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Measurement of Measurement of the Ratio of Inclusive Cross-Sectionsthe Ratio of Inclusive Cross-Sections
Dissertation presented byYıldırım D. Mutaf
Suny, Stony Brook
-jet
jet
pp Z b
pp Z
1.96 TeVs atat
25 February 2005
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 2
OutlineOutline
Introduction Introduction to Higgs
searches Higgs expectations from
Fermilab Motivation for studies on
Z+b production
Experimental Apparatus Introduction to Fermilab
RunII Program and DØ detector
Sub-detectors and particle identification methods
b-tagging primer
Measurement of σ(Z+b) / σ(Z+j) Methodology & jet
properties (with MC comparisons)
Extraction of the Z+b/Z+j ratio
Systematic uncertainties and final result
Improvements for Z+bb Studies Muon Isolation Likelihood B-tagging Optimizations Summary
IntroductionIntroduction
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 4
Higgs Searches at TevatronHiggs Searches at Tevatron
Tevatron Run II program is committed to both SM and non-SM Higgs searches Production cross-sections are
very low and analyses need large integrated luminosity
SM Higgs searches are pursued in W/Z associated production for low mass Higgs due to cleaner signatures bb decay mode (low mass)
Gluon fusion becomes relevant for high mass Higgs because of WW decay mode (high mass)
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 5
Higgs Expectations from TevatronHiggs Expectations from Tevatron
Higgs reach of Tevatron was first estimated rigorously in 1999
Many assumptions are made in this study with simplistic simulations
A similar study is performed recently in 2003
Full DØ and CDF detector simulations are used with more realistic reconstruction
Instead of parameterizations, used realistic detector performance and resolutions
• b-tagging efficiencies, di-jet mass resolution etc.
The conclusion is that more realistic Higgs searches can exceed earlier expectations
There is even more room to improve
Especially with more sophisticated analyses methods…
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 6
Motivation for Z+b Motivation for Z+b
Run II Luminosity is not yet enough for a Higgs analysis However, understanding the background processes for the “tiny”
Higgs signal is of paramount importance
At DØ, we recently performed a study of the b-jets produced with Z’s And measured the cross-section ratio for the production of Z+b-jet
to Z+inclusive jet processes
This measurement
• Adds to our understanding of the major Higgs backgrounds
– Also provides a validation of the b-tagging methods at DØ
– First step towards the measurement of the Z+bb cross-section
• And also provides an indirect probe of the b-quark density inside the proton
Experimental ApparatusExperimental Apparatus
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 8
TeVatron Upgrade ProgramTeVatron Upgrade Program
TeVatron collider complex is upgraded to pursue the challenges of Higgs searches as well as other interesting topics
Shorter bunch crossing Need faster readout/electronics
Larger CM energy (~10%) Increased production cross
section
Higher luminosity Need better triggering
Need radiation hard detectors
Run I Run II
Bunches in Turn 6 x 6 36 x 36Bunch Crossing (ns) 3500 396√s (TeV) 1.8 1.96∫ L dt (pb/week) 3.2 ~10
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 9
Run II PerformanceRun II Performance
DØ performance Easy to see the evolution from
commissioning to high efficiency data taking
TeVatron is delivering steady and high luminosities
Available data as of last week is ~ 550 pb-1
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 10
DDØØ Run II Experiment Run II Experiment
Upgrade of the very successful Run I experiment (1992-96) top discovery, largest W collection, QCD measurements …
Versatile detector ready for challenging physics tasks Higgs searches
• Direct searches, understanding backgrounds
Precision measurements of MW and Mt
• As well as other top & W properties (single top?)
Searches for Physics beyond SM• SUSY, Large Extra Dimensions, Technicolor…
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 11
DDØØ Run II Upgrade Run II Upgrade
Calorimetry New electronics/trigger Preshower detectors
• Central & Forward
Muon Detection New forward detectors New electronics &
scintillators for trigger
Tracking System Entirely new tracker with
several components• Tracking, vertexing,
btagging and triggering
DAQ / Trigger Entirely new systems
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 12
CalorimeterCalorimeter
Same detector as Run I but improved electronics and trigger system
Liquid Argon with interspersed Uranium absorbers
Accommodate 396 ns crossing
Uniform, hermetic calorimeter with fine segmentation and large coverage
|η|<4.2 with 0.1x0.1 ηφ towers
EM, FH and CH layers
Single particle energy resolutions
e: E/E ≈ 15%/√E
• e.g. 3.3% @ 20 GeV
: E/E ≈ 45%/√E
• e.g. 10.0% @ 20 GeV
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 13
Muon DetectorsMuon Detectors
Completely new forward muon system Also added central scintillators
Provides muon detection up to |η|<2.0 Cosmic ray rejection
Triggering with fast scintillators
Momentum measurement with the solid iron Toroid magnet
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 14
DDØØ Run II Tracking System Run II Tracking System
2 T solenoid magnetic field Super-conducting
New Silicon Microstrip Tracker (SMT) Necessary for improved dca resolution Tracking / vertexing / b-tagging
New Central Fiber Tracker (CFT) 8 double scintillating fiber layers Axial & stereo coverage up to |η|<2.4
(all layers) Incorporated into Level 1 trigger
system
Preshower Systems (Forward/Central) Electron / Photon discrimination Improve reconstruction via cluster
matching • central track and/or calorimeter towers
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 15
Silicon Microstrip Detector (SMT)Silicon Microstrip Detector (SMT)
~ 1.2 m
~ 10 cm
~ 800,000 read-out channels
Hybrid system (barrel & disk)
Essential piece of detector for track & vertex reconstruction > 95 % hit efficiency
r-φ hit resolution ~ 10 μm
z hit resolution ~ 40 μm
Enabling b-tagging up to |η|<2.4
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 16
Central Fiber Tracker (CFT)Central Fiber Tracker (CFT)
8 layers of scintillator fibers 1.8 meter or 2.6 meter long
Axial & stereo (3o angle)
77k read-out channels
Input to Level-1 track triggers
Including CFT
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 17
From Hits to TracksFrom Hits to Tracks
3D reconstruction of tracks from the hits in SMT & CFT DØ has several alternative
tracking reconstruction methods
Tracking in simulation and real detector shows difference
Hit
s i
n t
ran
sver
se p
lan
e
2D(r-φ)
3D(r-φ-z)
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 18
b Identification Primerb Identification Primer
b-tagging methods generally rely on the following properties of B hadrons
Semi-leptonic decays• Presence of soft muon in the jet
Long life-time of B hadrons• Reconstruction of the displaced
vertex where b decays
– Order of 1 mm decay length
– Vertex decay length resolution
• Track impact parameter distance from the primary vertex where the hard scatter occurs
– Track IP resolution
PV
IP
p
p
SV
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 19
Secondary Vertex MethodSecondary Vertex Method
Secondary Vertex Reconstruction Cluster tracks and form 2 track “seed”
vertices
• Add tracks to the seed according to final vertex χ2
Select vertices based on quality
• Collinearity and χ2 of the vertex fit
Define three types of vertices (loose, medium, tight) based on
• Momentum & IP of the vertex tracks
• Decay length & its significance (wrt PV)
Tagging b’s Tag a calorimeter jet if a secondary
vertex is matched to the vertex
TIGHT b-tag efficiency
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 20
Impact Parameter Method - IImpact Parameter Method - I
This method uses the signed impact parameter
significance of the tracks (IP/σIP) in the jets Negative IP tracks are mostly associated with PV and due to PV
uncertainties
Positive IP tracks are from the decays of long lived particles
Construct a likelihood for the tracks the probability of originating from Primary Vertex
IP /IP
PV IP
Jet Track
PV
IP
TrackJet
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 21
Impact Parameter Method - II Impact Parameter Method - II
The probability due to tracks from PV is flat by definition
Large & positive IP tracks peak around “zero-probability”
Combine the probabilities of the tracks in jets to get a jet probability
Cut on jet probability for b-taggingjet pT
jet η
TIGHT b-tag efficiency
Measurement of Measurement of σσ(Z+b) / (Z+b) / σσ(Z+j)(Z+j)
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 23
IntroductionIntroduction
As we pointed out before, b-quark production in association with Z boson has paramount importance for Higgs searches
Z+b-jet production has recently been investigated at DØ Besides studying general b-jet production and properties, we
performed a measurement of the following ratio:
Being a cross-section ratio, this measurement is in general insensitive to detector inefficiencies and systematic uncertainties
• Except the factors that affect b-jets and light jets differently (eg. b-tagging)
p p Z b
Rp p Z j
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 24
MethodologyMethodology
This analysis is performed in two different channels* Dimuon Analysis (Z→μμ) Dielectron Analysis (Z→ee)
* Actual dissertation research consists only the
dimuon channel Apart from the reconstruction of Z in
these channels, the hadronic (jet) parts of the two analyses are identical
Both analyses are based on data collected Aug’02-Sep’03 correspond to ~180 pb-1 luminosity
• however, the actual luminosity figure is irrelevant to this measurement
A schematic view of the Z+jet signature shown for
dimuon decay of Z
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 25
Dimuon Analysis (ZDimuon Analysis (Z→→μμμμ))
Muon Reconstruction at DØ Uses hits in the muon drift detectors and scintillators
• Muon detector track segments are reconstructed from the hit information
• Three muon criteria are used consistent with the quality of muon hits
– Tight, Medium & Loose
Scintillator timing cuts are used to reject the cosmic background
Optional central track matching
Muon & Z(μμ) Selection (specific to this analysis) Event must have triggered any MUON trigger (single muon, dimuon etc…)
2 Loose muons with both muons matched to reconstructed central tracks
• isolated i.e. pTrel with respect to closest jet
must be larger than 10 GeV
• |ημ| < 2.0 and pTμ > 15 GeV
65.0 < Mμ1μ
2< 115.0 GeV
Pμ
Pjet
PTrel
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 26
Dimuon Analysis (ZDimuon Analysis (Z→→μμμμ))
Z→μμ candidate with an additional jet
Shown above is the invariant mass of all selected dimuon candidates
In this dataset, we found about 11500 inclusive Z(μμ) candidate events within the [65-115] GeV mass range
underlying event
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 27
Dielectron Analysis (ZDielectron Analysis (Z→ee)→ee)
Electron Reconstruction at DØ Clusters of energy (simple cone algorithm) in EM layers of the calorimeter
• Energy deposited in EM layers should be > 90 % of the total energy inside cone
Shower shape consistent with electrons (not π0/γ)• χ2 from H-Matrix using discriminating variables like shower width, total energy,
fraction at EM etc…
Optional central track matching
Electron & Z(ee) Selection (specific to this analysis) Event must have triggered one of the two 2EM triggers 2 EM clusters
• isolated i.e. must be less than 20%
• |ηe| < 2.5 and pTe > 15 GeV
At least one of the electrons must have a reconstructed central track
80.0 < Me1e
2< 100.0 GeV
0.4 0.2
0.2
R RTOT EM
REM
E EE
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 28
Dielectron Analysis (ZDielectron Analysis (Z→ee)→ee)
Z→ee candidate with 2 jets (1 b-tagged)
Shown above is the invariant mass of all selected dielectron candidates
In this dataset, we found about 15600 inclusive Z(ee) candidate events within the [80-100] GeV mass range
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 29
Estimation of BackgroundEstimation of Background
Major backgrounds to Z(ee/μμ) +jet signature Drell/Yan continuum for dilepton production Multi-jet background (mostly important for tagged sample)
Dielectron Analysis Background is estimated as
a convolution of these two processes
• Fitting of the side-band mass distributions
Dimuon Analysis Background is evaluated for the
two contributions
• Multi-jet background is estimated from the isolation criteria
2 22
2 20 (1 ) (1 )
Q Z
Q Q Z Z
Q Q Z Z
N N N
N N N
N N N
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 30
Jets - IJets - I
Jet reconstruction and selection are identical in both the dimuon and dielectron analysis
Jet reconstruction at DØ Clustering of the calorimeter
towers starting with high energy
seeds (jet Emin = 8 GeV)
ΔR=0.5 standard cone algorithm jets are used in this analysis
Jets are passed through several quality criteria
Remove energy depositions in hadronic layers due to electron, photons etc.
Remove hot calorimeter towers…
Jets are applied Energy Scale (JES) corrections to account for
Calorimeter response and baseline subtraction (noise, pile-up etc.)
Out-of-cone showering, muonic decay energy compensation ...
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 31
Jets - IIJets - II
The kinematic cuts for jets in this analysis
|ηjet| < 2.5 and pTjet > 20 GeV
For b-tagging, we require the following for calorimeter jets In order to be “taggable”, the jets must be matched to clusters
of tracks made of at least 2 tracks (more track cuts are introduced)
In this analysis, we use the Secondary Vertex (SV) tagger Use TIGHT reconstructed SV’s with decay length significance >
7
Match taggable jets to SV’s if ΔRSV-JET<0.5
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 32
Jets - IIIJets - III
The number of inclusive Z+jet events in both channels are shown above…
Shown on right is the jet multiplicities for dimuon channel alone
Points are data with statistical error bars
Error boxes represent the uncertainty due to JES
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 33
Taggable Jet KinematicsTaggable Jet Kinematics
Taggable jet kinematics distributions observed in data are compared to expectations from Z+j ALPGEN MC Differences of jet reconstruction in
MC & Data are accounted for…
Expectations are overall in good agreement with jets observed in data
Jets in Z+j samples are a composition of different flavors Heavy jets : b/c
Light jets : u/d/s/g
Need to decompose the flavors to extract Z+b component
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 34
b-tagged Jetsb-tagged Jets
After b-tagging, we are left with 49 Z+b-tagged jet events Combined dielectron (27) & dimuon (22)
Not all of these events are “truly” b-quark events due to fake contamination from the b-tagging method
• εb ≈ 33.13 % , εc ≈ 8.42 % , εl ≈ 0.24 % (none zero)
(μμ)
εb(pT,η)
εl(pT,η)
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 35
Secondary Vertices (only from Secondary Vertices (only from μμμμ))
X 2 = 42.89 without b-content
X 2 = 30.89 with added b-content
X 2 = 34.59 without b-content
X 2 = 1.39 with added b-content
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 36
Cross-Check of SV b-taggingCross-Check of SV b-tagging
Decay length distribution comparison with expectation confirms that most these vertices are mostly from the decay of heavy hadrons
However, we performed a cross-check of the analysis by using different b-tagging methods
Soft Muon Tagging – Z(ee) Rather independent cross-check of
SV tagger (no reliance on track IP)
Measure tag efficiencies from MC and correct for muon and track reconstruction efficiencies in data
• Mistag measured from jets in data
Estimate that 10.3 ± 1.3 events should be tagged
• Consistent with 12 actually tagged
Impact Parameter Tagging – Z(μμ) Measure correlation between IP
and SV b-tagging algorithms in dijet data events for actual b-jets
• Use MC for charm and light jets
Out of 22 SV tagged events, expect 14.3 ± 3.4 events tagged by IP
• Consistent with 14 tagged events
79.2 %
56.1 %
22.4 %
B
C
L
C
C
C
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 37
Extraction of Jet Flavors & RatioExtraction of Jet Flavors & Ratio
# Z + taggable-jet events is 1658 (ee) & 1406 (μμ)
# Z + b-jet events is 27 (ee) & 22 (μμ) These numbers are not pure (composition is a mixture of b/c/l)
Zb Zq Bckg.
Zb
Zc
Zc Zq Bckg.
Zb Zc Zq B
Taggability
b-taggingZ+bjet
Z+taggable-jet An
alysis
Z+jet
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 38
Extraction of Jet Flavors & RatioExtraction of Jet Flavors & Ratio
Basically solve for the following equation to get the correct mixture of jet flavors…
Two equations, three unknowns (i.e. NB, NC, NL)
Introduce another equation by fixing NC/NB to most recent
NLO calculations*
bckg b B c C l Lbefore
bckg b b B c c C l l Lafter
N N t N t N t N
N N t N t N t N
(1.69 0.16)C BN N
* J. Campbell, K. Ellis, F. Maltoni, S. Willenbrock, Phys. Rev. D69 (2004) 074021
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 39
Measuring Jet Efficiencies - IMeasuring Jet Efficiencies - I
Taggability efficiency is measured in data (light-jet) to be 78.6 %
For heavy flavor jet taggability, we scale this with a correction factor obtained from MC 80.7 %
//
MCDATA DATA b cb c l MC
l
tt t
t
Jet reconstruction efficiencies are assumed to affect heavy and light jets in similar ways Assign a 2 % systematic uncertainty to this
assumption
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 40
Measuring Jet Efficiencies - IIMeasuring Jet Efficiencies - II
The efficiencies are corrected for the inclusiveness of the events i.e. jet multiplicities If you require one or more jets, the efficiency of such a selection
will basically be different for events having 1 jet, 2 jets, 3 jets …
Only apply this for light jets since all of the jets can be considered to be of the same flavor
• However for heavy jets, since we don’t know the exact flavor of each jet in the event, we don’t make the correction in this way
• Consult theory and take the ratio of Z+QQ in inclusive Z+Q events
and correct the heavy-jet efficiencies for this factor (fbb = 10.8 %, fcc =
6.7 %)
1 1
11 1
jetseventsNN
CORR jeti jeventsN
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 41
Extraction of Jet Flavors & RatioExtraction of Jet Flavors & Ratio
The ratio of cross-sections is defined as:
The number of events, Ni are added statistically from both
channels
Our result is comparable to the NLO prediction of 0.018 ± 0.004*
B
B C L
p p Z b NR
N N Np p Z j
0.0186p p Z b
Rp p Z j
* J. Campbell, K. Ellis, F. Maltoni, S. Willenbrock, Phys. Rev. D69 (2004) 074021
0.0234p p Z b
Rp p Z j
0.0211 0.0041 (stat)p p Z b
Rp p Z j
μμ
ee
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 42
Systematic Uncertainties Systematic Uncertainties
Measurement of Efficiencies b/c-tag efficiency 2.4 %
• The errors come from the measurement of the b-tagging efficiencies in data, errors related to fitting results etc
Mistag efficiency 5.8 %
• Due to the different results obtained from different selection of jets in data
Taggability efficiency 0.5 %
• The uncertainty factor from the fluctuations observed for the heavy-jet taggability correction factor obtained from MC
Jet Reconstruction efficiency 2.1 %
• The uncertainty associated with the assumption that the jet reconstruction efficiencies are similar for light and heavy jets whereas observed as different in MC
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 43
Systematic Uncertainties (more)Systematic Uncertainties (more)
Other major uncertainties Background Estimation 8.6 %
• Uncertainty due to estimation of the multi-jet background in Z+jet both before and after b-tagging (fit errors, assumptions etc.)
Z+Q or Z+(QQ) 5.4 %
• Uncertainty due to merging of two b-quarks from gluon splitting into a single jet cone
– Measure the increases in tag efficiencies for such cases from MC
– Obtain the relative fractions of Z+Q vs Z+(QQ) final states from theory
( ) ( )
( )
Q Q QQ QQ
Q QQ
N N
N N
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 44
Systematic Uncertainties (more)Systematic Uncertainties (more)
Other major uncertainties Jet Energy Scale
6.5 %• Uncertainty due to the correction of jet energies for detector
resolution etc.
Theoretical Input2.7 %
• Uncertainty due to fixing the NC to NB ratio to NLO calculations
ZQQ Correction Factor Uncertainty1.6 %
• Uncertainty due to correcting the heavy jet efficiencies for QQ contributions
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 45
Final ResultFinal Result
The overall effect of all the systematic uncertainties is ~ 12 %
The final result with the full systematic uncertainty
Again, theoretical prediction for this ratio is 0.018 ± 0.004 Our measurement is consistent with the theoretical NLO
predictions
0.00220.00250.0211 0.0041 (stat) (syst)
p p Z bR
p p Z j
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 46
Discussion Discussion
This result is the first Z+b measurement at hadron colliders and carry big importance for future studies We confirmed and demonstrated the b-tagging capabilities at
DØ Z+b production (both jet kinematics and the cross-section
ratio) is consistent with the theoretical predictions• Increasing confidence for Higgs background studies
Study of Z+b production also a good starting step for similar signature Higgs and other background studies
The ratio we measured has the largest contribution from the b-quark in the proton sea
• It’s an indirect experimental constraint on the b-quark PDF which is crucial for
– Single top production– hb(b) higgstrahlung process
Improvements for Z+bb Studies Improvements for Z+bb Studies
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 48
Introduction Introduction
DØ (TeVatron) is committed to Higgs searches Even though the challenges and time pressure are hard to beat
The initial measurements related to Higgs is produced for the similar signature backgrounds Like Z+bb, W+bb …
Current data-set is about 3 times larger than the data used for the ratio analysis Yet, we still would have 3 to 4 Z+bb events with similar
analysis strategy
These low yields require more sophisticated analysis methods
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 49
Introduction IIIntroduction II
At DØ, we’ve worked out a preliminary strategy to increase the yield for Z+bb
Employing simple multivariate techniques, the sensitivity to Z+bb signal is increased by a large amount
We studied the following optimizations: Muon Isolation
• Instrumental for multijet vs Z discrimination
B-tagging operating point optimization
• Finding the optimum combination of Signal (b-jet) and Background (mistag)
Optimization for two b-jet system
• Further b-tagging optimization specific to two b-jet system
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 50
Muon IsolationMuon Isolation
For muon isolation, several variables are investigated and their performances are compared in “signal” and “background” events Momentum scaling of the variables enhances their discrimination
power
Combine the most powerful variables in a single isolation discriminant
Using the background distribution of the isolation discriminant, we create an isolation likelihood
0.4 0.1 0.5
cell cell tracksN N Ncell cell trackT T T
R R Riso
E E p
fp
00
0
( )
( )( )
iso
fiso
iso
f df
P ff df
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 51
Isolation ProbabilityIsolation Probability
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 52
Isolation OptimizationsIsolation Optimizations
The correlations between the isolation probability of the two muons in the signal sample suggests simple combinations of the two…
At 95 % Z efficiency, we achieve as low as 3 % background rate with the multiplication of the isolation probabilities
Traditional “square-cut” strategy (AND) throws away the correlation between the two muons in the signal events
6-fold reduction in background rate at 95 % efficiency
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 53
B-tagging OptimizationB-tagging Optimization
We investigated if the TIGHT (or other available) b-tagging operating modes are most optimal for the study of Z+bb signal Test the efficiency of b-tagging methods on a toy Z+bb search
Use b/c and light jet MC
• Scale MC b-tagging efficiencies to data (simply assume 80% SF) The current operating points for the taggers are at 1.0%, 0.5% and 0.25% of mistag rates (per jet)
• These correspond to at least 10-4 reduction for Z+jj events
• The NLO Z+bb/Z+jj is about 1/50
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 54
B-tagging Optimization IIB-tagging Optimization II
We observe that the signal significance
Maximum at 3.6 % mistag rate Compared to 0.25 % mistag
rate we used for the ratio measurement….
More than 50 % improvement in the b-tagging efficiency
Significance SS B
Maximum signal significance @ 3.6 %
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 55
Two b-jet OptimizationTwo b-jet Optimization
Impact parameter b-tagging method uses a discriminant which is similar to the isolation probability we constructed Background (Mistags) : Flat probability
Signal (b-jets) : Probability peaking about “0”
It is again very natural to combine the probabilities of the two b-jets and benefit from the correlations between the two b-jets…
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 56
Two b-jet Optimization IITwo b-jet Optimization II
At the previously found optimum background level (operating point), the multiplicative combination of the two b-jet probabilities yield more than 50 % improvement in the signal (Z+bb) efficiency At the same background
level…
Again we observe that the traditional “square-cut” strategies (AND) perform far inferior when compared to a simple combination of the probabilities…
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 57
Summary of OptimizationsSummary of Optimizations
Using simple techniques, we have shown that The existing b-tagging operating points are too tight for Z+bb
search
The event efficiency can further be increased significantly using simple multi-variate techniques
Increases in the efficiencies (for Z+bb): LOOSE-LOOSE selection for b-jets Eff=~17%
Using more optimal mistag rate Eff=~27% (x1.60)
Using topological event probability Eff=~41% (x2.41)
Overall b-tagging optimizations can bring more than 2 times increase in the Z+bb selection efficiency Critical for such small cross-section processes…
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 58
ThanksThanks
DØ Collaboration & Fermilab
Co-authors of the ratio (Z+b/Z+j) analysis
K. Hanagaki
S. Choi
Supervisors
P. Grannis
J. Hobbs
Q. Li
A. Kharchilava
BACKUP SLIDESBACKUP SLIDES
Y. D. Mutaf Ph.D. Dissertation, 25 Feb. 2005 60
Muon Isolation VariablesMuon Isolation Variables
For muon isolation, several variables are investigated and their performances are compared Signal : Z+bb (MC Simulation)
Background : 2 high-pT jet events with ≥ 1 muon
( )
0.4 0.1
0.5
sin
cell cell
tracks
relT jet
N Ncell cellT T
R R
NtrackT
R
p p
Halo E E
TrkSum p