Transcript Study of tt production at NLO

Summary of the 2005 Rome
ATLAS Physics Workshop
M. Cobal, University of Udine
Physics Plenary, ATLAS Week, June05
Rome physics workshop
91 entries (out of
about 100 talks),
21 F plus
70 M
Speakers
age distribution
Entries / 2 years
12
10
8
6
4
2
76
72
68
64
60
56
52
48
44
40
36
32
28
24
20
0
Age (years)
Some numbers:
— ~450 participants
— 100 talks
— ~ 35 hours of presentations and discussions
June 05 ATLAS Week - M. Cobal
Sessions for physics groups
For all groups bulk of analyses performed on fully
simulated “Rome” samples







Concentrate on analyses possible with few fb-1
B-physics
— HUGE amount of work/results
Displace center of interest from exploration of ATLAS
Top
— Cannot do justice to everything presented!!
parametrised potential to:
Higgs
— Give general flavour of the workshop highlights
•Control
of detector
systematics
— Focus as
requested
in talk title
on earlyaffecting
physics
Standard Model
measurements and discovery
SUSY
Exotics
Heavy Ions
•Study of dependency of discovery potential from
achieved level of alignment calibration
•Development of strategies for estimate of
systematics on background evaluation
June 05 ATLAS Week - M. Cobal
A new point of view: Commissioning!
The game to play:
Understand detector /
Minimize MC dependency

Knowing the detector



Prepair with detector pessimistic
scenarios
Non-perfect alignment at startup,
e.g. in b-tagging
Dead regions in the calorimeter / noise
Unknown precise jet energy scale
Assess trigger dependencies
Redundancy between detectors
Straight tracks, etc.
Physics: available ‘candle’ signals in physics



Presence and mass of the W±, Z0, top-quark
Presence of b-jets
Balance in transverse plane, PT
Only after full understanding of these the road to discovery starts…
June 05 ATLAS Week - M. Cobal
Top
physics
Standard Model
• Minimum bias/Underlying event
• Before Rome: comparing existing models with SPS/Tevatron
and extrapolating to LHC
• Now: based on the full ATLAS software chain, explore how well
we can measure typical quantities:
• Studies on W
• Large statistics
• Basic benchmark process
• Aim at constraining proton PDFs
• Emphasis on understanding systematic detector effects
Charged particle density at  = 0


Minimum bias events (~20/beam cross)
Example of “very early” physics: only
need a few thousands interactions



“Soft” part of pp interactions not
described by PQCD
Constitutes unavoidable background
for all physics
Measure typical quantities using
full ATLAS chain:



LHC?
dNch/d
dNch/dpT
Large uncertainty track densities!
Multiple interaction model in PHOJET predicts a ln(s) rise in energy
dependence. PYTHIA suggests a rise dominated by the ln2(s) term.
June 05 ATLAS Week - M. Cobal
Charged particle densities

Generated vs reconstructed tracks

Explore special runs without
solenoid magnetic field?
dNch/d
dNch/d
1000 events
B=0
dNch/dPT
Black = Generated charged tracks
Blue = Reconstructed: NO TRT, NO solenoid
Red = Reconstructed: NO TRT, WITH solenoid
limited rapidity coverage
MeV
Can only reconstruct track
down to ~500 MeV PT
June 05 ATLAS Week - M. Cobal
Pdf determination using W bosons
W+ and W- Rapidity
ud  W 
du  W 


Uncertainty in pdf transferred to sizeable
variation in rapidity distribution electrons
Limited by systematic uncertainties
 To discriminate between conventional
PDF sets we need to achieve an
accuracy ~3% on rapidity distributions.
Error boxes:
The full PDF
Uncertainties
CTEQ61 (MC@NLO)
MRST02 (MC@NLO)
ZEUS02 (MC@NLO)
e-
MRST03 (Herwig+k-Factors)
e+
Stat ~6 hours
at low Lumi.

June 05 ATLAS Week - M. Cobal

Pdf determination using W bosons

Generator level for Ws

W+ and W- Rapidity
Full simulation
e+ e- Pseudo-Rapidity
eW+
e+
Selection Cuts applied
W-
W- /W+
Ratio
d / dyW (W  )
R ( yW ) 
d / dyW (W )
e- /e+ Ratio
Selection Cuts applied
June 05 ATLAS Week - M. Cobal
Charge Asimmetry
W Asymmetry
d / dyW (W  )  d / dyW (W  )
A( yW ) 
d / dyW (W  )  d / dyW (W  )


Charge Misidentification
dilutes Asymmetry
Correction:
e+e- Asymmetry
TRUE
Selection Cuts applied
A
A RAW  F   F 

1 F   F 
ARAW = Measured Asymmetry
ATRUE = Corrected Asymmetry
F- = rate of true emisidentified as e+
F+ = rate of true e+
misidentified as e-
June 05 ATLAS Week - M. Cobal
Systematics using Full Simulation
Charge misidentification
TRUE
A
A RAW  F   F 

1 F   F 
F-
ARAW = Measured Asymmetry
ATRUE = Corrected Asymmetry
F- = rate of true emisidentified as e+
F+ = rate of true e+
misidentified as e-

Detector Level
F
Use Z -> e+e- sample from
Full Simulation Rome production
~98K events, Herwig+CTEQ5L
data-like analysis (No MC-Truth)

Mis-ID
+

rate negligible?
June 05 ATLAS Week - M. Cobal
Underlying event
Df = f  fljet
Transverse <nch>
UE is defined as the
Transverse Region
—Soft component in hard scattering event
— On fully simulated jet sample compare
reconstructed and generated multiplicity.
Njets > 1,
|ηjet| < 2.5,
ETjet >10 GeV,
|ηtrack | < 2.5,
pTtrack > 1.0 GeV/c
Good agreement reconstructed/generated
Can use to tune MonteCarlo
Pt leading jet (GeV)
June 05 ATLAS Week - M. Cobal
W-mass

Aim to determine M(W) with precision of
15 MeV


Highest precision expected in Wν
Observables



Transverse mass MT
Missing PTmiss
PT lepton
June 05 ATLAS Week - M. Cobal
Top
physics
Top physics
•Top production: basic calibration tool for early physics
1500 tt->bW(ln)bW(jj) requiring 4 jets above 40 GeV/day at low L.
•Need to select clean top sample from the beginning
•Past work: show in fast simulation that top signal observable
with no b-tagging
•Rome work: perform signal and background analysis in full
simulation
tt(tot)
= 759 pb
tt(semi-lept: e,) ~ 30%
Nevents ~ 700 per hour
Reconstruct top w/o b-tag
TOP
CANDIDATE
Observe top quarks after ~1 week?
When no b-tag is yet present?
Hadronic top:
Three jets with highest vector-sum
pT as the decay products of the top
W boson:
Selection cuts:
Missing ET > 20 GeV
1 lepton Pt > 20 GeV
4 jets
PT > 40 GeV
Two jets with highest momentum
in reconstructed jjj C.M. frame.
Selection efficiency = 5.3%
Trigger efficiency not taken
into account yet
June 05 ATLAS Week - M. Cobal
Analysis including W+4jets
background
m(t)
S/B = 0.45
S
B
Number of events / 5.1 GeV
Number of events / 5.1 GeV
Observe both top and hadronic W peaks!
W+jets bckg is large (and has large uncertainty)
m(W)
S/B = 0.27
300 pb-1
W+jets and
MC@NLO signal
Top mass (GeV)
W+jets and
MC@NLO signal
W mass (GeV)
Use peak position M(W) for light jet energy calibration
June 05 ATLAS Week - M. Cobal
Various cuts to improve purity
Ask for: 70 < M(jj) < 90 GeV
m(t)
Top peak clearly visible
after 1 week of LHC data
m(t)
Top mass (GeV)
B-JET
CANDIDATE
Ask for: b signal probability
> 0.90 on 4th jet
Top mass (GeV)
June 05 ATLAS Week - M. Cobal
Use W in top events for jet calibration
Effect of a mis-calibration of jet energy dominant systematics
Several methods to calibrate. Simplest one:
part
RM
PDG
W
/ MW = 1 2
with
k =  j1 j 2 
Ei
 i = jet
Ei

compute R for k bins in E

True
apply k factors on R and recompute R n times =>  k =

 kn
EPart / E
n
R
E
E
June 05 ATLAS Week - M. Cobal
Top
Z+jets
EPart / E
EPart / E
Results after recalibration
After calib ‘Top’
E
E





Use Top sample to correct jet energies of Z+jet sample
TOP 12000 jets, Z+jet 8000 jets
Apply same cuts on jets energies
=> Top light jet scale seems to work for all light jets
In progress: repeat exercise with backgrounds
June 05 ATLAS Week - M. Cobal
Single top production

Three production mechanism



Some could be seen at Tevatron
At LHC ‘precise’ determination
of all of them
Main backgrounds

Non top events



Detailed simulation of single top only just
started. No realistic backgrounds yet.
NLO generator MC@NLO expected!
Z+jets, W+jets
Top-pair production
B-tagging essential in this case!
June 05 ATLAS Week - M. Cobal
Finding the Higgs particle
We have two options:
We find the Higgs at the LHC
Gain deep knowledge on the Standard Model
We do not find the Higgs at the LHC
Something serious wrong with our understanding of the Standard Model
and it is observable at LHC
In the absence of Higgs, the WW scattering amplitude violates unitarity
Inclusive H to NLO
H is very sensitive to
detector performance

Study impact of new layout
(initial/Rome) is underway
Energy reconstruction of
converted photons is critical
issue

E()/E(true)

Energy reconstruction of converted
and non-converted photons
Non-converted
E()/E(true)

converted
E()
June 05 ATLAS Week - M. Cobal
Inclusive H to NLO

NLO QCD corrections



Higgs production via MC@NLO generator
Higgs decay via HDecay program
Used QCD NLO corrections to background
pp+X
H+0j
TDR-like analysis with NLO σ


Signal significance possibly further enhanced
by 40%.
H may be a discovery channel on its own
for 10 fb-1
H+1j
June 05 ATLAS Week - M. Cobal
H 4 leptons





The HZZ*4leptons channel is the
golden channel for SM Higgs search
in the mass range
120 GeV < MH<~800 GeV
TDR studies on both e and  channels
Main backgrounds are:
 ZZ*/* (irreducible)
 Zbb, tt (reducible)
Background rejection based on cuts on leptons pT, reconstructed Z and
Higgs masses, lepton isolation based on calorimeter energies, impact
parameter significance
Current studies aim mainly at assessing the reconstruction and
selection performance
 4-muons channel
 4-leptons channel
June 05 ATLAS Week - M. Cobal
Normalized to 30 fb-1
H 4 muons



Preselection cuts as in TDR

First two leptons pT>20 and ||<2.5, second pair
pT>7 and ||<2.5
Likelihood for reducible background (Zbb and ttbar)
rejection

2 largest IP, 2 largest pT, 2 largest transverse
energies in a DR=0.2 cone
Likelihood for irreducible background (ZZ) rejection

Z invariant masses, angles between two Z’s
decay planes,  angles in Z’s frame
Higgs
Mass
(Gev)
DC1
Muid Comb
Mass res
(GeV)
DC2
Muid Comb
Mass res
(GeV)
TDR
Mass res.
(GeV)
130
1.68±0.02
1.9±0.1
1.42±0.06
150
1.88±0.03
2.0±0.1
1.62±0.06
180
2.50±0.02
2.9±0.2
2.20±0.06
June 05 ATLAS Week - M. Cobal
H4 - different group
Signal
QCD ZZ
Zbb
ttbar
NLO, Normalized to 30fb-1
June 05 ATLAS Week - M. Cobal
Significances
Significances using LO cross
sections, 10 fb-1:
Significances using LO and
NLO for 10 fb-1:
MH [GeV]
4e
2e2
4
combined
130
Rome
1.4
2.3
1.7
3.2
DC1
1.0
1.9
1.6
2.8
Rome
2.9
4.2
3.2
5.8
DC1
2.4
4.0
3.1
5.6
Rome
1.5
2.3
1.5
3.1
DC1
1.2
2.0
1.5
2.8
Rome
2.5
3.8
2.7
5.2
DC1
2.1
3.2
2.4
4.5
150
180
300
● NLO
◦ LO
-For large range of Higgs masses discovery after 10 fb-1 (one year?)
-Combining electron and muon channels essential
June 05 ATLAS Week - M. Cobal
A nasty one: HW+W-l+νl-ν

Event topology
Require forward jets
Reject central jets

Missing energy
Counting experiment


Two opposite leptons
No Higgs mass peak!

Discriminant variable is e.g.
angle φ between leptons

Background top-pair production
and di-boson production:
This decay mode significant in
region 150 < MH < 180 GeV


At MH=170 BR 100 times HZZ
Understanding of bckgr’s critical!


Develop clever methods to
assess backgrounds from data
Statistically can claim discovery
with ~5fb-1 of data
June 05 ATLAS Week - M. Cobal
Another one: ttH signal

Very interesting alternative to
Higgs discovery using photons


Determination largest Yukawa
coupling from production cross
section: (ttHttbb,tttt,ttWW)
g2ttHBR(Hbb,Htt,HWW)
Challenging channel:




4 b-jets
2 light jets
Missing energy
Isolated lepton

Backgrounds:


Top-pair production with extra
jets
Rely heavily on ID tracking and
b-tagging capabilities


Detailed knowledge
detector needed
Not done with realistic
simulation and
backgrouond
treatment yet…
Low Luminosity: 30/fb
June 05 ATLAS Week - M. Cobal
Search
for
SUperSYmmetry
Search for SuperSymmetry
Elegant extension to the ‘Standard Model’ that…
 stabilizes the Higgs mass; predict light Higgs mass.
unifies the coupling constants of the three interaction
 provides a candidate for dark matter
 is consistent with all electroweak precision data

Complex signatures: e, µ, t, jets, b-jets, Etmiss
 Good test for detector performance and reconstruction.
 Analyses divided by signature
SuSy parameter space

Various ways to create some order in the chaos of multi-parameter space



Select several mSUGRA points




Unified boson and fermion masses at GUT scale as in mSUGRA models:
Only 4 free parameters remain: m0, m½, tanβ, A0, sign =±
Consistent with WMAP data for
cold dark matter
Don’t believe mSUGRA, but use
it to suggest interesting possible
particle spectra
Typically σ>1 pb, so early
discovery physics
SU1
SU6
Analyze each of these points

E.g. point SU1:
SU3
SU2
June 05 ATLAS Week - M. Cobal
Hadronic SuSy topologies

Susy characterized by decays:

 Decay to jets, perhaps leptons, and
escaping LSP (missing ET)
 Events characterized by large
Meff = ETmiss+Σ|pT, jet|
All hadronic decay


Backgrounds given by SM
processes: Z and W-production,
top production, multi QCD jets
At TDR this background was
estimated

Convincing SuSy signal obtained
using parton shower MC’s
SU2
June 05 ATLAS Week - M. Cobal
Hadronic SuSy

However, it is well known that
parton showers underestimate
the high PT region

So complete background
estimation is redone


Use the right MC
generators!
Using ME approach where
possible
Susy signal effectively
disappeared in this channel
June 05 ATLAS Week - M. Cobal
SUSY:Background
Main difference from PT jet
For ETmiss> 700 GeV :
clear excess
ETmiss vital for SUSY
searches
High PT jets are emitted by
background as well:
not clear separation
June 05 ATLAS Week - M. Cobal
SUSY: s-transverse mass for SU1
In all possible ways and compute:
June 05 ATLAS Week - M. Cobal
One-lepton SuSy

Signal reduced by factor 5



Background reduced by factor
20-30
Dominant background are semileptonic top-quark pairs
Largest uncertainty in Meff
originates from estimation of
ETmiss
Simulation of 3-4%
calo dead channels

ETmiss distribution sensitive to
detector imperfections
June 05 ATLAS Week - M. Cobal
One-lepton SuSy: ETmiss estimate

Obtain the ETmiss distribution from
data using top events



By fixing the top mass in the
leptonic channel, predict ETmiss
Select top without b-tagging
ETmiss for top signal minus sideband



Add SuSy



Repeat procedure with SuSy
signal included
ETmiss distribution from data
Clear excess from SuSy at
high ETmiss observed: method
works!
Reduce combinatoricalEstimate
backgroundbackground
Normalise at low ETmiss, where
from data
SuSy signals are small
Example of reducing MC dependency
on ETmiss distribution
June 05 ATLAS Week - M. Cobal
Di-lepton SuSy


In most scenarios the first SUSY
decay reconstructed is leptonic
decay of neutralinos.
“Smoking gun”: excess of
opposite-sign lepton pairs with
an edge structure in invariant
mass


p
p
No mass peak themselves can
be reconstructed
~
g
q

~
q
q
~
c0 2
~
c0 1
~
l
l
l
Example at point SU3:
4.37 fb-1
No cuts
Muons
opposite sign
same sign
Muon reconstruction efficiency is
essential
June 05 ATLAS Week - M. Cobal
SUSY: SU1 Leptonic Signatures
Coannihilation point
MC Truth, lL
MC Truth, lR
D. Costanzo, F.Paige
20.6 fb-1, No cuts
Soft lepton
Hard lepton
MC Data
Two edges from:
c 20
~
 l lL  llc10
~
c  llR  llc10
0
2
Each s-lepton close in mass to one
of the neutralinos – one of the
leptons is soft
June 05 ATLAS Week - M. Cobal
SUSY: SU-2 Dileptons
1° edge
T.L.
6.9 fb-1
No cuts
2° edge Z
6.9 fb-1
No cuts
Focus-Point
Heavy scalars:
no scalar lepton in c decay
Direct 3-body decays:
c 20  llc10
c 30  llc10
The two edges measure
the two mass differences
Δm = m(cn0) -m(c10)
Two edges expected at 57.0 and 76.4 GeV
June 05 ATLAS Week - M. Cobal
SUSY: SU-2 Dileptons
SU2 SUSY production is:
 cc (direct) (4.5 pb)
Do not pass cuts to reject SM
2.6 excess
(little jets & ETmiss)
 gg →cc+jets (0.5 pb)
This can be separated
efficiently from SM
 After cuts (from fast sim),
only few events remain.
 Edge reconstruction in SU2
needs higher integrated
SU2 dilepton invariant mass,
luminosity.
after cuts to reject SM
6.9 fb-1
2j100+4j50+xE100
June 05 ATLAS Week - M. Cobal
Tau signatures in SuSy



Tau signatures (mostly hadronic decays) are important in
much of the mSUGRA parameter space, particularly at high
tan
At some points in the parameter space (e.g. funnel) can
only observe kinematic endpoints in  invariant mass
distributions
Can often see endpoints in m, mq, etc, but:



 triangular shape distorted due to ETmiss from ν
 statistics much lower due to -reconstruction efficiency
(expecially for soft-taus, coannihilation point)
typically achieve /jet  100 for a -reconstruction ε of 50%
June 05 ATLAS Week - M. Cobal
SuSy: Tau signatures

Black points: MC truth


(98.3 GeV)
4.9


fb-1
Typical distortion due to
escaping neutrino’s in tau decay
However, can still fit this
distorted distribution to obtain
edge point
 note the triangular shape
Red line: distribution from nonleptonic decay products

 (distorted shape)
4.9 fb-1
a strong di- edge has been identified in the bulk region and it looks
Possible to extract a useful measurement in the coannihilation region
June 05 ATLAS Week - M. Cobal
SUSY: b-tagging
June 05 ATLAS Week - M. Cobal
Exotics
Among the most popular:
- Alternatives to EW symmetry breaking
- Extended gauge symmetries
- Extra dimensions besides our 4D space time
No light Higgs at the LHC?

Scenario without ‘light Higgs’ particle: VL VL → VL VL violates unitarity at
scales ~TeV, reachable by LHC!

Increase in cross section damped e.g. by strong symmetry breaking
mechanism


VL VL → VL VL described at low energy by an effective theory
General parameterisation of the “new physics”. Can lead to resonances in
WW / WZ scattering
Signal:
Important backgrounds :
High pT bosons
• W+jets, Z+jets
Few/no jets in
central region
(no colour
exchange)
• ttbar
• qq→WZqq , WWqq
Forward tag jets
June 05 ATLAS Week - M. Cobal
Example resonances in Wlν, Wjj

Separation of signal from background difficult

Again ttbar background is essential ; need better undertanding
Scalar
Vector
no resonance
30 fb-1 of data
- Signal
- ttbar
June 05 ATLAS Week - M. Cobal
Exotics: H++
L-R symmetric model would be a natural extension of
the SM
 SU(2)L x SU(2)R x U(1)B-L
 predicts new fermions: heavy Majorana neutrino
 predicts new gauge bosons: WR
 predicts new Higgs sector
D R = ( D 0R , D R , D 
R )
D L = ( D 0L , D L , D 
L ) (if Lagrangian is invariant under L  R symmetry)
0

f1,2
, f1,2
June 05 ATLAS Week - M. Cobal
150 GeV
Exotics: H++
q
q
Example
WW
q

W
W+
l
D 
L
q
ee
WW+jets

ee
H++


l

- Signal: 150, 200, 500 GeV (~5K)
- Backgrounds: WW+jets (~5K)
- Fake rate: jet/e
Mass
(GeV)
150
Mean
GeV
149.2 ± 0.08
148.9 ± 0.09
Sigma
GeV
9.9 ± 0.06
11.9 ± 0.06
Expected
ee+
Selected
240
WW+jets
June 05 ATLAS Week - M. Cobal
20 ± 1.
0
Exotics: Narrow Resonance Z’ ee
June 05 ATLAS Week - M. Cobal
Exotics: Narrow resonance Z’ tt
ennnn
June 05 ATLAS Week - M. Cobal
Exotics: Narrow resonance G*ee
June 05 ATLAS Week - M. Cobal
Exotics: Little Higgs
New approach to the hierarchy problem  many new particles:
•T, heavy top
•New gauge bosons WH, ZH, AH
cannot distinguish 
treated together
0
•Higgs triplet f , f, f
Example…
1, 2 jets
1 TeV
Z
ZH
Fully simulated evts
q
H
q


+
1 ou 2
1, 2 jets
jets
q
q
W
WH
H


with Z/W  qq, primary vertex can be determined
Z vertex used to correct  of the photons
June 05 ATLAS Week - M. Cobal
Exotics: Little Higgs

Signal at M(H)=120 GeV

ZH/WH  Z/W H  qq 
1 TeV

xefficiency
xresolution
x=
FullSim
ATLFAST
arbitrary units
Signif. ATLFAST
Full reco 10.0.1
ATLFAST
arbitrary units
Estimation of
the
significance
M(ZH/WH) (GeV)
Efficiency
Resolution in mass
(GeV)
Significance
31.6 %
48
16.6
M(ZH/WH) (GeV)
Efficiency
Resolution in mass
(GeV)
Significance
June 05 ATLAS Week - M. Cobal
22.6 %
35
13.9
Not only science fiction!




First cosmic event in UX15!!
Barrel TileCal is complete
in the cavern.
Single tower trigger
First cosmic events
observed last Tuesday!
June 05 ATLAS Week - M. Cobal