search for heavy resonances in the beta final state - atlas final presentation · 2018-09-05 ·...

Columbia University

Search for Heavy Resonances in the VH→ qqbb̄Final State

ATLAS Final Presentation

Savannah King

2 August 2018

2

The Standard Model

Classifies the known elementary particles anddescribes their interactions by the fundamentalforces

I FermionsI Spin 1

2 , includes quarks and leptonsI Divided into 3 generations based on

characteristicsI Bosons

I Integer spin, responsible for propagatingthe fundamental forces

I W and Z bosons carry the weak interactionI Higgs boson is responsible for propagating

the Higgs field, which gives mass to massiveparticles

Figure: Standard Model ElementaryParticles

Savannah King | Search for Heavy Resonances in the VH → qqbb̄ Final State ATLAS Final Presentation

3CERN and the LHC

Figure: Main LHC experiments

European Organization for Nuclear Research(CERN)

I Located near Geneva, SwitzerlandI The site of the Large Hadron Collider, as well

as several other particle accelerators andone antiparticle decelerator

LHCI Largest particle accelerator in the worldI Main experiments: ATLAS (A Toroidal LHC

ApparatuS), CMS (Compact Muon Solenoid),ALICE (A Large Ion Collider Experiment), andLHCb (Large Hadron Collider beauty)


4The ATLAS Detector

46 m long, 25 m diameter general purpose particle detector

I Magnet system- Provides the magneticfield necessary to make momentameasurements

I Muon spectrometer- measures themomenta and direction of the muons

I Hadronic calorimeter- absorbs energyfrom particles that interact via thestrong interaction

I Electromagnetic calorimeter- detectsparticles that interact via theelectromagnetic force

Figure: Computer generated diagram ofthe ATLAS Detector


5The ATLAS Detector

I Calorimeter is composed of metalabsorbers and sensing elements

I Liquid Argon (LAr) calorimeter usesliquid argon as a sensing element

I Outer parts of the calorimeter usescintillating plastic tiles to detectparticles

I Inner detectorI Pixel detector-made up of silicon

pixels and has the highest granularityI Semiconductor tracker-made of

silicon stripsI Transition radiation tracker-utilizes

straw detectors that contribute tomeasuring the momentum ofparticles

Figure: Computer generated diagram ofthe ATLAS Detector


6

The ATLAS Detector

I Every type of particle has its ownsignature based on which parts of thedetector it does and does not interactwith

I Trigger system to reduce the flow ofdata from the detector

Figure: Particle identification in ATLAS


7Heavy VH→ qqbb̄ Resonances

I Attempts to address deficiencies in the Standard Model, such assensitivity of the Higgs boson mass to radiative corrections

I Several theories predict the existence of particles that couple with theHiggs Boson

I Looking for an unknown resonance, Y, decaying into a Higgs boson andaW or Z boson, which both decay hadronically

I Given its assumed decay modes, the particle Y must be a boson

Figure: Feynman diagram of Y → VH → qqbb̄


8

Data and Monte Carlo Samples

I 79.8 �−1 of proton-proton (pp) collision data at√s = 13 collected by

the ATLAS detector between 2015 and 2017

I Signals are modeled with Monte Carlo (MC) simulation

I Signal events generated with Madgraph5_aMC@NLO 2.2.2 interfacedto Pythia 8.186 for parton shower and hadronization

I NarrowWH→ qqbb̄ and ZH→ qqbb̄ samples with resonance massesof 1 TeV, 3.5 TeV, and 5 TeV


9Plots

η 2.5− 2− 1.5− 1− 0.5− 0 0.5 1 1.5 2 2.5

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810Data

WH, m(Y) = 1200 GeV

WH, m(Y) = 3500 GeV

WH, m(Y) = 5000 GeV

ATLAS Work in Progress = 13 TeVs

-1L = 79.8 fb∫

(a) η

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WH, m(Y) = 1200 GeV

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Mass

(c) Mass

Figure: Statistics for the data andWH → qqbb̄ signals


10

Plots

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Figure: Statistics for the data andWH → qqbb̄ signals


11

Search Strategy

[GeV]HM80 100 120 140 160 180 200

Hig

gs B

R +

Tot

al U

ncer

t

-410

-310

-210

-110

1

LH

C H

IGG

S X

S W

G 2

013

bb

ττ

µµ

cc

gg

γγ γZ

WW

ZZ

Figure: Higgs Branching Ratios

I At 125 GeV, the branching ratio of theH→ bb̄ channel is approximately 57%, sowe can identify Higgs bosons through theuse of b-tagging


12

Search Strategy

Figure: Boostedregime

I Higgs and V bosons are expected to have a very highkinetic energy and pt

I In a decay to two quarks, the angular separation of thequarks is related by the equation

∆R =√

∆Φ2 + ∆η2 ≈ 2mpt

where η is the pseudorapidity and Φ is the azimuthalangle

I Higgs candidates are identified by reconstructinglarge-R jets and searching for those that contain b-jets


13

Search Strategy

Figure: Boosted regime

I W and Z bosons have hadronic branching ratios ofapproximately 67.60% and 69.91% respectively

I Boosted V bosons are identified using substructuretechniques that search for the proper two-prongstructure within the large-R jet


14Event Reconstruction

anti-kt jet clustering algorithmI Identifies and reconstructs hadronically decaying V boson and Higgs

boson candidates

I Soft-resilient jet algorithm that produces conical jets

I Using radius R = 1.0 for reconstructions


15Event Reconstruction

Jet trimmingI Diminishes the effects of pile-up and soft radiation

I Constituents of each large-R jet reclustered into subjets with radiusR = 0.2

I Subjets with psubjett /pjett < 5% are removed

I Jets required to have pt > 200 GeV

I Mass of the final large-R jets is calculated using a combination oftracking and calorimeter information

Figure: Jet trimming to reduce pile-up and soft radiation activity


16

Event Reconstruction

CutsI Track jets (identified and measured by the ATLAS inner detector) are

constructed from charged particle tracks with pT > 0.4 GeV and|η| < 2.5

I MV2c10, a multivariate tagging algorithm, is used to determine whichtrack jets are b-jets (MV2c10 > 0.3706)

I b-tagging requirements result in an b-jet efficiency of 77% with amisidentification rate of about 2% for light-flavor jets and 24% forcharm jets


17Event Selection

I Events that are semi-leptonic are removed

I Selected events must have at least two large-R jets with |η| < 2.0 andinvariant massmJ > 50 GeV

I Two leading pT large-R jets within the event must have pT greater than450 GeV and 250 GeV respectively


18

Higgs Selection

I Leading pT large-R jet is tested for Higgs boson candidacy

I Jets with 95 GeV < mJ < 145 GeV are considered

I Events are then classified into two signal regions and one controlregion based on how many of the two leading track jets within thelarge-R jet are b-tagged


19

Higgs Selection

I Events in the “1-tag" (denoted SR1) and“2-tag" (denoted SR2) regions areconsidered Higgs candidates and areutilized in the analysis

I Events in which the leading pT large-Rjet passes the mass requirements buthas no b-tagged track jets fall in thecontrol region (denoted CR0)

I Used in background determination

1-tag SR

2-tag SR

0-tag CR sideband

sideband

sideband

Higgs boson candidate mass [GeV]

Num

ber o

f b-ta

gs

Figure: Signal regions and control region.The sideband, which is not utilized in thisanalysis, has similar expected event yieldto the signal region.


20

Higgs Selection

Mass [GeV]95 100 105 110 115 120 125 130 135 140 145

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Figure: Higgs candidate mass distribution for signals


21

V Boson Selection

Dβ=12 = ECF3(

ECF1ECF2

)3

where

ECF1 =∑i

pT,i

ECF2 =∑ij

pT,ipT,j∆Rij

ECF3 =∑ijk

pT,ipT,jpT,k∆Rij∆Rjk∆Rki

I Subleading pT large-R jet in each event istested for V boson candidacy

I 65 GeV < mJ < 105 GeVI V boson candidates must pass a

substructure cut based on the variableD2

I Uses energy correlation functions totag boosted objects with two-prongdecay structures


22

V Boson Selection

Mass [GeV]65 70 75 80 85 90 95 100 105

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(b) ZH → qqbb̄ signals

Figure: W/Z boson candidate mass distribution for signals


23

Data Driven Background Estimation

I Predominant background (approximately 90%) for the signal originatesfrom multijet events

I Multijet background is modeled directly from data

I The 0-tag (control) region is used to model the background in thesignal (SR1 and SR2) regions


24

Background and Data

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Figure: 1-tag data with background

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Figure: 2-tag data with background


25

Smearing Signals

I Want to test if VH→ qqbb̄ signals ofdifferent widths are detectable

I Lack of generated signals with greaterwidth, which corresponds to a shorterlifetime of the particle

I Signals are smeared using a smearingfunction- assumed to be a relativisticBreit-Wigner distribution

I Each signal is smeared twice byincreasing the signal width Γ by 5% and10%

σ2(E;µ, Γ) =µΓ

π

1(E2 − µ2)2 + (µΓ)2

h_J_Y_massEntries 24030Mean 1169Std Dev 80.06

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Entries 24030Mean 1169Std Dev 80.06

h_J_Y_massEntries 12000Mean 1143Std Dev 178.3

Reconstructed Y Mass

Mass [GeV]

Figure: Smeared 1.2 TeVWH → qqbb̄signal


26

Measuring Significance

I Significance of each signal and smeared signal with respect to thebackground is then calculated using the asymptotic formula

s =

√2(ns + nb) ln(1 +

nsnb

)− 2ns

I Significance utilizing a mass window of 1000 GeV (700 GeV for 1.2 TeVsignals) around each signal peak


27

Measuring Significance

Decay Resonance Mass Region Smear Significance(TeV) (% distribution width)

WH 1.2 SR1 0 35.3528WH 1.2 SR1 5 34.2042WH 1.2 SR1 10 33.0317WH 1.2 SR2 0 100.051WH 1.2 SR2 5 96.9684WH 1.2 SR2 10 93.8112WH 3.5 SR1 0 273.451WH 3.5 SR1 5 248.056WH 3.5 SR1 10 225.755WH 3.5 SR2 0 426.660WH 3.5 SR2 5 392.912WH 3.5 SR2 10 362.989

Table: Significance ofWH → qqbb̄ signals


28

1.2 TeVWH→ qqbb̄ signal

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Figure: 5 TeVWH → qqbb̄ signal with 2-tagdata


311.2 TeV ZH→ qqbb̄ signal

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Figure: 5 TeV ZH → qqbb̄ signal with 1-tag data

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Figure: 5 TeV ZH → qqbb̄ signal with 2-tagdata


34Results

I Significance of the narrower (original) signal is consistently higher thanthat of the wider signals

I As the signal becomes wider and flatter, it approaches the shape of thebackground and is thus less significant

I 1.2 TeV signals have markedly lower significance than the 3.5 and 5 TeVsignals


35Conclusions

I Signals of a range of different widths are detectable using our analysistechniques

I Signals of larger widths, which correspond to resonances with shorterlifetimes, are less distinguishable from background


36

Thanks!


search for heavy resonances in the beta final state - atlas final presentation · 2018-09-05 ·...

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