Transcript Measurement of the Standard Model WW→lνlν Production Cross

Measurement of the Standard Model
WW→lνlν Production Cross Section at
√s=7TeV in ATLAS experiment
S. Li1,2
Supervised by: Z. Zhao1 Y. Liu1 E. Monnier2
Center of Particle Physics and Technology,
University of Science and Technology of China1
& Centre de Physique des Particules de Marseille,
CNRS/IN2P32
Seminar@MPHY,USTC
09/09/2011
1
Support Note & Conf. Note
1.02 fb-1 Support Note and Conf. Note available on CDS:
ATL-COM-PHYS-2011-864: http://cdsweb.cern.ch/record/1366384/
ATLAS-COM-CONF-2011-125: http://cdsweb.cern.ch/record/1366687/
2
Outline
• Introduction
• Event Selection
• Background Estimations
• Systematic uncertainties
• 1.02fb-1 Results (full EPS datasets)
• Conclusion
3
Introduction
Motivation:
Irreducible background for H→W+W- search
Possible approach to new physics through aTGCs
7/19/2011
S. Li (USTC/CPPM)
4
WW→lνlν characteristics




Dileptonic decay channels allow signal extraction from large BG
Isolated high pT di-lepton final states are considered: ee, eµ, µµ
Cascaded W→τ+X → e/µ+X also included in addition to the promt W(e or µ) decays
Experiment signature in WW:
two OS isolated high pT leptons plus large Etmiss
 Backgrounds:


Top (both single and pair production): real leptons and MET
use data-driven normalisation


W+jets: 1 real + 1 fake lepton, real MET
use data-driven shape and normalisation


Drell-Yan Z/g*: real leptons + fake MET
use data-driven systematic uncertainty


Other Dibosons(WZ, ZZ, Wɣ/Zɣ)
based on MC
7/19/2011
S. Li (USTC/CPPM)
5
2010 results
 8 WW observed (1 ee, 2 µµ, 5 eµ) with 34pb-1
 6.85±0.07±0.66 signal Vs 1.68±0.37±0.42 bgd
 3σ evidence for WW processes
Documents: Conf. Note, INT Note, PRL draft (accepted)
With now 1.02fb-1 more precise measurements expected !
7/19/2011
S. Li (USTC/CPPM)
6
Object Selection
•
•
Selection strategy similar to 2010 analysis: 2010, 2011
Optimization driven by increased Luminosity, worse pileup effects and better S/B
 Muon definition:
 STACO Combined muon, pT>20 GeV, |η|<2.4, z0, d0 significance, other MCP
recommended cuts
 Isolation: pT(cone20)/pT< 0.1
 Electron definition:
 Tight, ET>20GeV (leading electron ET>25 GeV for ee and eμ), |η|<2.47 w/o
crack region, z0, d0 significance, OTX cleaning cut (acceptance loss weighted in
MC)
 Isolation: Etcone30_corrected<4GeV (electron energy leakage and pileup
corrections inside the isolation cone applied)
 Jet definition (antiKt4topo jet): EM+JES pT>30GeV and |η|<4.5)
 MET definition: MET_LocHadTopo with |η|<4.5, lepton energy smearing/rescaling
corrections as well as Mutag Muon correction are propogated
 Latest MCP/Egamma Energy Rescaling/smearings/eff SFs are applied.
7/19/2011
S. Li (USTC/CPPM)
7
Event Selection
• General pre-selection:




Combine Muons and Egamma streams with duplication removed
Official GRL: WZjets all channels, period D-H
Object overlap removal(e/e, e/µ, e/jet)
MET cleaning (reject larError events, reject SumET<0 events & events with jets in LAr Hole in
2011)
 PV (at least 3 associated tracks for the first vertex, Pileup Reweighting applied in MC using
official package PileupReweighting-00-00-13 accounting both in-time and out-time pileup in
2011)
 Trigger: EF_e20_medium (ee), EF_mu18_MG||EF_mu40_MSonly_barrel (μμ), OR of both
triggers (eμ), trigger matching applied
• Channel-specific selection:
 Exactly two prompt, isolated, opposite charge leptons with pT>20GeV
 Standard offline physics object –trigger objects matching cone size (0.15 for electrons and 0.1
for muons)
7/19/2011
S. Li (USTC/CPPM)
8
Event Selection
 Remove Drell-Yan contribution:


|Mll-MZ|>15GeV for ee and μμ
Mll>15GeV for ee and μμ, and Mll> 10 GeV for eμ
 Further remove Drell-Yan and QCD multi-jet contributions:


METRel > 45,40 GeV for µµ and ee;
> 25 GeV for eµ
 Remove top contribution:

Jet veto: no jets of ET > 30 GeV within |η| < 4.5
7/19/2011
S. Li (USTC/CPPM)
9
ee: METrel>40GeV
reject DY
eµ: METrel>25GeV
reject DY
µµ: METrel>45GeV
reject DY
Jet Veto:
reject Top
10
Cut flow with 1.02fb-1 data
414 candidates observed compared with 8 candidates last year
7/19/2011
S. Li (USTC/CPPM)
11
MC signal/bgd expectation(1.02fb-1)
All backgrounds
are estimated
using MC
simulation in this
table
S+B
7/19/2011
59.5
87.4
S. Li (USTC/CPPM)
233.0
380.0
12
Jet multiplicity after Z-veto and METrel
ee
µµ
WW
signal
region
eµ
Combined
13
DATA/MC comparison after Jet Veto
Δφ(l+l-)
pT(l+l-)
MT(l+l-, Etmiss)
7/19/2011
PT(l+l-, Etmiss)
S. Li (USTC/CPPM)
14
Data-driven Drell-Yan background
estimation
7/19/2011
S. Li (USTC/CPPM)
15
Data-driven Drell-Yan background estimation
•
•
DY background: lepton or jet energy not well measured
Data-driven method: (ATLAS note: ATL-COM-PHYS-2010-176)
 Assume the fraction of DY events after the METrel cut is the same
inside or outside of the Z mass window
 The non-DY backgrounds can be estimated either using MC
simulation or eμ events
 e/μ acceptance and efficiency differences accounted using
Z→ee/μμ events
•
•
MC closure test performed: good agreement between the
input and the estimated non-DY background has been
observed
Drell-Yan background determined from
Alpgen MC prediction and syst. uncertainties
determined by comparing METrel distribution
in DATA/MC within Z peak
ee
µµ
Estimated DY yields:
METrel in Z control
region
Data-driven results:
15.8±1.55±1.7
16.1±1.39±2.7
13.5±2.34±1.9
16
Data-driven W+jets background estimation
7/19/2011
S. Li (USTC/CPPM)
17
Data-driven W+jets background treatment
•
Wjets backgrounds estimated by scaling the number of events in the W+jet control sample,
Nlepton ID + Jet-Rich ID with a measured fake factor:
•
Fake factors(for both e and µ) measured from dijet sample driven from data:
•
Way-side jet pT requirement assigned to different
sub-samples and lepton fake factors are calculated
respectively with corresponding syst. addressed
e fake factor
µ fake factor
e fake factor
18
W+jets background estimation & Same Sign region validation
•
Final estimation(total contribution: 50.5±4.8(stat)±14.7(syst)):
•
•
•
Systematics: trigger bias, way-side jet pT sub-sample deviation, sample
dependence (W+jet vs dijet), real lepton contaminations, etc.
30% systematics assigned for the fake factor for both electron and muon
Validated in Same Sign Control Region:
•
Xchecked by Matrix method with good agreement (see backup)
7/19/2011
S. Li (USTC/CPPM)
19
Syst. study: Sample Dependence
•
Fake factors are different between Wjets and dijets sample, which is the major
syst. Uncertainty of the method. Take the deviation as the corresponding
syst.(40%)
•
Fake factors measured from HF samples(bbcc->e) look a bit surprising. Haven’t
assigned yet. Should not be a big deal because of smaller cross section.
Zoom in
20
Syst. study: Trigger Bias
•
•
•
•
Using JF17 pythia MC sample by removing signal component at truth level
Not dramatic systematic eff observed.
Statistics in numerator sample are still not that much
Assign overall deviation~10% conservatively as trigger bias systematic uncertainty
Fake Rate
No Trigger
EF_e20_etcut
0.018
0.021
21
Syst. study:
Electron Fake period dependence
• Almost negligible : take 8% conservatively
Syst.
Source
Trigger bias
Period
dependence
Sample
Dependence
Xsection
sum
uncertainty
10%
8%
40%
/
42%
22
Wjets background estimation: ee and eµ channel
Electron Fake rate



Fake rate
w/o MC subtraction
Fake rate
w/ MCsubtraction
0.0371±0.0002±0.0156
0.0257±0.0001±0.0108
Channel
ee
eµ(e-fake
only)
ee
eµ(e-fake
only)
Wjets background
w/o MC correction
8.30±0.56±3.4
9
13.35±0.71±5.
61
5.75±0.39±2.4
2
9.22±0.49±3.8
7
Wjets background
w/ MC correction
7.60±0.57±3.1
9
10.25±0.83±4.
30
5.26±0.39±2.2
1
7.08±0.57±2.9
7
ee channel: 5.26±0.39±2.21, eµ channel: 7.08±0.57±2.97
MC extraction effect is dramatic in Fake Rate estimation but not that much in Wjet
background yields correction(so neither pseudo nor pure data-driven)
Compatible with John’s results for periodD-H(1fb-1) those syst. need to be further studied:
23
Data-driven top background estimation
7/19/2011
S. Li (USTC/CPPM)
24
Data-driven top background estimation
• Full jet veto suppress top background(single top and ttbar)
• A semi-data-driven method:
 Njet≥2: control sample
 Assume Events fraction with Njet= 0 and Njet≥2 similar in data and MC (residual SM
backgrounds for Njet≥2 in data removed using MC simulation)
Njet= 0 top events: 58.6±2.1(stat)±22.3(syst)
JES (37%) dominant
MC estimation: 56.7 events
Cross checked with b-tagged top control
sample (see backup)
WW
signal
region
25
Other diboson backgrounds and final results
 Purely MC prediction (normalized to 1 fb−1).
 Zγ excluded from total contribution due to overlap with Z+jet backgrounds.
 Final results:
S+B
7/19/2011
63.9
98.8
S. Li (USTC/CPPM)
239.6
402.2
26
Systematics
• Syst. Sources accounted: (see backup for detailed summary)
•
Lepton Systematic:
 Lepton reconstruction and identification efficiencies
 Lepton isolation efficiency
 Lepton energy/momentum scaling and smearing
•
•
•
•
•
Jet Veto
MET syst.(in-time and out-time pileup included)
PDF uncertainty.
Dedicated syst. uncertainty from Data-driven background estimation
The luminosity uncertainty (3.7%, listed separately)
• The systematic uncertainty of the total cross section measurement is
13.4%, which includes the signal acceptance uncertainty (
)
of 6.8% and uncertainty of the background estimation (
) of
11.5%. The systematic error is calculated using propagation:
7/19/2011
S. Li (USTC/CPPM)
27
WW fiducial cross section
•





Fiducialcross section measured in the following phase space:
(Aww and Cww denotation and detail treatment see backup)
Lepton pT>20 GeV, |η|<2.4 for µ and |η|<1.37 or 1.52<|η|<2.47 for electron
Jet pT>30 GeV, |η|<4.5 and ΔR(e, jet) >0.3
ee channel: MET>40 GeV, mee>15 GeVand |m-mZ|>15 GeV
μμ channel: MET>45 GeV, mμμ>15 GeVand |m-mZ|>15 GeV
eμ channel: MET>25 GeV, meμ>10 GeV
Expectation:
46±3 pb and
5.84±0.37 pb
7/19/2011
S. Li (USTC/CPPM)
28
Conclusion
 WW cross section measurement in dileptonic channel using 1.02fb-1 data
 414 candidates observed in 2011 compared to 8 in 2010
 Data-driven methods used for almost all the backgrounds (Drell-Yan, top
and W+jets)
 Detailed studies done on systematic uncertainties for both signal and
backgrounds
 13.4% overall systematic uncertainty and 3.7% for Luminosity accounted
separately
 Measured xsection 48.2±4.0(stat.)±6.4(syst.)±1.8(lumi.) is consistent with
the theoretical prediction of 46±3 pb. Both inclusive and fiducial cross
sections measured for three channels
7/19/2011
S. Li (USTC/CPPM)
29
Ongoing work….
 Wrapping up baseline selection that has been updated
and frozen recently post EPS-HEP conference:
 Cut optimization aiming for a better s/b:
 Further suppress top backgrounds:
 B-jet veto with b-tagging technique to suppress the top
backgrounds
 Lower the jet-pt threshold
 CP recommendation updates(eff SFs, lepton smearing)
 Pending studies after 1fb-1 publication:
 DY background treatment:
 Combine the track and calo based MET to further suppress
the DY background(CMS recommendation)
 Pt(ll) and dφ(ll) possible approach
 Full dataset update with 2011 DATA and new MC
production
7/19/2011
S. Li (USTC/CPPM)
30
Backup
7/19/2011
S. Li (USTC/CPPM)
31
DATA & MC samples
•
PeriodD –H with L=1.02 fb-1 (Period B not
used), 3.7% Lumi. Uncertianty
•
Official GRL: Wzjets all channels
•
Unprescaled single lepton triggers:
EF_e20_medium,
EF_mu18_MG||EF_mu40_MSonly_barrel
7/19/2011
MC samples with p591 tags:
Full list
S. Li (USTC/CPPM)
32
Event Selection in 2011
(changes Highlighted)
•
•
Event selection strategy is similar to 2010 analysis: 2010, 2011
Optimization is driven by increased Luminosity and worse pileup effects as well as aiming for a
better S/B ratio.
• Object selection:
 Muon definition:
 Combined muon, STACO, pT>20 GeV, |η|<2.4, z0, d0 significance, other MCP recommended cuts
(pT_MS and pT_ID fractional difference used in 2010 analysis, adding more ID requirements in
2011)
 Isolation: pT(cone20)/pT< 0.1
 Electron definition:
 Tight, ET>20GeV (leading electron ET>25 GeV for ee and eμ), |η|<2.47 w/o crack region, z0, d0
significance, OTX cleaning cut (acceptance loss weighted in MC)
 Isolation: Etcone30_corrected < 4 GeV(electron energy leakage and pileup corrections inside the
isolation cone applied) (Etcone30 < 6 GeV used in 2010 analysis)

Jet definition (antiKt4topo jet): EM+JES pT>30GeV and |η|<4.5) (EM+JES pT>20 GeV and |η|<3 used
in 2010 analysis)
 MET definition: MET_LocHadTopo with |η|<4.5, lepton energy smearing/rescaling corrections as
well as Mutag Muon correction are propogated
•
Latest MCP/Egamma Energy Rescaling/smearings/eff SFs are applied.
7/19/2011
S. Li (USTC/CPPM)
33
Event Selection in 2011
(changes Highlighted)
• General preselection:




Combine Muons and Egamma streams with duplication removed
DQ
Object overlap removal(e/e, e/µ, e/jet)
MET cleaning (reject larError events, reject SumET<0 events & events with jets in LAr Hole in
2011)
 PV (at least 3 associated tracks for the first vertex, Pileup Reweighting applied in MC using
official package PileupReweighting-00-00-13 accounting both in-time and out-time pileup in
2011)
• Channel-specific selection:
 Trigger: EF_e20_medium (ee), EF_mu18_MG||EF_mu40_MSonly_barrel (μμ), OR of both
triggers (eμ), trigger matching applied correspondingly
 Standard offline physics object –trigger objects matching cone size (0.15 for electrons and 0.1
for muons)
 Exactly two prompt, isolated, opposite charge leptons with pT>20 GeV
7/19/2011
S. Li (USTC/CPPM)
34
Event Selection in 2011
(changes Highlighted)
 Remove Drell-Yan contribution:
 |Mll-MZ|>15GeV for ee and μμ (|Mll-MZ|>10GeV in 2010 analysis)
 Mll>15GeV for ee and μμ, and Mll> 10 GeV for eμ (no cut for eμ in 2010)
 Further remove Drell-Yan and QCD multi-jet contributions:
•
•
METRel > 45,40 GeV for µµ and ee (40 and 40 in 2011);
> 25 GeV for eµ (20 in 2011)
 Remove top contribution:
•
Jet veto: no jets of ET > 30 GeV within |η| < 4.5
7/19/2011
S. Li (USTC/CPPM)
(changes of jet definiton w.r.t. 2010)
35
Wjets Estimation: background yields
7/19/2011
S. Li (USTC/CPPM)
36
Wjets Estimation: Same Sign yields
7/19/2011
S. Li (USTC/CPPM)
37
WW cross section measurement strategy
•
The W+W− fiducial cross section and total cross section are determined from the three
dilepton channels (WW→eνeν, μνμν and eνμν) by maximizing log-likelihood functions shown
in the following equations:
• the coefficients AWW andCWW are definedas follows:
 AWW denotes the acceptance for the W+W− decays under consideration, defined as the
fraction of decays satisfying the geometrical and kinematical constraints at the generator
level (fiducial acceptance). This quantity can only be determined from Monte-Carlo
simulations. It is defined here after the decay leptons emit photons via QED final state
radiation; photons within a DR < 0.1 cone are added back to the deca leptons (“dressed”
leptons).
 CWW denotes the ratios between the total number of generated events which pass the final
selection requirements after reconstruction and the total number of generated events within
the fiducial acceptance. This corrections factor includes the efficiencies for triggering,
reconstructing, and identifying the W+W− decays falling within the acceptance.
7/19/2011
S. Li (USTC/CPPM)
38
Systematic uncertainties on WW signal acceptance
7/19/2011
S. Li (USTC/CPPM)
39
7/19/2011
S. Li (USTC/CPPM)
40
7/19/2011
S. Li (USTC/CPPM)
41
7/19/2011
S. Li (USTC/CPPM)
42
7/19/2011
S. Li (USTC/CPPM)
43
W+jet estimation using Matrix Method
7/19/2011
S. Li (USTC/CPPM)
44
Top background estimation from top control sample
using b-tagging
7/19/2011
S. Li (USTC/CPPM)
45