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Search for Large Extra Dimensions in the Diphoton Final State at the Large Hadron Collider

This dissertation defense discusses the Standard Model, hierarchy problem, Large Extra Dimensions, graviton phenomenology, virtual graviton production, direct graviton emission, and review of limits using data from the Large Hadron Collider. It explores the search for virtual graviton production in the diphoton final state at the LHC.

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Search for Large Extra Dimensions in the Diphoton Final State at the Large Hadron Collider

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  1. Search for Large Extra Dimensions in the Diphoton Final State at the Large Hadron Collider Duong Nguyen Brown University Dissertation defense April 27, 2011

  2. The Standard Model • The Standard Model (SM) describes interactions between fundamental particles • Spin-1/2 fermions: 6 quarks and 6 leptons arranged in 3 generations • Forces: strong, weak, electromagnetic and gravity (not included) • Unanswered questions in SM • Origin of 19 free parameters • Gravity is weak compared to other fundamental forces and totally neglected in the SM • No dark matter candidate • …

  3. Hierarchy Problem f f • Hierarchy problem refers to enormous difference between EWSB and Planck scale. • MPl/MEWSB ~ 1016 • Radiative correction to Higgs mass • Contains the ultra violet (UV) cutoff scale squred (Λ2) • UV cutoff is ~MPl, the Higgs mass is ~100 GeV. Thus, high degree of fine tuning is needed (10-34) to protect Higgs mass. • Some possible solutions: extra dimensions, supersymmetry, strong dynamics. ????

  4. Large Extra Dimensions • Large Extra Dimensions, ADD (Arkani-Hamed, Dimopoulos, Dvali) model [Phys. Lett. B 429 (1998) 263]. • SM is constrained in 3+1 dimensions • Gravity propagates through entire multidimensional space and its strength is diluted -> fundamental Planck scale is observed R: size of extra dimensions (EDs) nED: number of EDs MD: fundamental Plank scale MPl: gravitational scale • If MD~1 TeV

  5. Phenomenology Energy spacing 1 meV-100 MeV ADD • Graviton’s energy is quantized • Because gravitons propagate in the compact extra dimensions similar to particle-in-the-box problem. • Results in a tower of graviton excitations, referred as Kaluza-Klein (KK) modes • Small energy spacing between KK modes: ~1 meV to 100 MeVnon-resonance excess above the SM spectrum • Gravitational coupling is enhanced tremendously because of large number of excited KK modes • Gravitational strength contribution from each KK mode is GN~1/MPl2 • At 1 TeV, given R~1 fm and nED=7 as many as 1028 modes can be excited

  6. Virtual graviton production • Drell-Yan like virtual graviton production decaying to diphotons or difermions. • Non-resonant enhancement in Drell-Yan or diboson spectrum at high invariant mass • Decays of spin-2 graviton to spin-1/2 fermions is suppress. Thus, the diphoton is the dominant channel • Cross section • ED effects are parameterized by • MS is an ultraviolet (UV) cutoff to avoid UV divergence of KK modes.

  7. Direct graviton production • Gravitons are emitted and escape to the extra dimensions  appears as missing transverse energy • Results in a single jet (monojet channel) or a gauge boson associated with a large missing transverse energy. • If the gauge boson is the photon  monophoton channel • Direct graviton emissions depend directly to MD • It is suppressed by a factor

  8. Review of the Limits • 95% CL lower limits on MD • 95% CL lower limits on MS

  9. The Large Hadron Collider • LHC is the highest energy proton-proton collider: 7 TeV (designed 14 TeV), 40 MHz collision rate.

  10. LHC Performance in 2010 • Luminosity increased rapidly during the last month of data taking • Integrated luminosity in 2010 runs: • Delivered 47.03 pb-1 • Recorded 43.17 pb-1 • 36 pb-1 is used in this analysis • Peak luminosity: 2.0x1032/cm2/s

  11. CMS detector

  12. Particle Detection at CMS

  13. CMS Electromagnetic Calorimeter (ECAL) 0.5 m 2.6 m 6.4 m Endcap Dee Barrel Super Module Pb-Si Preshower 23cm 25.8Xo 22cm 24.7Xo 22cm 24.7Xo Endcap crystal, 1 type, 3x3 cm2 at rear Barrel crystal,34 types, ~2.6x2.6 cm2 at rear PbWO4 Light emission: 80% in 25 ns Radiation length: X0 = 0.89 cm Molière radius: RM = 2.10 cm Emission peak: 425nmGood radiation resistance to very high doses 23cm 25.8Xo

  14. Photon Reconstruction at CMS • Energy reconstruction: clustering energy deposits in ECAL crystals • A 5x5 crystal array contains about 96.5% (97.5%) of unconverted photon energy in EB (EE) • About 50% photons are converteduse hybrid clustering, multi5x5 clustering • EB: hybrid clustering (for converted photons) or 5x5 crystal array • EE: multi5x5 clustering • Position determination • Energy-weighted average of position of crystals

  15. Photon Identifications Isolation Cone ΔR=0.4 γ Signal Cone ΔR=0.06 • Separate photons from jets: isolations and shower shape • Had/EM: ratio of between hadronic and electromagnetic energy within • Tracking isolation (TrkIso): of track associated with the primary event vertex surrounding the photons within hollow cone of (a rectangular strip of is excluded) • ECAL isolation (EcalIso): surrounding the photons within (and excluding strip) • HCAL isolation (HcalIso): surrounding the photon within • Shower shape: the weighted width in of the shower. • Separate photons from electrons: vetoing on the existence of pixel seeds in the pixel detector

  16. Analysis Strategy Signal • Performe the search for virtual graviton production in the large extra dimension framework. • The final state is high invariant mass diphoton (about few hundreds GeV and above) • Major backgrounds are dijet, photon+jet and diphotons • Look for excesses over the SM background prediction at high invariant masses • If no excess has found, set the limit on the fundamental Planck scale • Background estimation • Photon+jet and dijet: Use data-driven method based on photon misidentification rate • Diphoton: Use Monte Carlo estimation and apply a k-factor to correct for NLO precision • Drell-Yan background is negligible • Estimate photon efficiency for limit calculation Backgrounds

  17. Kinematic optimization (I) • Identify cuts on eta of photons and invariant mass of diphotons to obtain highest sensitivity. • Eta distributions: ADD signal is central, SM diphoton is flat and QCD+photonJet is more forward->a sensitive region includes EB and possible a part of EE. • Signal is enhance at high invariant mass -> set invariant mass cut high enough for the best sensitivity.

  18. Kinematic optimization (II) • Mgg > 500 GeV and |eta| < 1.4442

  19. Event selection • Event cleaning • Primary vertex selection: NDOF > 4, d0 < 2 cm, |z| < 24 cm • Trigger • Single photon trigger used for fake rate estimation • Double photon trigger used for plotting invariant mass • Kinematic: pT > 30 GeV, |eta| < 1.4442 • PhotonID

  20. Photon Efficiency (I) • Photon reconstruction and identification efficiency estimated in MC is (90±2)%

  21. Photon Efficiency (II) • Total photo efficiency in data is (87.8±2.3)% which includes • MC efficiency above (90±2)% • A scale factor of 1.010±0.012 to correct for the different between efficiency in MC and data • Efficiency of the pixel seed veto: (96.6±0.5)% • The diphoton efficiency is (77.1±4.5)%

  22. Photon Misidentification Rate • Jets can misidentified as photons especially when they fragment into leading π0. • Defined as the ratio between number of tight photons and loose photons. • Tight photon passing photon selections • Loose photon passing looser selections than above with one of isolations is inverted • Corrected for direct photon by photon purity Red: photon sample Blue: muon sample Gren: jet sample • Systematic uncertainty is 20%

  23. QCD and Photon+jet background • Using the photon misidentification rate, one can estimate the SM backgrounds as: PhotonJet QCD • T is the tight photon and L is loose photon and x(y) refers to the pT of first and second object, respectively. Therefore: • NTTxy: number of tight tight photon with pT of x and y • NLLxy: number of loose loose photon with pT of x and y • NLTxy: number of loose photon with pT of x and tight photon with pT of y • NTLxy: number of tight photon with pT of x and loose photon with pT of y • is the number of diphoton events which is estimated from MC • The first and second parentheses correspond to QCD and photon+jet backgrounds, respectively.

  24. Background Estimation Results • Good agreement between SM background estimation and data • Observe zero event in the signal region (mgg > 500 GeV) • Systematic uncertainty • 20% and 40% for photon+jet and dijet backgrounds, respectively (from 20% uncertainty on the misidentification rate) • 22% for diphoton backgrounds (from NLO K-factor uncertainty k=1.3±0.3) • Uncertainty on the total backgrounds • Add linearity photon+jet and dijet uncertainties (they are 100% correlated) • This linearity sum is added in quadrature with diphoton uncertainty

  25. Diphoton invariant mass • Hatched band shows the systematic uncertainty on the background • No excess of event over the SM predictions

  26. Kinematic Distributions

  27. Systematic Uncertainty

  28. Limit setting • Perform limit setting for a counting experiment with > 500 GeV • Apply Bayesian procedure • Use Poisson likelihood for model density • Lognormal prior for nuisance parameter • Flat prior for cross section • 95% upper bound • Upper bound on the <0.11 pb at 95% C.L.

  29. Translating Limit (I) • Translate model-independent upper limit on S to lower limits on MS • Use SHERPA to model the large extra dimension effect • Relate S and • Derive MS limits from limits depending on conventions for F • For n=2 in HLZ convention, derive MS limits from 1/MS4

  30. Translating limits (II) • Conventions for F to derive MS limit • Limits with UV cutoff • At LHC, energy scale of hard scattering can exceeds the UV cutoff MS when MS < 7 TeV • SHERPA doesn’t take into account the non-perturbative effects above MS conservative set cross section to zero for M>MS • MS limits summary • The most stringent limits are highlighted

  31. Event display (I) Mgg = 319 GeV

  32. Event display (II) Mgg = 432 GeV

  33. Conclusion • We performed the search for LEDs in the diphoton channel at the LHC with 36 pb-1. We found no excess of events over the SM prediction. • We set the limits on ultra violet cutoff MS in the range of 1.6-2.3 TeV, which extend the current limits of Tevatron except nED=2 case • We also quote the model-independent cross section which is 0.11 pb for two photon in the ECAL barrel with • The limits will be much further extended with upcoming 2011 run. Thank you

  34. Eta, phi correlation

  35. Template fit • Side-band for bkgr template • SigmaIetaEta: 2 + 0.001*pT < trkIso < 3.5 GeV • Conversion: 0.011 < sigmaIetaEta < 0.013 • Isolation sum: 0.011 < sigmaIetaEta < 0.013

  36. Drell-Yan • Electron fake rate is estimated from data. • Nee and Neg is the number of electron-electron and electron-photon in the Z mass window (85-95 GeV) • feg = (3.4+/-0.2)x10-2 • Apply feg to MC high mass DY spectrum

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