 2 2.5 3 3.5
ATLAS Preliminary
= 8 TeVs
1 L dt = 14.3 fb∫
e + jetsboosted
[TeV]reco
ttm
Data
/Bkg
0.51
1.5
0 0.5 1 1.5 2 2.5 3 3.5
Eve
nts
/ T
eV
1
10
210
310
410
510
610Data tt
Multijets W+jets
Other Backgrounds
0 0.5 1 1.5 2 2.5 3 3.5
ATLAS Preliminary
= 8 TeVs
1 L dt = 14.2 fb∫
+ jetsµboosted
[TeV]reco
ttm
Data
/Bkg
0.51
1.5
0 0.5 1 1.5 2 2.5 3 3.5
James Ferrando Searches for tt̄ resonances 43/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
mtt̄
Z’ mass [TeV]
0.5 1 1.5 2 2.5 3
) [p
b]
t t
→ B
R(Z
’×
Z’
σ
210
110
1
10
210
310
Obs. 95% CL upper limit
Exp. 95% CL upper limit
uncertaintyσExp. 1
uncertaintyσExp. 2
Leptophobic Z’ (LO x 1.3)
Obs. 95% CL upper limit
Exp. 95% CL upper limit
uncertaintyσExp. 1
uncertaintyσExp. 2
Leptophobic Z’ (LO x 1.3)
ATLAS Preliminary
1 = 14.3 fbdt L ∫
= 8 TeVs
mass [TeV]KK
g
0.5 1 1.5 2 2.5
) [p
b]
t t
→K
K B
R(g
× K
Kg
σ
210
110
1
10
210
310
Obs. 95% CL upper limit
Exp. 95% CL upper limit
uncertaintyσExp. 1
uncertaintyσExp. 2
KaluzaKlein gluon (LO)
Obs. 95% CL upper limit
Exp. 95% CL upper limit
uncertaintyσExp. 1
uncertaintyσExp. 2
KaluzaKlein gluon (LO)
ATLAS Preliminary
1 = 14.3 fbdt L ∫
= 8 TeVs
benchmark models excluded up to 1.8 TeV (Z ′) and 2.0 TeV (gKK)
James Ferrando Searches for tt̄ resonances 44/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
CMS 8 TeV Search
New CMS search CMS - Phys. Rev. Lett. 111 (2013) 211804
Combines all-hadronic (boosted)and l+jets (resolved andboosted), channels
Boosted l+jets:
Separated into separatechannels by b-tagno isolation required forleptonsrequire at least two jetsBuild tt̄ combination via achi2, cut on the chi2 toreduce backgrounds
[GeV]ttM0 500 1000 1500 2000 2500 3000 3500
Eve
nts
/ 10
0 G
eV
-110
1
10
210
310
410 Data
ttothersZ' 2 TeV
= 8 TeVs, -1CMS, 19.7 fb
= 0b-tag
Nµe+
(a)
James Ferrando Searches for tt̄ resonances 45/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
CMS 8 TeV Search
New CMS search CMS - Phys. Rev. Lett. 111 (2013) 211804
Combines all-hadronic (boosted)and l+jets (resolved andboosted), channels
Boosted l+jets:
Separated into separatechannels by b-tagno isolation required forleptonsrequire at least two jetsBuild tt̄ combination via achi2, cut on the chi2 toreduce backgrounds
[GeV]ttM0 500 1000 1500 2000 2500 3000 3500
Eve
nts
/ 10
0 G
eV-110
1
10
210
310
410 Data
ttothersZ' 2 TeV
= 8 TeVs, -1CMS, 19.7 fb
1≥ b-tag
Nµe+
(b)
James Ferrando Searches for tt̄ resonances 45/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
CMS 8 TeV Search
New CMS search CMS - Phys. Rev. Lett. 111 (2013) 211804
Combines all-hadronic (boosted)and l+jets (resolved andboosted), channels
All-hadronic:
Requires two high-pT large-Rjets - that are “top-tagged”with a sophsiticated taggertwo leading back-to-backb-tagged small-R jets
[GeV]ttM1000 1500 2000 2500 3000 3500
Eve
nts
/ 10
0 G
eV-110
1
10
210
310
410Data
ttNTMJZ' 2 TeV
= 8 TeVs, -1CMS, 19.7 fb
all-hadronic
(c)
James Ferrando Searches for tt̄ resonances 46/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
CMS 8 TeV Search
New CMS search CMS - Phys. Rev. Lett. 111 (2013) 211804
Combines all-hadronic (boosted)and l+jets (resolved andboosted), channels
Resolved l + jets:
require at least four jetsseparate into b-tag categoriesbuild a χ2 and cut on itFit a smoothyl falling pdf tothe SM background
[GeV]tt
(d) M600 800 1000 1200 1400 1600 1800 2000
Eve
nts
/ 50
GeV
210
310
410
510 1≥
b-tag, Nµe+
= 8 TeVs, -1CMS, 19.7 fb
DataBackgroundZ' 750 GeV
James Ferrando Searches for tt̄ resonances 47/ 65
IntroductionRecent Searches
The FutureSummary
Early ATLAS searches @ 7 TeVATLAS searches with the Full 7 TeV dataATLAS and CMS searches with 8 TeV data
CMS 8 TeV Search
[TeV] Z'M0.5 1 1.5 2 2.5 3
) [p
b]
t t
→ B
(Z'
× Z
'σ
Up
per
lim
it o
n
-310
-210
-110
1
10
210Expected (95% CL)
Observed (95% CL)
Z' 1.2% width
Expectedσ1±
Expectedσ2±
= 8 TeVs, -1CMS, 19.7 fb
All-hadronic combined with the separate resolved and boostedl+jets results in different kinematc regimes.
James Ferrando Searches for tt̄ resonances 48/ 65
IntroductionRecent Searches
The FutureSummary
Towards Run IILHC upgradeUpgrade Schedule
Proud historybright future?
Boost 2012 report - arXiv:1311.2708
What’s next?
Towards LHC run-II
Prospects with theupgraded LHC
James Ferrando Searches for tt̄ resonances 49/ 65
IntroductionRecent Searches
The FutureSummary
Towards Run IILHC upgradeUpgrade Schedule
Towards Run II
How much luminosity is neeed at 13 TeV to be competitive withcurrent data?
0 500 1000 1500 2000 2500 30000
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2 @ 8 TeV-1 @ 13 TeV vs 20fb-11 fb
(13
TeV
)/(8
TeV
)b
s/
(GeV)ttm
(13 TeV)t t→Z' Current Observed LimitCurrent Expected Limit
0 500 1000 1500 2000 2500 30000
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2 @ 8 TeV-1 @ 13 TeV vs 20fb-15 fb
(13
TeV
)/(8
TeV
)b
s/
(GeV)ttm
(13 TeV)t t→Z' Current Observed LimitCurrent Expected Limit
(simple extrapolation using cross sections for Z ′ and NLO tt̄ inappropriate mtt̄ range)
James Ferrando Searches for tt̄ resonances 50/ 65
IntroductionRecent Searches
The FutureSummary
Towards Run IILHC upgradeUpgrade Schedule
Towards Run II
0 500 1000 1500 2000 2500 30000
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2 @ 8 TeV-1 @ 13 TeV vs 20fb-16 fb
(13
TeV
)/(8
TeV
)b
s/
(GeV)ttm
(13 TeV)t t→Z' Current Observed LimitCurrent Expected Limit
0 500 1000 1500 2000 2500 30000
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2 @ 8 TeV-1 @ 13 TeV vs 20fb-17 fb
(13
TeV
)/(8
TeV
)b
s/
(GeV)ttm
(13 TeV)t t→Z' Current Observed LimitCurrent Expected Limit
Reach should start to increase as we approach 6-7 fb−1
James Ferrando Searches for tt̄ resonances 51/ 65
IntroductionRecent Searches
The FutureSummary
Towards Run IILHC upgradeUpgrade Schedule
Upgrades
James Ferrando Searches for tt̄ resonances 52/ 65
IntroductionRecent Searches
The FutureSummary
Towards Run IILHC upgradeUpgrade Schedule
Upgrade Schedule
Example, ATLAS upgrade schedule:
Phase I
Installation Date: 2018-19
Detector upgrades:µ-trigger, L1 Calo-trigger,FTK, new Small wheel formuons , new forwarddetectors . Various readoutimprovements. (Maintainperformance at higherluminosity)
Lumi 2.2× 1034 cm−2s−1,300-400 fb−1 by 2022,µ = 55-80
Phase II
Installation Date: 2022-24
Detector upgrades: SplitL0/L1 trigger, numeroustrigger and readout upgrades,improved HLT, RPCprecision upgrade, completetracker replacement. (Maintain/improveperformance at higher µ,improve resistance toradiation damage)
Lumi 5× 1034 cm−2s−1, upto 3000 fb−1, µ = 140-200
James Ferrando Searches for tt̄ resonances 53/ 65
IntroductionRecent Searches
The FutureSummary
Towards Run IILHC upgradeUpgrade Schedule
Upgrade studies
ATLAS upgrade performance and physics prospects have beenstudied in:
Phase-I LOI: CERN-LHCC-2011-012
Phase-II LOI: CERN-LHCC-2012-022
tt̄ resonance search: ATL-PHYS-PUB-2013-003
Studies of tt̄ resonance searches done with a parametrisation ofdetector response, not at the full-simulation level.
James Ferrando Searches for tt̄ resonances 54/ 65
IntroductionRecent Searches
The FutureSummary
Towards Run IILHC upgradeUpgrade Schedule
Upgrade Studes
tt̄ in simplified l+jets (boosted) and dilepton selections
James Ferrando Searches for tt̄ resonances 55/ 65
IntroductionRecent Searches
The FutureSummary
Towards Run IILHC upgradeUpgrade Schedule
Upgrade Studes
tt̄ in simplified l+jets (boosted) and dilepton selections
James Ferrando Searches for tt̄ resonances 56/ 65
IntroductionRecent Searches
The FutureSummary
Towards Run IILHC upgradeUpgrade Schedule
Upgrade Studes
tt̄ in simplified l+jets (boosted) and dilepton selections
James Ferrando Searches for tt̄ resonances 57/ 65
IntroductionRecent Searches
The FutureSummary
Towards Run IILHC upgradeUpgrade Schedule
Upgrade Studes
tt̄ in simplified l+jets (boosted) and dilepton selections
Exclusion reach for benchmarks could extend as far as 5-6TeV after the phase=II upgrade
Of course we hope for a discovery before then
James Ferrando Searches for tt̄ resonances 58/ 65
IntroductionRecent Searches
The FutureSummary
Summary
ATLAS + CMS haveperformed severalsearches for newparticles decaying to tt̄
Deployed newtechniques for theidentification of heavyboosted objects
No evidence for newparticles → tt̄ yet
Need to look carefullyat the data for allkinds of new physics, itmay not contain whatwe expect...
James Ferrando Searches for tt̄ resonances 59/ 65
ATLAS All-hadronicMore on l+jets
Top Tagging
Back-up
James Ferrando Searches for tt̄ resonances 60/ 65
ATLAS All-hadronicMore on l+jets
Top Tagging
All-hadronic
Initial jet
C/A
C/A
mj1/mjet < µfrac
mj2/mj∗2 < µfrac
James Ferrando Searches for tt̄ resonances 61/ 65
ATLAS All-hadronicMore on l+jets
Top Tagging
All-hadronic
Make exactly three
jets
Top candidate
mab = mW (1 ± 0.15) (a, b = j1, j2, j3)
James Ferrando Searches for tt̄ resonances 62/ 65
ATLAS All-hadronicMore on l+jets
Top Tagging
l+jetsBackgrounds
James Ferrando Searches for tt̄ resonances 63/ 65
ATLAS All-hadronicMore on l+jets
Top Tagging
l+jetsBackgrounds
James Ferrando Searches for tt̄ resonances 64/ 65
ATLAS All-hadronicMore on l+jets
Top Tagging
top-tagging
Progress in top tagging: ATLAS-CONF-2013-084
James Ferrando Searches for tt̄ resonances 65/ 65