Search for high transverse momentum Higgs
boson produced in association with a vector boson
and decaying into a pair of b-quarks using the
Center-of-Mass tagger with the ATLAS detector
Carlos Miguel Vergel Infante Advisor: Soeren Prell
POS: Chunhui, Chen, Mani Mina, Alejandro Travesset-Cases, Kerry Whisnant. Iowa State University
Outline
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Introduction to the Standard Model [Chapter 2 and 3]
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Experiment (ATLAS) [Chapter 4, 5 and 6]
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Physics Analysis [Chapter 8 and 9]
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Conclusions [Chapter 10]
Search for high transverse
‘‘Lego Blocks’’
‘What is everything made of?’
Democritus: Everything is made of indivisible
blocks
Search for high transverse
momentum Higgs boson produced in association with a vector boson and decaying into a pair of b-quarks using the Center-of-Mass tagger with the ATLAS detector
Standard Model
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Four forces: EM, Weakinteraction, Strong Interaction and Gravity (pending!)
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Bosons intermediate theinteraction between particles
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W and Z bosons = V bosonsSearch for high transverse
momentum Higgs boson produced in association with a vector boson
and decaying into a pair of
What about the Higgs
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Higgs allows particles to have mass•
Discovered in July 4th, 2012! Properties still under studySearch for high transverse
Does the SM explains everything?
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Well… no…•
We don’t understand yet dark matter nor dark energy -> that’s why we called them dark•
Adding gravity still pending•
What can the Higgs tell us?Search for high transverse
Understanding the Higgs
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Theory connects with experiment in the branching fractions!•
BF = prob. of Higgs decaying into some other particles.•
! biggest BF! Only observed until last year…•
! with only 0.2% was used for the discovery!H → b¯b
H → γγ
Search for high transverse
Understanding the Higgs
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Theory connects with experiment in the branching fractions!•
BF = prob. of Higgs decaying into some other particles.•
! biggest BF! Only observed until last year…•
! with only 0.2% was used for the discovery!H → b¯b
H → γγ
Search for high transverse
Understanding !
H → b¯b
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About 10 billion events with (at least)one b-quark per Higgs produced at LHC
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Quarks are more complicated to ‘see’ in the detector than electrons andmuons. Plus tagging!
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Executive summary:LARGE BACKGROUND!
Search for high transverse
Best way to search !
H → b¯b
?
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Ideal: no more than two quarks(jets) + good discriminant
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Best production mode: VH•
Only two b-quarks (from Higgs)•
V bosons can decay into leptonsSearch for high transverse
momentum Higgs boson produced in association with a vector
boson and decaying into a pair of
!
VH → b¯b
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ZH can also be produced by gg!•
! ~ 20%•
! ~ 21% (no taus)•
! ~ 13% (no taus)•
Search is divided in three channels: zero, one and two lepton channelZ → νν
W → lν
Z → ll
Search for high transverse
momentum Higgs boson produced in association with a vector
boson and decaying into a pair of
!
Observation
VH → b¯b
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Biggest decay finally observed!•
Results are compatible with the Standard Model predictionSearch for high transverse
momentum Higgs boson produced in association with a vector
boson and decaying into a pair of
!
Observation
VH → b¯b
•
Biggest decay finally observed!•
Results are compatible with the Standard Model predictionSearch for high transverse
momentum Higgs boson produced in association with a vector
boson and decaying into a pair of
b-quarks using the Center-of-Mass tagger with the ATLAS detector
Highly boosted Higgs!
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Background falls faster than signal at higher energies. Then, bettersignal-background ratio!
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Improvements in the precision of the measurement is possible!•
Observation analysis have limitations at high boosted energies! (more later)Search for high transverse momentum Higgs boson
Highly boosted Higgs!
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Even more interesting: some Beyond the SM theories (BSM) predict deviation from the SM Higgs at high energies!Search for high transverse momentum Higgs boson
Summary so far
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VH is the best way to search for !•
Boosted analysis could show some new physics!H → b¯b
Search for high transverse
momentum Higgs boson produced in association with a vector boson and decaying into a pair of
Large Hadron Collider at CERN
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Circumference: 27 km•
CoM energy up to 14 TeV in proton-proton collisions•
Biggest particle accelerator in the World•
Main experiments: ATLAS and CMSSearch for high transverse
ATLAS Detector
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Four sub-detectors: 1. Inner Detector 2. EM Calorimeter 3. Hadronic Calorimeter 4. Muon Spectrometer•
Neutrinos are not detected directly with the detectorSearch for high transverse
Neutrinos and missing
transverse energy (MET)
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The initial momenta in the transverse plan to the beam line is zero•
Then, final momenta has to be zero•
In the SM, MET is assumed to be neutrinosSearch for high transverse
Let’s talk (finally) about Jets
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Quarks and gluons cannot exist by themselves (color confinement)•
They create shower of particles. Messy!•
Jets are measured with all the ATLAS sub-detectors!•
Energy deposits are clustered in cones with a specific radiusSearch for high transverse
Flavor Tagging
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Jets are formed by quarks and gluons. So which one? (tagging)•
b-quarks travel ~5 mm in thedetector before decaying. Then, secondary vertices give indication of b-jets
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c-jets are also quite similar…biggest background
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A multivariable algorithm (MV2c10) classifies (tags) jets as: b-, c-, orSearch for high transverse
So what’s the problem
with highly boosted Higgs?
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For low energies, the twob-quarks from the Higgs boson are well-separated
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For higher energies, the b-jets overlap•
Easier to have a large jet (fat jet) with both b-quarks in it.•
Goal: feed the MV2c10 algorithmSearch for high transverse
What to do with
overlapping subjets?
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Go from fixed-Radius to Variable-Radius (VR) depending on the subjet’s momenta (! )p
TSearch for high transverse
Novel technique:
Center-of-Mass tagger (CoM)
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Tracks and calorimeter deposits are moved to the rest frame of the fat jet•
The two b-quarks appear now as back-too-back. Easy to distinguish them!Search for high transverse
momentum Higgs boson produced in association with a vector boson and decaying into a pair of b-quarks using the Center-of-Mass tagger
CoM vs VR
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b-tagging working point set at 70%•
Better signal efficiency for CoM for the same working point!•
Higher background rejection too!Search for high transverse
momentum Higgs boson produced in association with a vector boson and decaying into a pair of b-quarks using the Center-of-Mass tagger
Summary so far
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VH is the best way to search for!
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Boosted analysis could show some new physics!•
CoM tagger is expected to improve the background rejection and keeping high signal efficiency!•
Goal: compare VHbb with CoM and with VRH → b¯b
Search for high transverse
momentum Higgs boson produced in association with a vector boson and decaying into a pair of
Signal Signature
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Searching for events with at least 1 fat jet with 2 b-tagged subjets•
0-lepton: events with large MET, no leptons•
1-lepton: one lepton (electron or muon) and MET•
2-lepton: two leptons (2 electrons or 2 muons) with a combined mass comparable to the Z massSearch for high transverse
Main Backgrounds
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The main backgrounds are W+jets, Z+jets, and ttbar•
Their larger cross section and the presence of real b-quarks can fake the signal final stateSearch for high transverse
Analysis Strategy
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Using ATLAS run 2 data equivalent to a total integrated luminosity of 139.0/fb at 13 TeV of center-of-mass energy•
Separate events between channels and between the ! of the vector bosons•
Give the large ttbar contamination in 0- and 1-lepton channel, separate events between signal and control regionsp
TSearch for high transverse
However…
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CoM is a topological tagger with the purpose of tagging !•
In the CoM analysis, VR is used for the jets outside of the leading fat jetH → b¯b
Search for high transverse
Background Composition
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Main backgrounds in 0- and 1-lepton channels are ttbar and V+jets•
Control regions filled with ttbar•
2-lepton channel background is Z+jets. No need to ttbar control regionSearch for high transverse
momentum Higgs boson produced in association with a vector boson and decaying into a pair of b-quarks using the Center-of-Mass tagger with the ATLAS detector
Low
!
p
T
!
Likelihood Fit
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A Poisson fit over all bins is performed. Each bin is expected to have:!
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where ! represents exactly the SM prediction!•
Two fits are performed:1. Background-only hypothesis (! )
2. ! allow to float with the value that optimizes the fit
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The compatibility between both fits are analyze.E
i= μs
i+ b
iμ = 1
μ = 0
μ
Search for high transverse
Two Important Variables in the Fit
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! (signal strength) tells us about the measurement with respect to the SM prediction!•
Significance (in terms of standard deviations) tells use about theincompatibility between measurement and background-only hypothesis
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5 sigmas are equivalent to a discoveryμ
Search for high transverse
Results
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CoM improves the significances by 12.9% compared to VR!•
Main improvement in the 0-lepton channelSearch for high transverse
momentum Higgs boson produced in association with a vector boson and decaying into a pair of b-quarks using the Center-of-Mass tagger with the ATLAS detector
Summary
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VH is the best way to search for !•
Boosted analysis could show some new physics if !•
First search for highly-boosted ! for ATLAS Run 2 data has been presentedH → b¯b
μ ≠ 1
VHb¯b
Search for high transverse
momentum Higgs boson produced in association with a vector boson and decaying into a pair of b-quarks using the Center-of-Mass tagger with the ATLAS detector
What's next?
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Search was performed using the VR analysis as baseline. Not optimized forCoM!
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Promising idea: changing the working for VR outside of the leading fat jet tomove more ttbar events from signal regions to control regions (more ideas in document)
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Relaxing the b-tagging working point from 70% to 85% for VR improves theselection an extra 4%
Search for high transverse
