Kenneth Wraight
Fully-leptonic ttbar + jets events at ATLAS
• Introduction to top quarks at ATLAS
• MC based di-leptonic cross-section measurement
• Study of effect of jet algorithm choice
• Study of effect of ISR model variation on simulated cross-section
Thanks to Craig Buttar and Sarah Allwood-Spiers
Part I
Introduction
to top quarks
at ATLAS
2
The ubiquitous top quark
• The strong force is measured in top
pair production.
• The weak force is measured in top
decay and single top production.
• Fundamental top parameters like
mass, spin and charge still require
precise measurement .
• Tops decay before hadronisation,
passing spin information to
daughters. This provides unique
environment for Beyond Standard
Model searches.
• Ultimate test of multi-scale QCD
calculations used to predict top and
new physics properties.
3
The top quark as background
LHC will be the world’s first proper top quark factory:
~1 ttbar event per second for σ=833pb at L=1033cm-2s-1
Background to...
associated Higgs production
multi-jet SUSY decays
new & exotic physics
4
Diameter = 25m, Length = 46m, Weight = 7000 tonnes
LHC
ATLAS (the detector)
Muon
detectors
Calorimeters
ATLAS
Magnets
Inner detector
ATLAS is a general purpose detector designed to measure
collision products in order to test the Higgs theory and look
for signatures of ‘New Physics’.
5
Part II
MC based
di-leptonic
cross-section
measurement*
*based on work done for Atlas Public Note: ATL-PHYS-PUB-2009-086
6
At 14 TeV total σ(LHC) = 125 ± 25 mb
σ(ttbar) = 833 ± 12% pb
 try to extract ~1 signal event from 20 million total.
Signal Vs. Background
• Dileptonic (e/μ) events are
only ~5% of the ttbar crosssection.
BUT
• Clean channel
• Distinctive trigger
• No need to reconstruct top mass
Z+jets

W+jets

7
Event Characteristics
fully leptonic  2 opposite charge leptons (e/μ)
2 neutrinos  large missing energy
2 b quarks + ISR/FSR  2 or more jets
8
Experimental Object Definitions (@ 10TeV)
• Electrons:
–
–
–
–
shower shape requirement
isolation: etcone20 < 6 GeV
pt > 20 GeV
0 < |η| < 1.37 or 1.52 < |η| < 2.47
• Muons:
–
–
–
–
• Jets & MET:
match inner det. & spectrometer
isolation: etcone20 < 6 GeV
pt > 20 GeV
|η| < 2.5
–
–
–
–
Cone jets
pt > 20 GeV
|η| < 2.5
MET>20/35GeV
• Overlap*:
– select electrons
– remove jets within
ΔR < 0.2 of electron
– remove muons within
ΔR < 0.3 of jet
*electrons & jets share same container 9
Samples & Event Selection (@ 10TeV)
sample
σ * k-factor
ttbar (non hadronic)
202.86 * 1.07
Weν +jets
13253.84 * 1.22
Wμν +jets
13242.22 * 1.22
Wτν +jets
13237.72 * 1.22
Zee +jets
1183.66 * 1.22
Zμμ +jets
1182.41 * 1.22
Zττ +jets
1178.57 * 1.22
Wbb
14.64 * 1.22
WW
15.62 * 1.69
WZ
1.37 * 1.42
ZZ
4.87 * 1.81
single top (t-channel)
41.12 * 1.05
single top (W-channel)
14.41 * 0.99
• single flavor channel (ee, μμ):
– trigger: 2 high-pt leptons
(EF_e15_medium , EF_mu15,
resp.)
– two opposite charged leptons,
pt > 20 GeV & |η|<2.5
– Etmiss > 35 GeV
– ≥ 2 jets of pt > 20 GeV
– Z-pole veto 86 < mll < 96 GeV
• mixed flavor channel (eμ):
– trigger: 2 high-pt leptons
(EF e15 medium or EF mu15, resp.)
– two opposite charged leptons,
pt > 20 GeV & |η|<2.5
10
– Etmiss > 20 GeV
*quoted errors are from
sample statistics only
cumulative*:
exclusive*:
di-electron channel selection cutflow (for L=200pb-1 @10TeV)
↑ATLAS work in Progress↓
11
non-fully leptonic ttbar, Zee, Zμμ, Zττ, Weν,
Wμν, Wτν, Wbb, WW, WZ, ZZ, t-chan, W-chan
Ns=228 Nb=45
Results
ATLAS work
in progress
•
The selection procedure above results in
the following S/√(S+B) ratios for 200pb-1:
ee = 13.1, μμ = 16.3 and eμ = 24.5,
cf.
ee = 13.2, μμ = 16.4 and eμ = 24.6 from
the Pub Note
• with the following cross-sections:
ee = 216.96 ± 23.9 pb, μμ = 217.03 ± 19.1 pb, eμ = 216.67 ± 13.4 pb
 σlep comb.= 217.41 ± 10.0 pb,
cf. 217.06 pb*
*MC@NLO sample cross-section*K-factor
--from Pub. Note: ATL-PHYS-PUB-2009-086
12
Part III
Study:
systematic effect of
jet algorithm choice
13
effect of jet algorithms
• A few jet algorithms about these days...
–
–
–
–
•
•
•
•
Atlas Cone (old favourite) – cone based, unsafe
Kt – cluster based, IR&Collinear safe
anti-Kt – cluster based, IR&Collinear safe
SIS cone – cone based, IR&Collinear safe
effects come from resolution & JES differences between algorithms
observed distributions can change
signal & background acceptances can change
S/(S+B) & significance determine importance of acceptance changes
• R= 0.4 for kt cut-off and SIS cone size.
• Pseudo-event samples and selection same as in Top PubNote.
• Main backgrounds (based on Pub Note selections) ...
- ee : Zee + jets && Wenu + jets
- mumu : Zmumu + jets
- mix : Z tautau + jets
14
Signal & background... for L=200pb-1 @ 10TeV
alg.
cone
Kt
anti-Kt
SIS cone
chan.
ee
μμ
eμ
ee
μμ
eμ
ee
μμ
eμ
ee
μμ
eμ
S
259
350
778
249
338
761
250
337
761
252
336
765
B
30.7 51.8 28.4 24.1 49.9 25.8 27.2 50.3 26.9 31.4 48.3 27.4
S/
0.89 0.87 0.97 0.88 0.87 0.97 0.90 0.87 0.97 0.89 0.88 0.97
S+B
S/
15.2 17.5 27.4 14.8 17.2 27.1 15.0 17.1 27.1 15.0 17.1 27.2
√(S+B)
↑ATLAS work in Progress
• Varying jet algorithm choice does not substantially effect signal or (main)
background distributions.
• Acceptance variance is within level of a few percent which is comparable with other
systematics
• Relative (i.e.S/(S+B)) selection is constant across algorithms.
• S/√(S+B) varies slightly across algorithms. All compare well with Pub. Note:
15
ee: 14.2, μμ: 17.3, eμ: 26.2
Part IV
Study:
systematic effect of ISR
model variation on
simulated cross-section
This research project has been supported by a Marie Curie Early Stage
Research Training Studentship of the European Community’s Sixth
Framework Programme under contract number (MRTN-CT-2006035606-MCnet)
Special thanks to Peter Skands
16
320 = Perugia 0 Pythia tune: benchmark
321 = Perugia Pythia tune: larger ISR phase-space & harder hadron’n
322 = Perugia Pythia tune: smaller ISR phase-space & softer hadron’n
ISR effects on ttbar pt spectra (hadron level)
ATLAS work
in progress
ttbar system
ATLAS work
in progress
tops
ATLAS work
in progress
leptons
ATLAS work
in progress
W bosons
Initial differences in ttbar system washed-out by decay-chain
17
320 = Perugia 0 Pythia tune: benchmark
321 = Perugia Pythia tune: more pert. activity, less non-pert. particles
322 = Perugia Pythia tune: less pert. activity, more non-pert. particles
ISR effects on ttbar hadrons & jets
ATLAS work
in progress
ATLAS work
in progress
# cone jets
hadron level FS
hadrons
atlfast
# hadrons
ATLAS work
in progress
hadron pt
ATLAS work
in progress
selected Cone
jets
cone jet pt
No. of jets determined by hadron pt not multiplicity
18
320 = Perugia 0 Pythia tune: benchmark
321 = Perugia Pythia tune: more pert. activity, less non-pert. particles
322 = Perugia Pythia tune: less pert. activity, more non-pert. particles
ISR effects on sample selection
• 100k events for each sample
• used (close to) Pub. Note selection (see above)
sample
ee
μμ
eμ
comb.
ttbar
9390
+5.7% -3.5%
10975
+3.2% -4.2%
23631
+3.2% -2.7%
43996
+3.7% -3.2%
W
+jets
629
9.7% -25.6%
--
947
+3.5% -28.9%
1576
+6.0% -27.6%
Z
+jets
463
453
+10.6% -29.2% +6.4% -23.8%
3
919
+100% -33.3% +8.8% -26.6%
WW
+jets
323
451
+27.2% -17.0% +7.1% -22.8%
913
1687
+21.5% -15.3% +18.7% -17.6%
Effects of ISR more important in background estimation
19
Summary
• Completed a MC based cross-section measurement
– including all relevant backgrounds (except QCD di-jets)
– results in accordance with published results
• Investigated the effects of jet algorithm choice
– on ttbar signal and major channel backgrounds
• Investigated the effects of ISR variations
– on ttbar signal and selected background
• Currently on MCnet studentship at CERN
– working on min. bias & UE focussed analyses
20
Backup
21
non-fully leptonic ttbar, Zee, Zμμ, Zττ, Weν,
Wμν, Wτν, Wbb, WW, WZ, ZZ, t-chan, W-chan
Ns=228 Nb=45
selection stack plots (di-electron channel) – Jets & Etmiss
ATLAS work
in progress
ATLAS work
in progress
ATLAS work
in progress
ATLAS work
in progress
22
non-fully leptonic ttbar, Zee, Zμμ, Zττ, Weν,
Wμν, Wτν, Wbb, WW, WZ, ZZ, t-chan, W-chan
Ns=228 Nb=45
selection stack plots (di-electron channel) – Electrons
ATLAS work
in progress
ATLAS work
in progress
ATLAS work
in progress
23
Systematics (details)
--from Pub. Note: ATL-PHYS-PUB-2009-086
24