1. Alessandro Tricoli W production at LHC By
In collaboration with Monika Wielers
Rutherford-Appleton Laboratory
ATLAS UK Physics Meeting at Durham,
18th September 2006

2. Alessandro Tricoli, RAL
Overview
Introduction
W production mechanism
Importance of W production as standard candle process
Reconstruction of W->en events and background rejection
Standard ATLAS W Selection as in the TDR
Trigger-aware selection of W->en events against background
MWT and electron rapidity, ET distributions
Alternative approach for W selection: CMS-like
Jet Multiplicity with Cone and KT algorithms in W->en events
effect of jet-finder choice on W selection
Distortion on W-Asymmetry due to W selection cuts
Conclusions and Outlook
ATLAS UK Physics Meeting- Durham, 18th Sept. 2006
Alessandro Tricoli, RAL

3. Alessandro Tricoli, RAL
W± Production at LHC
LHC
pp -> W± + … W p p
Cabibbo
Suppressed
Valence-Sea
and mainly Sea-Sea :
largest contribution
Sea-Sea:
next largest contribution
(Cabibbo dominating), whereas ~5% at Tevatron
(17%)
(23%)
Cabibbo Suppressed Contribution 1-3% at LHC
ATLAS UK Physics Meeting- Durham, 18th Sept. 2006
Alessandro Tricoli, RAL

4. Why accurate W reconstruction is important
W production is a standard candle process:
theoretically and experimentally well known
large statistics:
~21 W->ln events per sec. at 1033 cm-2 s-1 lumi. (10 times larger than Z production)
Together with Z it is a control-process for detector calibrations/performances:
trigger selection efficiencies
particle ID efficiency: calorimeter and tracker performances
missing ET calibration (i.e. W->ln)
jet reconstruction (i.e. in situ JES calib. with W->jj in tt events)
luminosity monitor
Control-process for physics performances:
Theory cross check: electro-weak corrections, LO-NLO-NNLO approximations etc.
PDF constraining (see Mandy’s talk):
low-x gluon using W+ and W- rapidity distributions
low-x valence quarks using W± asymmetry
W+jets process is background to new physics
important to accurately determine its kinematics parameters and quantify uncertainties
See:
ATLAS Calorimeter
Calibration Workshop
5-8 Sept. 2006
ATLAS UK Physics Meeting- Durham, 18th Sept. 2006
Alessandro Tricoli, RAL

5. Alessandro Tricoli, RAL
Analysis scope
W reconstruction linked to various performance and physics groups:
egamma, muon, jet/Etmiss/tau
SM, Higgs, SUSY, Exotics
This work focuses on the electron channel W->en
Immediate aim is to contribute to CSC papers:
W/Z inclusive, W/Z+jets and PDF
ATLAS UK Physics Meeting- Durham, 18th Sept. 2006
Alessandro Tricoli, RAL

6. Alessandro Tricoli, RAL
Data Sets
Signal (W->en) and Backgrounds
generated/simulated with PYTHIA Athena (2)
Reconstruction ATHENA v11.0.5
Offline Analysis: AOD/AAN-tuples
Sample
Dataset
Sim rel
N. Gen.
Events
 (filt.) (pb)
We
5104
15,000
11270
W
5106
4,608
3462
Zee
5144
1,889
1419
Z
5146
109 82
Di-jet (QCD)
5802
144,456
2.05·108
Top background negligible
ATLAS UK Physics Meeting- Durham, 18th Sept. 2006
Alessandro Tricoli, RAL

7. Alessandro Tricoli, RAL
W Event Selection
Standard (ATLAS-TDR) W Selection :
Trigger menu e25i applied:
one isolated e±, tuned for efficiently select e± with ET> 25 GeV
L1, L2 and EF
only exception QCD, due to poor stat.: no trigger sel.
Standard Electron Identification: isEM and cluster-track matching requirement
cracks removal h= and |h|<2.4
Electron ET>25 GeV
Missing-ET >25 GeV
Jet Veto Cuts: Jets PT<30 GeV, Event Recoil PT< 20 GeV
60 GeV < MWT <100 GeV
pnT in MWT is not estimated by Missing-ET (from
cells), but by the vector sum of hard objects.
Alternative (CMS-TDR-like)
ATLAS UK Physics Meeting- Durham, 18th Sept. 2006
Alessandro Tricoli, RAL

8. Efficiencies for We From Monika Wielers
Use W events generated with Pythia, Herwig and
Generated/simulated with (2), reconstructed with
Select events with ET(e)>25GeV, ||<1.37 or 1.52< ||<2.47 at generator level
Efficiencies very comparable between Pythia and Herwig
gives slightly higher efficiencies, but still consistent within errors,
e.g. small differences in L1 isolation
Pythia
Eff %
Herwig L1
98.3
98.5
98.7
+L2
90.2
91.1
91.7
+EF
83.3  0.9
83.0  1.1
84.0  1.1
+offline
80.6  0.9
80.7  1.1
81.6  1.1
Offline only
90.2  1.0
89.6  1.1
90.4  1.2
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

9. Missing ET in W->en events missETTruth – missETFinal:
Compare:
Missing ET Truth (MET_Truth)
Standard ATLAS reconstructed Missing ET (MET_Final) from cells
CMS-like pnT reconstructed from hard objects (ele&jets)
electrons selected by isEM
jets selected if there is no electron within DR<0.4 and PTJet>20 GeV
Fit differences missETTruth – missETFinal and missETTruth - pTn
missETTruth - missETFinal
missETTruth - pTn
MET_Truth
MET_Final
pnT
Fit
missETTruth – missETFinal:
Mean= ± 0.07 GeV
s = ± 0.07 GeV
2/NDF = / 22
missETTruth – pTn:
Mean= ± 0.15GeV
s = ± 0.16GeV
2/NDF = 419.5/62
Asymmetric,
Large tails
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

10. W Transverse Mass ATLAS-like vs CMS-like
W -> tn
Z -> e-e+
Z -> t+t-
QCD
W -> en
Cumulative
Bkg
W Transverse Mass ATLAS-like vs CMS-like
ATLAS-like
ATLAS-like
after
e-ID only
After
Miss ET >25GeV
CMS-like
after
e-ID only
60< MWT< 100 GeV
CMS-like analysis: larger QCD and Z->e+e- background (no Miss ET cut)
ATLAS-like analysis: electron ET and missing ET cuts efficiently reject backgrounds
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

11. MWT, and electron h, PT ATLAS Jet Veto Cuts
ATLAS-like
After Jet Veto Cuts
W -> tn
Z -> e-e+
Z -> t+t-
QCD
W -> en
Cumulative
Bkg
After Jet Veto Cuts
ATLAS-like
ATLAS-like
After Jet Veto Cuts
(GeV)
Electron
Jet veto cuts efficiently reject fake electron background
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

12. Event Selection and Bkg Rejection
Selection cuts
We
Nevnts (Cut Reduction)
W
Nevnts (Signal Contamination)
Zee
Z
Di-jets
Trigger, e-selection, crack removal
7390
365
(4.9%)
1784
(24.1%) 18
(0.2%)
52,760
+ Electron ET>25 GeV Full Sim
6531
(-11.6%)
253
(3.8%)
1557
(23.8%) 14
4,686
(71.8%)
+ 60 GeV < MWT < 100 GeV Full Sim
(CMS-like)
4567
(-30.1%) 86
(1.9%)
913
(20%) 6
(0.1%)
660
(14.5%)
+ ET(miss) > 25 GeV Full Sim
Previous ATLFAST study
5678
(-17.0%)
(-15%)
167
(2.9%)
(1.6%) 19
(0.3%) 8
426
(7.8%)
+ no jet with ET > 30GeV Full Sim
4710
(-11.8)
115
(2.4%)
(1.2%) 2
(0.0%) 7 36
(0.8%)
+ pT(recoil) < 20 GeV Full Sim
3712
(-21.2%) 90 1
(0.02%) 28
QCD background difficult to estimate: rate and shape very sensitive to selection cuts
Good agreement between Full Simulation and previous ATLFAST estimates on Bkg contamination
CMS-like has larger Bkg contamination than the standard (ATLAS-TDR) selection
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

13. Accompanying Jets in W->en events Jet Multiplicity
Before Jet Veto Cuts
(after Missing ET cut)
CONE R=0.4
KT D=1.0
After Jet Veto Cuts e n
jet p
Large discrepancy between
Cone and KT algorithms
KT D=1.0 has larger jet multiplicity than Cone with R=0.4
Cone has more events with
no jet in the event
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

14. Impact of Jet Algorithms on Jet Veto Cuts
Using KT instead of Cone for the Jet Veto cuts
ATLAS-like Selection cuts
We
W
Zee
Z
Di-jets
+ no jet with ET > 30GeV
CONE R=0.4
KT D=1.0
Difference (KT-CONE)
4710
3934
(-16%)
115
105
(-9%) 2 1 7 36
+ pT(recoil) < 20 GeV
3712
3384 90 87
(-3%) 28
The choice of Jet Algorithm has impact on the W selection efficiency
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

15. Impact of selection cuts on W-Asymmetry
Look at the e± Asymmetry
From W± decay
Rome Full Sim. Data: 67K W->en events, Herwig+Jimmy, CTEQ5L, Athena
e-Asym. Ae
Ae(h) Before W sel
cuts (only ele-ID)
Ae(h) After W sel cuts
Shape of asymmetry distribution in h is crucial to constrain PDF
W Selection cuts deform Ae shape:
ele-ET cut mainly responsible for this distortion
effect reproducible with analytic calculations (Mandy)
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

16. Impact of selection cuts on W-Asymmetry
Rome Full Sim. Data
Difference between Ae(h)
at Gen. (no cuts) and Det (all cuts) levels
Difference between Ae(h)
at Gen. (all cuts) and Det (all cuts) levels
Mean = ± 0.034
Mean = ± 0.052
electron reconstruction and W sel. cut distortions ~27% on average (h-dependency)
distortion seems to be reproducible at generator level (higher stat needed)
if true with higher stat and if same cuts applied on analytic calc.
=> data unfolding can be small
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

17. Conclusions and Outlook
W->en trigger/offline selection comparable between Pythia, Herwig.
Only slightly higher with
Background contamination is small ~3%
QCD background difficult to estimate due to large cross section
QCD events are very sensitive to the chosen cuts
higher stat and/or better tools needed
CMS-like selection can be used as a cross-check for data commissioning
Missing ET from hard objects can be used
to cross check MET_Final at detector start up.
but CMS-like selection leaves larger background contamination
it can be improved (i.e. removal of 2 electron events, see Z->e+e-)
Jet Multiplicity and ET are very much algorithm dependent
The efficiency of Jet Veto cuts strongly depends on the Jet Algorithm
W-Asymmetry for PDF constraining:
Distortions in h due to selection cuts can be under control if reproducible on calculations as it is at MC-generator level
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

18. Alessandro Tricoli, RAL
EXTRAS
ATLAS UK Physics Meeting- Durham, 18th Sept. 2006
Alessandro Tricoli, RAL

19. Alessandro Tricoli, Oxford University
Reconstruction
Reconstruction done using
Run trigger and offline reconstruction
Select events triggered by e25i
L1, L2 (IDscan as L2 track algo), offline as prototype for EF
Use EF optimisations as prototype for offline e-identification
Eff* %
e± with ET=25 GeV
We with ET>25 GeV
Rate L1
96.2
97.7
6.5 KHz
L2 Calo
95.1
95.4
1.1 KHz
L2 ID
92.9
93.7
510 Hz
L2 Match
90.6
90.2
160 Hz
EF Calo
87.6
87.7
80±11 Hz
EF ID
82.2
83.9
46±8 Hz
EF Match
80.0
82.4
42±8 Hz
(*) eff given with barrel/endcap crack excluded, ||<2.47
Alessandro Tricoli, Oxford University

20. Accompanying Jets in W->en events Jet ET
All jets in the event
Highest-ET jet in the event
Before Jet Veto Cuts
(after Missing ET cut)
Before Jet Veto Cuts
(after Missing ET cut) ?
<ET>=31.9
<ET>=31.0
<ET>=20.1
<ET>=26.6
After Jet Veto Cuts ?
After Jet Veto Cuts
<ET>=14.7
<ET>=16.0
<ET>=17.4
<ET>=16.5
If all jets are considered: KT R=1.0 jets are softer than Cone R=0.4
If the highest-ET jet in the event is considered: KT jets are harder than Cone (?)
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL

21. Electron ET and h (ATLAS-like)
After Missing ET > 25 GeV
After Missing ET > 25 GeV
ATLAS-like
ATLAS-like
(GeV)
After Jet Veto Cuts
After Jet Veto Cuts
ATLAS-like
ATLAS-like
(GeV)
Jet veto cuts efficiently reject background especially QCD
CERN ATLAS SM Group, 6th Sept. 2006
Alessandro Tricoli, RAL