search for new phenomena in the dilepton ﬁnal state using

Search for new phenomena in the dilepton final state using

proton-proton collisions

Simon SchmitzDLR, HPC-SC

Standard Model of Particle Physics (SM)

https://comm

ons.wikim

edia.org/wiki/File:Elem

entary_particle_interactions_in_the_Standard_Model.png


Mathematical formulation:

https://comm

ons.wikim




• Gravity• Dark matter and dark energy• Neutrino masses• Matter-antimatter asymmetry• …


What the SM not describes:

https://comm

ons.wikim




• Gravity• Dark matter and dark energy• Neutrino masses• Matter-antimatter asymmetry• …


What the SM not describes:

New Phenomenahttps://com

mons.w

ikimedia.org/w

iki/File:Elementary_particle_interactions_in_the_Standard_M

odel.png

Physics Beyond the Standard Model

https://twiki.cern.ch/tw

iki/bin/view/C

MSPublic/H

LTDiM

uon2017and2018



iki/bin/view/C

MSPublic/H

LTDiM

uon2017and2018

New Resonance?



iki/bin/view/C

MSPublic/H

LTDiM

uon2017and2018

New Resonance?

• Grand Unified Theories (GUTs)➡ Many predict heavier versions of the Z boson

• Supersymmetry (SUSY)➡ Extends SM by new symmetries (not relevant for this analysis)

Theoretical motivation

Large Hadron Collider

• Beam divided in 2808 bunches containing around 1.2 ⨉ 1011 protons

• Produced proton-proton collisions at up to √s = 14 TeV and frequency of 40 MHz

• Currently in Long Shutdown 2 (preparing for Run 3 starting in 2021)

https://cds.cern.ch/record/1443022/files/CER

N-ATS-2012-064.pdf

https://de.wikipedia.org/w

iki/Datei:C

ern-accelerator-complex.svg

p p

Large Hadron Collider (LHC)

https://cds.cern.ch/record/2302977

p pEvent



p pEvent

l+

l-



p pEvent

l+

l-


ATLAS Detector


https://atlas.cern

Proton-Proton Collisions

hard scattering process

incoming protons

underlying event

parton shower

hadronization

https://doi.org/10.1007/978-3-319-41045-6_4

ATLAS Detector

• Tracking system and muon spectrometer based on silicon and gaseous detectors➡ Momentum measurement

• Electromagnetic and hadronic calorimeter system in sampling type➡ Energy measurement

https://atlas.cern/discover/detector

Particle Signatureshttp://collider.physics.ox.ac.uk/

• Not shown: hadronization of quarks causes „jets“ which can fake particles

Analysis of Dilepton Final States

• All processes apart from fake background can be estimated with Monte-Carlo (MC) simulations

• Fake background is estimated with data-driven approach

• Invariant mass is used as discriminant variable (m2 = E2 - p2)

• Comparisons between data and MC simulations reveal new phenomena

Signal Process

Background ProcessesFake BackgroundDrell-Yan Top Quarks Diboson

Kinematical Distributions

Even

ts

0

10

20

310×

DataDrell-YanTopDibosonFake

-1Data, 13 TeV, 36.1 fb > 120 GeVeem

eleading η

2− 0 2

Dat

a / M

C

0.5

1.0

1.5

Even

ts

0

10

20

310×

DataDrell-YanTopDiboson

-1Data, 13 TeV, 36.1 fb > 120 GeVµµm

µleading η

2− 0 2

Dat

a / M

C

0.5

1.0

1.5

Even

ts0

5

10

15

310×

DataDrell-YanTopDibosonFake

-1Data, 13 TeV, 36.1 fb > 120 GeVeem

eleading φ

2− 0 2

Dat

a / M

C

0.5

1.0

1.5

Even

ts

0

5

10

15

310×

DataDrell-YanTopDiboson

-1Data, 13 TeV, 36.1 fb > 120 GeVµµm

µsubleading φ

2− 0 2

Dat

a / M

C

0.5

1.0

1.5

Azimuthal angley

x

Detectorɸ

Pseudorapidity

Detectory

zθ

Invariant Mass Spectra

• No obvious resonance was observed➡ Statistical interpretation

Even

ts

2−10

1

210

410

610

710 DataDrell-YanTopDibosonFake

-1Data, 13 TeV, 36.1 fb

[GeV]eem100 1000

Dat

a / M

C

0.5

1.0

1.5

Even

ts

2−10

1

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710 DataDrell-YanTopDiboson

-1Data, 13 TeV, 36.1 fb

[GeV]µµm100 1000

Dat

a / M

C

0.5

1.0

1.5

Statistical Interpretation• In a counting experiment the observed data is distributed

according to a Poisson distribution

• If d is the number of observed events in data and b the number of expected background events the probability describing the discrepancy between d and b can be expressed via

• The bins in the invariant mass spectrum are scanned with windows of different widths

• A p-value is calculated for each of these windows➡ BumpHunter method

BumpHunter Result

• Dielectron: largest discrepancy at around 2.4 TeV with local p-value of 2.7σ

• Dimuon: largest discrepancy at around 1.3 TeV with local p-value of 1.4σ➡ Global p-values insignificant

[TeV]eem1

Even

ts

1−10

1

10

210

310

410DataBackgroundMost Significant Window

-1Data, 13 TeV, 36.1 fb

[TeV]µµm1

Even

ts

1−10

1

10

210

310

410DataBackgroundMost Significant Window

-1Data, 13 TeV, 36.1 fb

Exclusion Limits• Up to which mass can a resonance be exclude with a certain

credibility?

• A MC simulation of signal process is used to estimate the expected number of background and signal events n

• The following likelihood can be constructed where Θ is the signal strength and u the uncertainties

• However, the interesting quantity is

• Integration over uncertainties

Exclusion Limits

• Intersection of theoretical cross section and observed limit define lower limit on Z’ mass (credibility interval of 95%)

• The E6-motivated model yields a limit of 4.06 TeV

• The SSM yields a limit of 4.35 TeV

[TeV]χZ'm

2 4 6

[pb]

Bσ

4−10

3−10

2−10

1−10

-1Data, 13 TeV, 36.1 fbCombined

Expected Limitσ 1±Expected σ 2±Expected

Observed LimitχZ'

[TeV]SSMZ'm

2 4 6 [p

b] B

σ

4−10

3−10

2−10

1−10

-1Data, 13 TeV, 36.1 fbCombined

Expected Limitσ 1±Expected σ 2±Expected

Observed LimitSSMZ'

Conclusion• The SM is not complete➡ New physics has to be out there

• Presented the ATLAS experiment at the LHC

• Showed some details of the analysis of dilepton final states

• Statistical interpretation of the invariant mass spectrum has been presented

• Exclusion limits have been derived

search for new phenomena in the dilepton ﬁnal state using

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