Elif Aslı Albayrak Ph.D. Thesis Defense October 7 th , 2011

1. 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 Albayrak • Ph.D. Thesis Defense • October 7th, 2011

2. 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

3. LHC and CMS Experiment

4. 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 • AToroidal LHC ApparatuS (ATLAS) • Compact Muon Solenoid (CMS) • The Large Hadron Collider Beauty Experiment (LHC-b) • ALarge 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.

5. The Compact Muon Solenoid (CMS) • The CMS is designed to discover Higgs particle and new physics beyond the Standard Model (SM). Total weight : 12500 T Overall diameter : 15.0 m Overall length : 21.5 m Magnetic field : 4 Tesla

6. 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.

7. 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.

8. R&D STUDIES FOR HE UPGRADE AT CMS

9. 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.

10. 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.

11. 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.

12. Selection of Wavelength Shifter • 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. Plain quartz plate Fermilab Thin Film Laboratory WLS evaporation setup Fermilab Thin Film Laboratory ZnO:Ga sputtering system and guns

13. 2 μm pTp deposited quartz plate 0.2 μm ZnO:Ga deposited quartz plate plain quartz plate 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) Both pTp and ZnO:Ga enhance the light collection by at least a factor of 4.

14. 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.

15. 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

16. 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.

17. Hadronic Capabilities Detector linearity Longitudinal Shower Profile Hadronic Energy Resolution Data Geant4 simulations Data Geant4 simulations A good agreement between data and simulation. Solid line -> simulation Points -> data 1% hadronic response linearity • 1% hadronic response linearity. • 15% hadronic resolution at 350 GeV pion beam. • On a bigger scale it can reach up to current HE resolution. • 8% at 300 GeV pion beam.

18. Electromagnetic Capabilities Electromagnetic Energy Resolution Detector linearity Longitudinal Shower Profile Data Geant4 simulations Data Geant4 simulations A good agreement between data and simulation. Solid line -> simulation Points -> data 3% em response linearity • 3% electromagnetic response linearity. • Above 120 GeV both simulation and data converge to 5.6% • It can be used as a radiation hard EM calorimeter for future colliders.

19. 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.

20. Motivation Supersymmetry Analysis Data Driven Background Estimations Result and Interpretation Conclusion INCLUSIVE SEARCH FOR NEW PHYSICS AT CMS WITH JETS AND MISSING MOMENTUM SIGNATURE

21. 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

22. SUSY Particles Standard Model Particles Sfermions Spin 1 Fermions Spin 1/2 Sbosons Spin 1/2 Bosons Spin 0,1 Squarks Quarks Force Carriers SUSY Force Carriers Sleptons Leptons The Supersymmetry (SUSY) • 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.

23. 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.

24. 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

25. Motivation Supersymmetry Analysis Data Driven Background Estimations Result and Interpretation Conclusion INCLUSIVE SEARCH FOR NEW PHYSICS AT CMS WITH JETS AND MISSING MOMENTUM SIGNATURE

26. 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.

27. 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

28. 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.

29. 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.

30. MHT and HT in Data and MC

31. 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.

32. Motivation Supersymmetry Analysis Data Driven Background Estimations Result and Interpretation Conclusion INCLUSIVE SEARCH FOR NEW PHYSICS AT CMS WITH JETS AND MISSING MOMENTUM SIGNATURE

33. 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

34. 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.

35. lost lepton 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.

36. signal region: non-isolated leptons control region: isolated, identified and accepted leptons signal region: not-accepted leptons signal region: non-identified leptons 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.

37. 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 taubackground. • Each muon in the control sample is replaced by a taujet. • 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.

38. 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.

39. Rebalance: Response function (Gaussian core) Smear: Response function (Full response) Jet3 Jet1 Jet2 missing momentum missing momentum missing momentum Jet3 Jet2 Jet3 Jet1 Jet1 Jet2 Rebalance & Smear Method Reconstructed Events: QCD+ttbar+w+Z+...+BSM Seed Events: A good estimation for the true well balanced QCD events (MHT ~ 0). 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.

40. intrinsic (MC truth) imbalance γ+Jet pT Balance Technique • CMS reconstruct photons with an excellent energy resolution (∼ 1%). • σ(pTJet/pTγ) a good estimator of the jet pT resolution. Varies as a function of SecondJetPt independent from the extra jet activity in the event.

41. |Δϕ(γ,Jet)|> 2.7 γ leading jet ➤ pT > 10 GeV, |η| < 1.3 Dataset and Event Selections • γ+jet resolution measurements are based on 36.1 pb−1 of pp collision data collected during 2010. • γ+jetdata is collected withdifferent 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 resolutions are measured in different η and pT bins depending on γ pT and leading jet η in the events.

42. Measurement of Jet Resolutions: γ+Jet Resolutions are measured both in data and MC for back-to-back γ+jet events . To study additional jet activity resolutions are measured in the fraction of pTJet2/pTγ. component of jet resolution as a function of pTJet2 / pTγ Intrinsic resolution is independent of any additional activity in an event. Within the statistics MC well predicts data. Reducing secondary jet activity narrows measured distributions.

43. 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 pTJet2 / pTγ . measured resolutions Data/MC ratio intrinsic resolutions intrinsic resolution is expected to be flat in MC and therefore is fitted with a zero degree polynomial before Data/MC ratio is calculated.

44. 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.

45. 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.

46. 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.

47. 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.

48. 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 • treatment of multi-parton final states: Different MC event generators (PYTHIA, HERWIG, MADGRAPH) are used. • modeling of hadronization: The hadronization parameter in PYTHIA is turned on and off. • modeling of kT kick: 1 GeV smearing of second particle jet pT is studied. Relative changes on the measured Data/MC ratio

49. 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. PhotonJet / QCD

50. 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. 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. without pileup correction with pileup correction