R&D STUDIES FOR CMS HE AT THE SUPER LHC CONDITIONS &
INCLUSIVE SEARCH FOR NEW PHYSICS AT CMS WITH JETS AND MISSING MOMENTUM SIGNATURE
Elif Aslı AlbayrakPh.D. Thesis Defense
October 7th, 2011
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Outline
Introduction to LHC and CMS Experiment
Part I: Detector Studies
R&D STUDIES FOR HE UPGRADE AT CMS
Part II: Physics Analysis
INCLUSIVE SEARCH FOR NEW PHYSICS AT CMS WITH JETS AND MISSING MOMENTUM SIGNATURE
LHC and CMS Experiment
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The Large Hadron Collider
The LHC is the largest proton-proton (pp) collider designed to run with 14 TeV center of mass energy and 1034cm-2s-1 peak luminosity.
It also provides heavy ion collisions to study the quark-gluon plasma state of the matter.
There are four experiments at LHC
A Toroidal LHC ApparatuS (ATLAS)
Compact Muon Solenoid (CMS)
The Large Hadron Collider Beauty Experiment (LHC-b)
A Large Ion Collider Experiment (ALICE)
The CMS is one of the general purpose experiments, designed to study the physics of pp collisions at 14 TeV at the LHC.
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The Compact Muon Solenoid (CMS)
• The CMS is designed to discover Higgs particle and new physics beyond the Standard Model (SM).
Total weight : 12500 TOverall diameter : 15.0 mOverall length : 21.5 mMagnetic field : 4 Tesla
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The CMS Detector
The CMS has four main subsystems dedicated to measure the energy, momentum and position of photons, electrons, muons and all the other products of 14 TeV pp collisions:
The magnet
Its bending power allows to determine charge/mass ratio of the tracked particles.
Length/radius ratio and high magnetic field (3.8 T) provides a good momentum resolution.
The muon system
Reconstructing muons, measuring their momentum with a high accuracy and using them for trigger information.
The tracker
Measure charged particle trajectories with high efficiency and provide precise reconstruction of secondary vertices originating from LHC collisions.
The calorimeters
Consists of electromagnetic (ECAL) and hadronic components (HCAL).
Measure the energy of electrons, photons and jets with a high precision.
High accuracy measurement for the missing transverse energy.
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The Hadronic Calorimeter (HCAL)
The HCAL is a compact calorimeter and composed of
Barrel (HB)
Endcap (HE)
Forward (HF)
The barrel and the endcaps are sampling calorimeters.
• surround the ECAL and the tracker system.
• cover the pseudorapidity range up to |η| < 3.0.
The forward calorimeter consists of steel absorbers and quartz fibers embedded in it and extends the coverage up to |η| < 5.0.
R&D STUDIES FOR HE UPGRADE AT CMS
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The Hadronic Endcap (HE) Calorimeter
Consists of two large structures at each end of the hadronic barrel detector.
Each HE consists of 14 η towers with 5°φ segmentation.
covers the pseudorapidity region 1.3 < |η| < 3.0, which contains about 34% of the particles produced in the final state.
in the current design 19 layers of plastic scintillators (3.8 mm) are placed between the 7.8 cm brass absorber plates.
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Radiation Problem
If LHC discovers the Higgs boson or new physics we will need higher number of events to study rare events such as MSSM Higgs, Higgs coupling to itself.
Higher number of events higher luminosity runs LHC upgrade.
With LHC luminosity upgrade the accumulated radiation will damage the CMS and the other detectors.
Scintillator tiles used in CMS HE will loose their efficiency and stop providing light collection.
As a solution to radiation damage problem, we proposed p-terphenyl (pTp) deposited quartz plates to replace the scintillator tiles.
Advantage: quartz plates are radiation hard.
Disadvantage: light production for quartz plates, photons from Cherenkov process, creates acutely less photons than a scintillation process.
• To increase the light collection efficiency, R&D studies are performed on the quartz plates.
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Light Enhancement
Light collection created with Cherenkov process increases with 1/λ2.
More photon can be collected if we use a wavelength shifter method with UV absorption spectra.
For this purpose different wavelength shifters including p-terphenyl(pTp) , 4% gallium dopped zinc oxide (ZnO:Ga), o-terphenyl (oTp), m-terphenyl (mTp) and p-quarterphenyl (pQp) were tested.
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Different wavelength shifter (WLS) materials are deposited on quartz plates and the coated plates are tested for the light collection efficiency.
Evaporation and RF techniques are used to deposit the WLS materials on the quartz plates.
Coated plates are prepared at University of Iowa and Fermilab Thin Film Laboratory.
Selection of Wavelength Shifter
Fermilab Thin Film Laboratory ZnO:Ga sputtering system and guns
Fermilab Thin Film Laboratory WLS evaporation
setup
Plain quartz plate
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Selection of Wavelength Shifter
Quartz plates coated with different thickness of wavelength shifters are tested at
Fermilab Meson Test Beam Facility (Nov 07 and Feb 08)
CERN H2 area (August 2007)
2 μm pTp deposited quartz plate0.2 μm ZnO:Ga deposited quartz plate
plain quartz plate
Both pTp and ZnO:Ga enhance the light
collection by at least a factor of 4.
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Selection of Wavelength Shifter
The pTp and ZnO:Ga enhance the light production by a factor of 4.
oTp, mTp, and pQp did not perform as well as pTp and ZnO:Ga.
Since ZnO:Ga is more difficult to deposit on the quartz plates and does not provide an advantage compared to pTp deposited quartz plates, we decided to focus on pTp.
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Radiation Hardness
Different methods were used to test the radiation hardness of pTp
The radiation hardness tests with proton (Indiana University Cyclotron Facility and CERN beam lines) and neutron (Argonne).
The 90Sr activated light outputs of pTp samples before and after irradiation were compared (University of Mississippi CMS Laboratories).
16% light output lost after 200 kGy of proton irradiation.
After 200 kGy radiation damage level slows down.
After 400 kGy still have more than 80% of the initial light collection
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The Calorimeter Capabilities
A calorimeter prototype was built with 2 μm pTp deposited (one side) quartz plates .
The 15cm x 15cm x 5mm quartz plates are used.
For hadronic (electromagnetic) configuration 7cm (2cm) absorbers were used between each layer.
The prototype was tested for
hadronic capabilities with 30, 50, 80, 130, 200, 250, 300, and 350 GeV pion beams.
electromagnetic capabilities with 50, 80, 100, 120, 150 and 175 GeV electron beams.
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Hadronic Capabilities
Detector linearity Hadronic Energy ResolutionLongitudinal Shower Profile
DataGeant4 simulations
DataGeant4 simulations
1% hadronic response linearityA good agreement betweendata and simulation.
Solid line -> simulationPoints -> data
•1% hadronic response linearity.1% hadronic response linearity.•15% hadronic resolution at 350 GeV pion beam.15% hadronic resolution at 350 GeV pion beam.•On a bigger scale it can reach up to current HE resolution. On a bigger scale it can reach up to current HE resolution. •8% at 300 GeV pion beam.8% at 300 GeV pion beam.
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Electromagnetic Capabilities
Detector linearityElectromagnetic Energy
ResolutionLongitudinal Shower Profile
DataGeant4 simulations
DataGeant4 simulations
3% em response linearityA good agreement betweendata and simulation.
Solid line -> simulationPoints -> data
•3% electromagnetic response linearity.3% electromagnetic response linearity.•Above 120 GeV both simulation and data converge to 5.6%Above 120 GeV both simulation and data converge to 5.6%• It can be used as a radiation hard EM calorimeter for It can be used as a radiation hard EM calorimeter for future colliders.future colliders.
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Results
Test beam results and Geant4 simulations showed that pTp deposited quartz plates are perfect candidates to replace the current HE scintillator tiles. Both quartz and pTp radiation hard and cost efficient
pTp deposited material loses only 20% of the initial light collection after 400 kGy proton irradiation.
well above higher luminosity conditions (25Mrad = 250 kGy)
pTp deposited quartz plates increase the light yield by at least factor of 4.
The pTp deposited quartz plate calorimeter is a good option in terms of accomplish the current HE calorimeter performance.
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INCLUSIVE SEARCH FOR NEW PHYSICS AT CMS WITH JETS AND MISSING MOMENTUM SIGNATURE
Motivation
Supersymmetry
Analysis
Data Driven Background Estimations
Result and Interpretation
Conclusion
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Motivation
The Standard Model (SM) can explain our nature’s working mechanism with a high accuracy but there are still unanswered questions such as
Why some force carriers have mass but others do not?
How does the electroweak symmetry breaking mechanism work?
Can gauge couplings be unified at a high mass scale?
What is the source of dark matter in the universe?
Many beyond SM physics theories such as Supersymmetry, extra dimensions, Technicolor, and fourth family try to address these questions.
Supersymmetry (SUSY) is favorite explanation for most of the theorist because it can lead to incorporation of gravity to particle physics
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The Supersymmetry (SUSY)
Sfermions Spin 1Sfermions Spin 1 Sbosons Spin Sbosons Spin 1/21/2
Squarks
Sle
pto
ns
SU
SY Fo
rce C
arrie
rs
SUSY ParticlesSUSY Particles
Fermions Spin 1/2Fermions Spin 1/2 Bosons Spin Bosons Spin 0,10,1
Qu
arks
Lepto
ns
Force
Carrie
rs
Standard Model ParticlesStandard Model Particles
SUSY is a symmetry that relates fermions and bosons.
Introduces a spectrum of new particles which are the superpartners of SM particles.
Superpartners have the same masses (unbroken symmetry) and quantum numbers with SM particles but differ by half spin difference.
Sparticles are not observed in nature ➜ SUSY must be broken.
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The Minimal Supersymmetric SM (MSSM)
Minimal extension of the SM with minimal particle content.
Respects the same SU(3)C x SU(2)L x U(1)Y gauge symmetries as does the SM.
Assumes that the interaction between particles conserves R-parity.
R = (-1)3(B-L)+2S, which is a multiplicative quantum number with spin S, baryon number B, and lepton number L
All the superpartners are created in pairs.
The lightest supersymetric particle (LSP) is stable and weakly interacts with particle.
LSP is a candidate for the cold dark matter in the universe.
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Experimental Signature
Multijet events with large missing momentum is the most generic experimental signature for R-parity conserving SUSY. Long cascade decays of
sparticles ⇒ multijets
LSP will escape the detector ⇒ missing momentum
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INCLUSIVE SEARCH FOR NEW PHYSICS AT CMS WITH JETS AND MISSING MOMENTUM SIGNATURE
Motivation
Supersymmetry
Analysis
Data Driven Background Estimations
Result and Interpretation
Conclusion
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Inclusive Search for New Physics
Multijet events with a large missing transverse momentum (MHT) search for 36 pb−1 of pp collision data collected with the CMS detector from 2010 March to 2010 November.
The event selection starts with a loose requirement (baseline selection). Later on tighter requirements are applied in order to define search selections.
The variables used in this analysis
MHT: magnitude of the negative vectorial sum of the transverse momenta of the jets with pT > 30 GeV and |η| < 5.0
HT: scalar sum of the transverse momenta of the jets with pT > 50 GeV and |η| < 2.5.
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The SM Backgrounds
The largest SM backgrounds for multijet and large MHT analysis are coming from Z(→ )+jets
W+jets and ttbar
QCD multijet events with large missing momentum from leptonic decay of jets
There are also contributions due to jet mismeasurements, and noise or dead components from the detector
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Simulations
Monte Carlo (MC) simulated background and signal samples produced with a detailed Geant4 CMS detector simulations are used in this analysis.
The predicted cross sections for MC samples are normalized to next-to-leading (NLO) or next-to-next-to-leading-order (NNLO) cross sections when available and the event yield is normalized to total integrated luminosity of 36 pb−1.
Background Samples
The QCD multijet, ttbar, W, Z, γ+jets, dibosons and single top samples are generated with PYTHIA and MADGRAPH generators.
Signal Sample
The LM1 (CMS low mass SUSY point), with constrained MSSM (CMSSM) parameters m0 = 60 GeV, m1/2= 250 GeV, A0 =0, tanβ =10, and sign(μ) > 0, is used as our benchmark point.
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Event Selection
Data used in this analysis are collected with HT trigger, defined as the scalar sum of the transverse momenta of the calorimeter jets with pT > 20 GeV.
At least 3 jets with pT > 50 GeV and |η| < 2.5.
|∆φ(jet1,2,[MHT)| > 0.5 rad and |∆φ(jet3,[MHT)| > 0.3 rad (to reduce QCD background).
Veto on isolated muons and electrons (to reduce EWK background).
HT > 300 GeV and MHT > 150 GeV (baseline selections)
Search Selections
MHT > 250 GeV (High MHT), motivated by the search for generic dark matter candidate coupled with high background rejection.
HT > 500 GeV (High HT), sensitive to higher object multiplicities like SUSY cascade decays.
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MHT and HT in Data and MC
MHT HT
BeforeMHT > 150
GeVcut
AfterMHT > 150
GeVcut
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Event Yield in Data and MC
Event yield after trigger selection, cleaning and event selections are
The simulations are only used for verification.
All the SM backgrounds in this analysis are estimated by using data-driven methods.
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INCLUSIVE SEARCH FOR NEW PHYSICS AT CMS WITH JETS AND MISSING MOMENTUM SIGNATURE
Motivation
Supersymmetry
Analysis
Data Driven Background Estimations
Result and Interpretation
Conclusion
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Data Driven Background Estimations
Z( ) + jets
W and ttbar
QCD
Rebalance and Smear Method (R&S)
Jet Resolution Measurements
γ + jet pT balance
• Gaussian Measurements
• Method
• Uncertainty Measurements
• non-Gaussian Measurements
Combination with dijets
Results of R&S Method
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Z( )+jets Background Estimation
Irreducible background to jet + MHT search.
Estimated by using γ+jet sample at high pT.
Similar electroweak correspondence between Z boson and γ.
σ(Z→ +jets)/σ(γ+jet) provides a good handle to estimate MHT spectrum.
Only direct photons are related to Z production.
Contribution from fragmentation photons and isolated neutral pions and η mesons are background
Number of predicted events for Z→ +jets are calculated by multiplying the number of γ+jets events with Z/γ correction factors.
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W and ttbar Background Estimation
The W+jets and ttbar events are not rejected by the lepton veto if
a lepton from W or top-quark decay is
outside the geometric or kinematic acceptance
not reconstructed
not isolated
a tau lepton decays hadronically (τh).
The sum of lost-lepton and τh W+jets and ttbar.
lost lepton
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The W/ttbar ➞ e,μ+X (Lost Lepton)
Events where a W boson decays leptonically and not rejected by the lepton veto.
To estimate the number of events in the signal region a control sample with exactly one well identified and well isolated muon is used and corrected for non-isolated, non-identified and not-accepted leptons.
control region:isolated, identified
and accepted leptons
signal region:non-identified leptons
signal region:non-isolated
leptons
signal region:not-accepted leptons
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The W/ttbar ➞ τh + X
A control sample with exactly one muon with pT > 20 GeV and |η| < 2.1 is used to estimate the hadronic tau background.
Each muon in the control sample is replaced by a tau jet.
The control sample is corrected for kinematic and geometric acceptance of the muons
muon trigger, reconstruction and isolation efficiencies
relative branching fractions of W decays into muons or hadronic tau jets.
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QCD Background Estimation
Due to its high production cross-section QCD multijet events are the largest background to jets + MHT final state.
A well balanced QCD multijet event at parton level can be reconstructed with a significant missing energy due to
large fluctuations in the calorimeter response to the energy of jets
the semi-leptonic decays of heavy flavor quarks
dead or malfunction channels
These effects manifest themselves as deviation from Gaussian nature of jet resolutions.
It is very important to measure the complete response functions of jets (Gaussian core + non-Gaussian tails) to be able to estimate the QCD background to jets+MHT final state from the data.
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Rebalance & Smear Method
missing momentum
Jet1
Jet2
Jet3
Rebalance:Response function
(Gaussian core)
Seed Events:A good estimation for the true
well balanced QCD events (MHT ~ 0).
Jet2
Jet1
Jet3
missing
momentum
Smear:Response function
(Full response)
Inclusive QCD Prediction:Smeared seed events with high missing momentum.
Response functions are measured by two different approaches based on pT balance.
lower pT region: γ+jet pT balance method
higher pT region: dijet asymmetry method
Measured resolutions are used to derive Data/MC correction factors depending on jet pT and jet η.
Jet1
Jet3
Jet2
missing
momentum
Reconstructed Events:QCD+ttbar+w+Z+...+BSM
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γ+Jet pT Balance Technique
CMS reconstruct photons with an excellent energy resolution (∼ 1%).
σ(pTJet/pT
γ) a good estimator of the jet pT resolution.
intrinsic(MC truth) imbalance
Varies as a function of SecondJetPt
independent from the extra jet activity in the
event.
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Dataset and Event Selections
γ+jet resolution measurements are based on 36.1 pb−1 of pp collision data collected during 2010.
γ+jet data is collected with different trigger paths based on γ pT.
Events with well identified photons are used in this analysis.
HCAL, ECAL, tracker isolations, and shower shape requirements are applied to the data to suppress QCD background.
Events with a track seed in the pixel detector are vetoed to discriminate photons from electrons
leading jetγ
pT > 10 GeV, |η| < 1.3
➤
|Δϕ(γ,Jet)|> 2.7 resolutions are measured in resolutions are measured in different different ηη and p and pTT bins depending bins depending
on on γγ p pTT and leading jet and leading jet η η in the in the events.events.
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Measurement of Jet Resolutions: γ+Jet
To study additional jet activity resolutions are measured in the fraction of pT
Jet2/pTγ.
Within the statistics MC well
predicts data.
Reducing secondary jet
activity narrows measured
distributions.
Intrinsic resolution is independent of
any additional activity in an
event.
component of jet resolution as a function of pT
Jet2 / pTγ
Resolutions are measured both in data and MC for
back-to-back γ+jet events .
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Method to Derive Data/MC Ratio
Intrinsic resolutions are calculated both for data and MC.
Assumption : imbalance component in data is same as MC and subtract this same component from measured resolutions in quadrature for various bins of pT
Jet2 / pTγ .
measured resolutions intrinsic resolutions Data/MC ratio
intrinsic resolution is expected to be flat in MC and therefore is fitted with a zero degree polynomial before Data/MC ratio is
calculated.
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Data/MC Ratio for Intrinsic Resolutions
The measured ratio is consistent with being independent of pT and is fitted with a zeroth degree polynomial.
•To complete resolution measurements for Gaussian core, the following systematics uncertainties on the measured Data/MC ratio were studied; Variation of the extrapolation fit range.• Effect of |Δϕ(γ,Jet)| requirement• Uncertainty of jet energy corrections.• Particle level imbalance • Flavor difference between γ+jet and dijet events • Pileup subtraction.
For each item the Data/MC ratio is recalculated and the relative difference
between the new and the nominal ratio is used in systematical uncertainty
assignments.
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Variation of Extrapolation Fit Range
Data/MC ratio for intrinsic resolutions is recalculated for the two fit regions and relative difference between two ratio are used to define uncertainty.
0.0 <|η|< 1.1
1.1 <|η|< 1.7
1.7 <|η|< 2.3
2.3 <|η|< 3.5
Higher ExtrapolationRange 1.079 1.108 1.076 1.184
ΔR (uncertainty up) 0.009 0.003 0.01 0.002
Lower ExtrapolationRange 1.065 1.095 1.061 1.169
ΔR (uncertainty down) 0.005 0.007 0.005 0.0129
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Effect of |Δϕ(γ,Jet)| Requirement
Resolutions are measured for back-to-back γ+jet events with |Δϕ(γ,Jet)|> 2.7 rad.
To study the effect of this requirement resolutions are measured in MC for two different Δϕ selections (2.1 and 2.7 rad) and their ratio is calculated.
Since the deviation from 1 on the measured MC(1)/MC(2) ratio is less than 1% the effect of ∆φ requirement is not
taken into account in systematical error calculations.
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Uncertainty of Jet Energy Corrections
To determine the systematical uncertainty due to JEC, the jet energy corrections applied on the reconstructed jets are varied by up and down with the official uncertainties provided by CMS JEC group.
0.0 <|η|< 1.1
1.1 <|η|< 1.7
1.7 <|η|< 2.3
2.3 <|η|< 3.5
JEC up 1.080 1.106 1.104 1.189
ΔR (uncertainty up) 0.01 0.004 0.038 0.006
JEC down 1.044 1.078 1.042 1.123
ΔR (uncertainty down) 0.026 0.024 0.024 0.06
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Particle Level Imbalance
MC imbalance resolution is used to calculate intrinsic resolutions in data (➩ assuming MC closely describes data).
Various sources can lead to different imbalance component in data and MC
1. treatment of multi-parton final states: Different MC event generators (PYTHIA, HERWIG, MADGRAPH) are used.
2. modeling of hadronization: The hadronization parameter in PYTHIA is turned on and off.
3. modeling of kT kick: 1 GeV smearing of second particle jet pT is studied.
0.0 <|η|< 1.1
1.1 <|η|< 1.7
1.7 <|η|< 2.3
2.3 <|η|< 3.5
Multi-parton Final State 0.017 0.03 0.064 0.001
Hadronization 0.009 0.005 0.023 0.038
Modeling of kT Kick 0.013 0.014 0.021 0.024
Assigned Uncertainty ±2% ±3% ±4% ±4%
Relative changes on the measured Data/MC ratio
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Flavor Difference Between γ+jet and Dijet Events
Intrinsic resolutions are measured in γ+jet and QCD MC samples and their ratio is calculated.
For the 20 - 300 GeV pT region where the resolutions are measured from γ+jet
sample, the intrinsic resolutions differ by ∼ 3%.
If MC is wrong by 30% (conservative assumption) this ratio can vary from 2-4%
Since the final measurement is Data/MC ratio, common biases and systematics
cancel and residual uncertainty becomes ~1%.
This systematic is included on measured Data/MC ratio when γ+jet and dijet results
are compared.Photo
nJe
t /
QC
D
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Pileup Subtraction
To study the effect of additional large number of soft pp collisions on measured Data/MC ratio
Data/MC ratio is recalculated with a data sample where jets are additionally corrected for pileup.
Since the MC samples were produce without pileup effect the jets in MC sample are not corrected for pileup.
without pileup correction with pileup correctionAssigned
Uncertainty
0.0 <|η|< 1.1 ±4%
1.1 <|η|< 1.7 ±2%
1.7 <|η|< 2.3 ±2%
2.3 <|η|< 3.5 ±5%
Since the data sets used in this analysis are not corrected for additional pileup, the assigned uncertainties are not included in the final Data/MC ratio.
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Measurement of Jet Resolutions: γ+Jet
Contributions from extrapolation, JEC and particle level imbalance are accepted to be uncorrelated and final systematical uncertainty is calculated as the quadrature sum of the individual components.
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Estimation of non-Gaussian Component
The resolutions in the MC sample are stretched according to the measured Data/MC ratio.
Resolutions measured in data and MC are compared, and the number of events outside the 2.5 σ range is counted in the bins of γ pT to compare resolution tails.
Not enough statistics to measure the resolution tails with precision
A constant fit to the ratio is consistent within the errors with the study using dijet sample, and provides a cross check.
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Combined Measured Resolutions
Dijet asymmetry is measured as (pTJet1 - pT
Jet2)/(pTJet1 + pT
Jet2) as a function of jet η and jet pT.
An extrapolation to no second jet activity is performed to eliminate secondary jet activities.
Data/MC ratio measured in γ+jet and dijet samples.
Combined Data/MC ratio in various η ranges.
Data/MC correction factor for resolution tails.
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Results of R&S Method
The measured Data/MC ratios from data driven jet resolution measurements are used to correct the MC truth resolution.
Corrected MC truth resolutions are used to smear the jets in the seed sample.
Smeared seed events are used
to estimate kinematic distributions (MHT and HT) of QCD sample
to predict the number QCD background after event selections
R&S method on MC
R&S method on data
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INCLUSIVE SEARCH FOR NEW PHYSICS AT CMS WITH JETS AND MISSING MOMENTUM SIGNATURE
Motivation
Supersymmetry
Analysis
Data Driven Background Estimations
Result and Interpretation
Conclusion
56
Results
No excess is observed over predicted number of events ➡ Limit
Settings
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Within the CMSSM
tanβ=10, μ>0, A0 =0
Depending on squark and gluino mass a 95% CL upper limit on the production Depending on squark and gluino mass a 95% CL upper limit on the production cross section is obtained for 2-3 pb range. cross section is obtained for 2-3 pb range.
Gluino masses below 500 GeV are excluded at 95% CL for squarks with mass Gluino masses below 500 GeV are excluded at 95% CL for squarks with mass below 1 TeV.below 1 TeV.
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Within the Simplified Model Spectra
Simplified Models remove the complexity of the physics models with several parameters.
Characterize the experimental data in terms of a small number of basic parameters.
With simplified models, the experimental results can be translated to any desired framework.
Two benchmark simplified models are studied for jets+MHT signature.
pair produced gluinos, where each gluino decays to two light quarks and the LSP
pair produced squarks, where each squark decays to one jet and the LSP.
The simplified model samples are simulated with PYTHIA generator for a range of particle masses which are involved in the decays of chosen benchmarks.
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Within the Simplified Model Spectra
High MHT Selection
A 95% CL exclusion limits on new particle production cross A 95% CL exclusion limits on new particle production cross section for 0.5-30 pb range, depending on the masses of new section for 0.5-30 pb range, depending on the masses of new
particles.particles.
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INCLUSIVE SEARCH FOR NEW PHYSICS AT CMS WITH JETS AND MISSING MOMENTUM SIGNATURE
Motivation
Supersymmetry
Analysis
Data Driven Background Estimations
Result and Interpretation
Conclusion
61
Conclusion
A search on new physics with jets+MHT signature based on 2010 pp data collected with CMS at LHC is performed.
Data driven methods are used to measure jet energy resolutions and estimate SM backgrounds.
Data showed a good agreement with SM predictions.
No excess is observed over predicted background.
Upper limits are derived in the R-parity conserving CMSSM with A0 = 0, μ > 0, and tanβ=10.
Gluino masses below 500 GeV are excluded for squarks with mass less than 1 TeV (CMSSM).
Upper limit on 2-3 pb range production cross section.
Set limits on σxBr for simplified models.
Upper limit on 0.5-30 pb production cross section depending on the new particles’ mass.
Backup Slides
63
Simplified Models: Selection Efficiency
The efficiencies are shown as a function of gluino/squark and LSP mass.
The models are only valid when the gluino mass is greater than the LSP mass.
The signal acceptance increases with higher mass splitting.
The acceptance is low on diagonal
mass splitting is low
jets are produced with low transverse momentum.
MHT > 250 GeV, HT > 300 GeV
MHT > 250 GeV, HT > 300 GeV
MHT > 150 GeV, HT > 500 GeV
MHT > 150 GeV, HT > 500 GeV
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Simplified Models: Experimental & Theoretical Uncertainties
experimental experimental experimental experimental
theoretical theoretical theoretical theoretical
The sources of the systematic uncertainties Experimental Uncertainties: jet energy scale and resolution, the lepton veto, the
cleaning, and the trigger selection.
Theoretical Uncertainties: The initial and final state radiation, and the parton distribution functions