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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
Standard Model of Particle Physics (SM)
Mathematical formulation:
https://comm
ons.wikim
edia.org/wiki/File:Elem
entary_particle_interactions_in_the_Standard_Model.png
Standard Model of Particle Physics (SM)
• Gravity• Dark matter and dark energy• Neutrino masses• Matter-antimatter asymmetry• …
Mathematical formulation:
What the SM not describes:
https://comm
ons.wikim
edia.org/wiki/File:Elem
entary_particle_interactions_in_the_Standard_Model.png
Standard Model of Particle Physics (SM)
• Gravity• Dark matter and dark energy• Neutrino masses• Matter-antimatter asymmetry• …
Mathematical formulation:
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
Physics Beyond the Standard Model
https://twiki.cern.ch/tw
iki/bin/view/C
MSPublic/H
LTDiM
uon2017and2018
New Resonance?
Physics Beyond the Standard Model
https://twiki.cern.ch/tw
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
p p
Large Hadron Collider (LHC)
https://cds.cern.ch/record/2302977
p pEvent
Large Hadron Collider (LHC)
https://cds.cern.ch/record/2302977
p pEvent
l+
l-
Large Hadron Collider (LHC)
https://cds.cern.ch/record/2302977
p pEvent
l+
l-
Large Hadron Collider (LHC)
ATLAS Detector
https://cds.cern.ch/record/2302977
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
210
410
610
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