Diboson resonance search in the all-hadronic final state On behalf - - PowerPoint PPT Presentation

ATLAS Experiment Université de Genève

Diboson resonance search in the all-hadronic final state

Sofia ADORNI BRACCESI CHIASSI On behalf of the ATLAS collaboration Moriond 16-23 March 2019

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI 1

VV ➜ JJ physics motivation

✸ Searching for heavy resonances decaying in WW, ZZ or WZ ➜ sensitive to BSM physics (spin-0/1/2 resonances) ✸ Why focus on fully hadronic decay products?

• High sensitivity in the high

mass regime (BR(W→qq) ≃ 3 x BR(W→l𝛏), BR(Z→qq) ≃ 10 x BR(Z→ll))

• Probe all decay modes in one

analysis

• Sensitive to the unexpected

(generic bump hunt)

Large-R jets

500 1000 1500 2500 [GeV]

T

0.1 0.2 0.3 0.4 0.5 0.6 0.7 resolution

2

jet D Fractional

LC Topo TCCs

ATLAS Simulation Preliminary

qqqq → R=1.0, WZ

T

anti k >200 GeV

T jet

|<2.0, p

jet

η | = 13 TeV s

Generated jet p 2000

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI 2

Track-CaloClusters

✸ Track-CaloClusters (TCC) (reference) are a type of particle flow designed for the high energy regime:

• The calorimeter has better energy resolution but poor

spatial resolution

• The tracker has poorer transverse momentum (pT)

resolution but good angular resolution ➜ use tracker angles and calorimeter energy scale very roughly: TCC 4-vec = (pTcalo, 𝛉track, 𝛠track, Ecalo) ✸ At high pT the hadronic decays of the W/Z are very collimated (reaching the granularity limits of the calorimeter)

0.5 1 1.5 2 2.5 3 3.5 4 [TeV] 100 200 300 400 500 600 700 800 900 1000 Background rejection Jet p [TeV]

T

s=13 TeV ATLAS Simulation Preliminary 0.5 1 1.5 2 2.5 3 3.5 4 [TeV]

T

Jet p 0.2 0.4 0.6 0.8 1 1.2 1.4 W-tagging efficiency

Mass efficiency cut D2 efficiency cut nTrk efficiency cut Total efficiency

ATLAS Simulation Preliminary =13 TeV s

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI 3

New W/Z tagger

✸ Fully hadronic mode = large QCD dijet background ➜ use Jet SubStructure (JSS) to distinguish background and signal (mass, D2 and ntrk) ➜ JSS variables are more powerful with TCC jets ✸ New tagger optimised for best analysis significance ➜ novel tagging strategy

• replaces previous fixed efficiency tagger, non-optimal for analyses

Events / 0.1 TeV

3 −

10

2 −

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1 −

10 1 10

2

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3

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4

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Data Fit Fit + Bulk RS m=1.5 TeV Fit + Bulk RS m=2.6 TeV ZZ or WW SR /DOF = 3.1/3

2

χ

[TeV]

JJ

m

1.5 2 2.5 3 3.5 4 4.5 5

Significance

2 − 2

ATLAS Preliminary

s = 13 TeV, 139 fb-1 Events / 0.1 TeV

3 −

10

2 −

10

1 −

10 1 10

2

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3

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Data Fit Fit + HVT model A m=2.0 TeV Fit + HVT model A m=3.5 TeV WZ or WW SR /DOF = 6.0/4

2

χ

[TeV]

JJ

m

1.5 2 2.5 3 3.5 4 4.5 5

Significance

2 − 2

ATLAS Preliminary

s = 13 TeV, 139 fb-1

WW + WZ SR WW + ZZ SR

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI 4

Results and exclusion limits

✸ No significant excess in any of the observed channels ✸ WW + WZ: HVT model B (A) excluded up to 4.4 (4.1) TeV ✸ WW + WZ: Radion excluded up to 3.2 TeV ✸ WW + ZZ: Bulk RS excluded up to 2.8 TeV

m(V’) [TeV] 1.5 2 2.5 3 3.5 4 4.5 5 WW+WZ) [fb] → B(V’ × V’) → (pp σ

1 −

10 1 10

2

10

3

10

)

• 1
• Phys. Lett. B 777 (2018) 91 (36.7 fb

)

• 1
• Phys. Lett. B 777 (2018) 91 (Scaled to 139 fb

)

• 1

Current Result (139 fb

ATLAS Preliminary = 13 TeV s qqqq → VV

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI 5

Conclusions

✸ We didn’t observe any significant excess ✸ Improvement in sensitivity equivalent to redoing the 36.7 fb-1 study on entire HL-LHC dataset of ~3000 fb-1 ! ✸ This very large improvement is due to the combination of two major innovations

• Use of Track-CaloClusters as inputs to

jet reconstruction

• Use of new tagger (optimised for

significance + use of Ntrk variable)

✸ The gain in sensitivity observed goes well beyond statistics : this analysis really shows the potential of new methods for reconstruction, tagging and statistics analysis

HVT V’ → WW + WZ

x4 x4 x2

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI

BACKUP

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI

Event selection

Trigger Trigger on lowest unprescaled large-R jet trigger (year-by-year) Quality GRL, DQ checks, jet cleaning Leptons Veto events with leptons of pT > 25 GeV and |𝛉| < 2.5 Jet Kinematics (TCC jets) Leading pT > 500 GeV (for trigger), subleading pT > 200 GeV (for calibration), m > 50 GeV (for calibration), |𝛉| < 2.0 (for tracks) mJJ > 1.3 TeV Trigger fully efficient at 1.3 TeV (for background) mJJ < 7.0 TeV Upper limit fixed by common range with other analyses for heavy resonance combination |𝞔y| < 1.2 Reducing t-channel QCD jet pair production Jet pT asym < 0.15 Signal is balanced Boson tagging W/Z selections defined as X < mJ < Y, D2 < Z, ntrk < K Reduces QCD background ~5 orders of magnitude W/Z mass windows overlap ➜ signal regions are not orthogonal

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI

Systematic uncertainties

Background fit normalisation and shape (~25% (100%) at 3(5) TeV) Main uncertainties Boson-tagging signal efficiency (~25%) Jet pT scale (JPTS) ~5% ISR - FSR (3% for HVT, 5% for RSG) PDF (1%, up to 12% for HVT) Remaining uncertainties Luminosity scale (2.1 %) JPTR (<1%)

Tagging and jet uncertainties approved by the JSS group

60 80 100 120 140 160 180 200 [GeV]

J

m 10000 20000 Events / 5 GeV

Data Fit Fit bkd. W/Z+jets W+jets Z+jets

• 1

=13 TeV, 139 fb s V+jets control region

ATLAS Preliminary

777 ± Fitted W/Z+jet events: 17112 0.04 ± = 0.92

Tag

s

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI

Boson tagging SF using V+jets

✸ The boson tagging efficiency is evaluated in data enriched W/Z+jet events ✸ We tag one jet (leading or subleading) and anti-(D2)tag the other one ✸ We fit the distribution we get using a signal+background function ✸ We obtain the scale factor and the uncertainty from differences between data and MC

• STag = 0.92 ± 0.04 (stat) ± 0.02 (closure) ± 0.03 (tt) ± 0.02 (fit) ± 0.05 (high pT) ± 0.1 (theory)

= 0.92 ± 0.13

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI

Background modelling

• ABCD method

✸ The fit range is from 1.3 TeV to 8 TeV ✸ Validation was done in a dedicated control region created with the ABCD method and parametrised tagging efficiencies of QCD

• Fit was able to describe the expected

mJJ spectra in all fit control region ✸ The behaviour of the fit at high masses was checked and we are confident that the extension of the fit range is valid

Events / 0.1 TeV

3 −

10

2 −

10

1 −

10 1 10

2

10

3

10

4

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Data Fit

ATLAS Preliminary

s = 13 TeV, 139 fb-1 WZ CR /DOF = 3.9/5

2

χ

[TeV]

JJ

m

1.5 2 2.5 3 3.5 4 4.5 5

Significance

2 − 2

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI

Background modelling

✸ To perform the bump hunt we first have to fit the background

• We use a parametric function:

✸ The choice of the fit function could possibly have an impact on the analysis

• Comparison of two working fit functions on pseudo experiments showed significant effect only at high

mass (where limiting factor is the lack of statistics) ✸ The difference observed for this specific choice is significantly smaller than the uncertainty on the fit ➜ neglected in statistical treatment ✸ Why extend the range?

• From fit up to 6 TeV (limits up to 5 TeV) ➜ fit up to 8 TeV (limits up to 7 TeV)

✸ We are preparing for the full Run2 “grand combination”

• Background is understood
• Very large uncertainties: expectation is << 1 in this regime with 100% uncertainty

✸ Comparison of mean and spread between data and MC gives us confidence that they are well modelled.

dn dx = p1(1 − x)p2−ξp3x−p3

x = mJJ 13[Tev]

with

) [TeV]

KK

m(G

1.5 2 2.5 3 3.5 4 4.5 5

WW+ZZ) [fb] →

KK

B(G × )

KK

G → (pp σ

2 −

10

1 −

10 1 10

2

10

3

10

4

10 ATLAS Preliminary

• 1

= 13 TeV, 139 fb s qqqq → VV

Observed 95% CL upper limit Expected 95% CL upper limit σ 1 ± Expected limit σ 2 ± Expected limit = 1

PI

M Bulk RS, k/

m(V’) [TeV]

1.5 2 2.5 3 3.5 4 4.5 5

WW+WZ) [fb] → B(V’ × V’) → (pp σ

2 −

10

1 −

10 1 10

2

10

3

10

4

10 ATLAS Preliminary

• 1

= 13 TeV, 139 fb s qqqq → VV

Observed 95% CL upper limit Expected 95% CL upper limit σ 1 ± Expected limit σ 2 ± Expected limit = 1

V

HVT model A, g = 3

V

HVT model B, g

ATLAS Experiment Université de Genève Sofia ADORNI BRACCESI CHIASSI ✸ WW + WZ: HVT model B (A) excluded up to 4.4 (4.1) TeV ✸ WW + WZ: Radion excluded up to 3.2 TeV ✸ WW + ZZ: Bulk RS excluded up to 2.8 TeV

HVT V’ → WW + WZ GKK → WW + ZZ Radion → WW + WZ