measurement of the ratio of inclusive cross-sections

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


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)


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…


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


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


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


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…


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


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


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


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


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


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


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)


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


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


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


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)


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


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


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


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


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


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


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


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 ...


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


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


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


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)


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


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


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



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


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


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



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


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


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


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


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


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


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


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


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


Isolation ProbabilityIsolation Probability


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


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


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 %


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…


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…


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…


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


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

measurement of the ratio of inclusive cross-sections

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