Validation of Higgs to bb tagging techniques via Z➝bb

Juan Varela

University at Albany, SUNY

Nevis Laboratories Summer REU Program

Columbia University

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Columbia Team

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Prof. Parsons

Prof. Brooijmans

Inês Ochoa Julia Gonski

Dan Williams Elena Busch Images: https://www.nevis.columbia.edu/~atlas/

Outline

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● Standard Mode (SM) of particle physics● Higgs Boson● Large Hadron Collider; ATLAS Experiment● My research ● Conclusions● Acknowledgements● References

Standard Model of Physics

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● Best understanding of the fundamental particles and most of fundamental forces

● Two basic types of particles- Fermions and Bosons○ Fermions

■ Quarks: up bottom charm down top strange

■ Leptons: Tau Electron Muon Respective Neutrinos

○ Bosons■ Gluon, W, Z, photon

● While it describes most of the known universe very well, certainly has its flaws ○ Neutrinos having mass, implementing gravity

Fig. 1 The Standard Model of Physics

Image:http://united-states.cern/physics/standard-model-and-beyond

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Fig. 2 : Mass of all Fundamental Particles in the Standard Model

Fig. 6: Higgs boson decaying into two photons

Image: http://atlasexperiment.org/HiggsResources/

Motivation● ATLAS and CMS published papers in 2012

of the discovery of a particle in the 126 GeV mass range that was a boson consistent with the theorized Higgs Boson

● Many Higgs boson properties have been measured since the discovery: mass, spin, some branching ratios, but it's very important to study all properties with increased precision

● Its properties are predicted to be different in different theoretical models○ Opens up search for new physics

● Methods to detect H must be as well understood as possible

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Fig. 3: Measured mass spectra of Higgs candidates reconstructed in events containing two photonsImage: https://atlas.cern/updates/physics-briefing/precise-measurement-higgs-boson-mass

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Fig. 4 : Standard Model Higgs boson decay branching ratios Image: https://twiki.cern.ch/twiki/bin/view/LHCPhysics/CrossSections

Higgs Boson Branching Ratios

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● At Higgs mass of 125 GeV, bb decay has the greatest branching ratio (BR)

● Leading decay (BR ~ 58%) is H ➝bb● Z➝bb (BR~14%) provides a

standard candle to Higgs tagging

● Main Goal:

Use Z→bb process to validate performance of techniques used to identify H→bb.

The Large Hadron Collider (LHC)

● Located near Geneva, Switzerland● 27 km ring of superconducting magnets● Largest particle accelerator in the world● 4 Main Experiments:

○ CMS○ ALICE○ LHCb○ ATLAS

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Fig. 5: Map of the LHCImage: http://edu-observatory.org/olli/CosmicRays/CERN-Brochure-2009-003-Eng.pdf

The ATLAS Experiment ● ATLAS : A Toroidal LHC ApparatuS● 46m long, 25m in diameter, and

weighs ~7000 tons (roughly weight of Eiffel Tower)

● Four Main Components:○ Inner Detector○ Calorimeters○ Muon Spectrometer ○ Magnet System

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Fig. 6: Diagram of the ATLAS detector Image: http://atlas.cern/discover/detector

ATLAS Coordinate System

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Fig. 7: ATLAS Detector Coordinate SystemImage: https://www.researchgate.net/figure/Coordinate-system-used-by-the-ATLAS-and-CMS-experiments-at-the-LHC-33_fig4_326689955

Fig. 8: Eta and Theta VariableImage: https://wiki.physik.uzh.ch/cms/_detail/latex:theta-eta.png?id=latex%3Atikz

My Research

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● Types of Jets ● Tagging Methods ● Signal and Background● Jet Reconstruction● Event Selection

Large-R Calorimeter Jets

● Jets are clusters of objects that can be tracks, calorimeter deposits, etc

● Large-R Jets / Fat Jets○ ΔR = 1.0

● Boosted jets ≡ Jets with greater transverse momentum; decays product of resonances get bunched together in the lab frame.

● Boosted Large-R jets sought for in tagging of boosted heavy bosons like Higgs, W, Z and top quarks○ If bosons are not boosted, large-R

jets would not contain their decay products 13

Fig. 9: Boosted Large-R Jets

Variable-R track-jets● Sub-jets / constituents within the large-R

jet ● Clusters of jets which are measured in the

inner detector● R = ⍴ / pT

○ ⍴ is a constant = 30 GeV ○ pT is the transverse momentum of jet

● The radius gets smaller for higher pT jets and wider for lower pT jets, with a maximum radius of 0.4

● Track-jets are meant to reconstruct the b-jets from the Higgs decay.

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Fig. 10: VR Track Jets

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Fig. 11: Number of Associated Track Jets for Large-R Jet

D2 Variable

● Variable useful in identifying jets with two-prong substructures○ Uses only calorimeter information

● Defined from energy correlation functions○ Dependent on the momentum of each constituent,

separation between different pairs ● Looking for a two-pronged structure since the Z boson,

and the Higgs boson, decays into the bb pair

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Fig. 12. : N-point Energy Correlation Functions

Fig.13. : Large-R jet constituents

MV2c10 variable

● Multivariate discriminant variable that helps to determine if a track-jet is coming from a b-quark (or a c- or a light quark)

● 77% ε (efficiency) working point○ 70% ε WP also utilized

● Properties of b-hadrons:○ Relatively large b-hadron mass (~5 GeV)○ Significant b-hadron lifetime (~1.5 ps)

● B-hadrons travel a few mm in the detector before decaying○ Gives rise to secondary vertices and

other useful topologies used for tagging

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Fig. 14: MV2c10 Variable Effectiveness

Fig. 15: Jet properties MV2c10 takes into consideration

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● B-jet discrimination with highest efficiency for MV2c10 variable, lower for light-flavour jets

● Tends to higher values (closer to 1) and corresponds to b-jets, while light jets tend to -1.

Different Tagging Methods

● Single b-tagging○ Looks at track jets and tags individually○ Utilizes the MV2c10 variable

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Fig.16: Diagram of Track Jet

Different Tagging Methods● Single b-tagging

○ Looks at track jets and tags individually○ Utilizes the MV2c10 variable

● X⟶bb○ Large-R Jet Tagging ○ Tags up to three-pronged structures

■ Only looking for two pronged (Z⟶bb), up to three pronged to discriminate against background (tt decay, final state radiation)

○ Uses:■ Substructure variables based on

calorimeter information■ Same inputs as used for MV2c1

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Fig. 17 : Large-R Jet substructureImage:

https://indico.fnal.gov/event/8672/session/4/contribution/39/material/slides/0.pdf

Data and Monte Carlo Samples

● Proton-proton collisions from the 2017 LHC run, at a center of mass energy of 13 TeV.

● A total integrated luminosity of approximately 47 fb−1 was recorded by ATLAS● Signal:

○ Z(→bb)+γ● Background:

○ QCD background (photons + jets) and W + γ○ Negligible contributions from jets faking photons, electrons faking photons and

tt.

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Fig. 18: Leading Feynman Diagrams of various background decays

Images: https://www.nevis.columbia.edu/~miochoa/Zbb_REU2019.html

Selecting Z(⟶bb) + γ events ● Select Z➝bb large-R jets produced in

association with a photon● Z➝bb candidate:

○ Highest pT large-R jet● One photon and at least one large-R jet

● Photons cannot be within the large-R jet, hence the ∆R cut

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Fig. 19: Leading Feynman Diagram of Z boson decay

Selecting Z(⟶bb) + γ events● Single-photon trigger: transverse energy above 140 GeV and loose photon

identification requirements.● Track-jets associated to the large-R jet

○ Leading Photons and Jets■ Photons and jets with highest transverse momentum

○ Identifying track-jets with b-hadrons with two different methods:■ b-tagging track-jets (2-tag region) using 70% and 77% efficiency working

points● 70 % and 77% ε WPs typically used for Higgs Tagging

■ X⟶bb tagging Large-R jet: using dedicated Higgs tagger trained against QCD jets, using 60% efficiency working point

○ 2-tag region also tested with an additional cut on variable D2 (jet substructure variable to select jets with 2-prong decays)■ D2 > 1.3

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B-Tagged Track Jet for each Large R Jet

Fig. 20 : Number of B-Tagged Track Jets for each Large-R Jet at 77% ε WP

● Scale factor of 1.34 derived for pre-tagged plots ○ Scaling up the ᵞ + jets

background since it is not very well modeled

● Different scale factors derived for each specific tagging method

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pT for Photons and Large-R Jets

Fig. 21: Transverse momentum of photons (right) and B-Tagged Large R Jets (left) at 70% ε WPs

Fig. 22: Transverse momentum of photons (right) and B-Tagged Large R Jets (left) at 77% ε WPs

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Large-R jet mass pre-tagging

Fig. 23: Mass (MeV) of Large-R Jet with highest pT

Large-R jet mass after 2b tagging

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● Jet mass peak ~ 90 GeV in Z + photon backgrounds after tagging indicative of both tagged jets result of Z boson decay

Fig. 24: Mass (MeV) of Large-R Jet after B-Tagging at 70% (left) and 77% (right) ε WPs

Large-R jet mass after 2b tagging w/ D2 cut

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Fig. 25: Mass (MeV) of Large-R Jet after B-Tagging, with D2 cut, at 70% (left) and 77% (right) ε WPs

X→bb Tagging

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Fig. 26: Mass (MeV) of Large-R Jet after Xbb Tagging at 70% (left) and 77% (right) ε WPs

Signal/Background Comparisons

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Pre-Tagged(Leading jet)

2b-Tagged (w/o D2 cut)

X→bb

S/B 0.004 0.038 0.079

S/B in mass window

0.010 0.097 0.149

● Only for 77% efficiency

● 2b-Tagging w/o the D2 cut taken into consideration

● Leading jet used as baseline for pre-tagged masses

● X→bb is the most efficient in tagging

Conclusions● Main goal: To validate the different tagging techniques used to identify H⟶bb decay

by using them in the identification of the Z⟶bb decay● We compared techniques - Single BTagging and Xbb Tagging- to identify H⟶bb

decays using Z⟶bb decays in ATLAS proton-proton collision data from 2017● Normalization of photon+jets is not known very well, and it changes as we require

b-tagging. Result of not knowing what is the flavor composition of the jets in these events (how many are b's and how many are light).

● Improvements / Future Analysis:○ Photon+jets is not very well modelled in simulations. So in practice , this analysis

would need to come up with ways of doing a data-driven estimate in future for better modeling.

○ Work being done on an X⟶bb mass decorrelated tagger

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Acknowledgements● This material is based upon work supported by the National Science Foundation

under Grant No. NSF PHY-1659528● Columbia University● Nevis Laboratories ● Prof. Parsons & Prof. Brooijman● Inês Ochoa

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References[1] ATLAS Collaboration, Performance of jet substructure techniques for large-R jets in proton-proton collisions at sqrt(s) =

7 TeV using the ATLAS detector, (2013) , [arXiv:1306.4945v1]

[2] http://edu-observatory.org/olli/CosmicRays/CERN-Brochure-2009-003-Eng.pdf

[3] ATLAS. ’ATLAS.’2019, http://atlas.cern/

[4] CERN. 'CERN.' Home, 2019, home.cern/.

[5] Ochoa, Ines. ’REU2019: Validation of Higgs to bb Tagging Techniques with Z→bb decay.’ - CodiMD, 2019, www.nevis.columbia.edu/ miochoa/Zbb-REU2019.html

[7] Yu, Felix, et. al; Anatomizing Exotic Production of the Higgs Boson, (2014), [arXiv:1404.2924]

[8] CMS Collaboration, Search for the standard model Higgs boson produced through vector boson fusion and decaying to bb-bar, (2015), [arXiv:1506.01010v3]

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