Same sign dilepton events with jets and large missing transverse energy at the LHC with CMS

1. Same sign dilepton events with jets and large missing transverse energyat the LHC with CMS Marc Weinberg University of Wisconsin Preliminary Examination Marc Weinberg, University of Wisconsin

2. Outline of talk • Introduction • The LHC and CMS • Kinematics and reconstruction • Searching for supersymmetry (SUSY) • SUSY signals in CMS • Summary and plans Marc Weinberg, University of Wisconsin

3. The Standard Model (SM) • Constituents of matter • Quarks and leptons • All fermions • Force-mediating particles • Photon, W and Z, gluons • All bosons • Higgs boson • Gives mass to SM particles • Not yet discovered • Tests and predictions • Predicted: W and Z, gluons, top and charm quarks • EW sector of SM tested precisely Marc Weinberg, University of Wisconsin

4. Problems with the SM • Dark matter • All SM particles excluded as dark matter candidates! • Hierarchy problem • From mW, mZ: • From loop-order: • Quadratically divergent correction! Red = baryonic matter (from X-rays) Blue = total mass (from grav lensing) Marc Weinberg, University of Wisconsin

5. Supersymmetry • Central idea: each boson paired with a fermion, each fermion paired with a boson • Every SM particle has “superpartner” yet to be discovered • SUSY must be broken symmetry • R-parity: multiplicative quantum number • Devised to explain stability of proton • Also: • Neutralinos: ; mixtures of neutral gauginos • Charginos: ; mixtures of charged gauginos Marc Weinberg, University of Wisconsin

6. How SUSY helps the SM • Predicts TeV scale superpartners • Solves hierarchy problem: makes EW scale Higgs reasonable • Fermion loop gives negative sign • All quadratically divergent terms cancel! • R-parity • Conserved in many models: forces stable lightest supersymmetric particle (LSP) • Neutral LSP is dark matter candidate! • Precise gauge coupling unification • Unification approximate in SM Marc Weinberg, University of Wisconsin

7. Searching for SUSY • Missing ET • R-parity conserving models: LSPs pair produced • Stable LSPs carry energy out • Jets • Decays of colored superpartners • production expected to be dominant • Leptons • Can be same sign • Decays of • Produced in later stages of decay chain Marc Weinberg, University of Wisconsin

8. Why same sign dileptons? • SM ss dilepton backgrounds are small • QCD; heavy flavor production, neutral B mixing • Top production; semi-leptonic t and b decays • Electroweak single boson + jets • Hadron in jet fakes electron or decays into muon • Electroweak diboson + jets • SUSY sources of same sign dileptons • Gluino-gluino: Majorana particle—equal probability of positive/negative charged lepton in decay • Squark-squark: charge correlated with proton valence quarks • Other superpartner pairs: gluino-squark, chargino-squark, etc. • Other non-SM sources of same sign dileptons • Little Higgs models • Universal extra dimensions (UED) Marc Weinberg, University of Wisconsin

9. How do we look for new physics? • The Large Hadron Collider • 27 kilometer ring near Geneva, Switzerland • Proton-proton collisions • 7 TeV / beam Marc Weinberg, University of Wisconsin

10. LHC collisions • 7x higher than Tevatron • Search for new massive • particles up to m ~ 5 TeV • 102x higher than Tevatron • Search for rare processes with • small σ (N = Lσ) Marc Weinberg, University of Wisconsin

11. Experiments at the LHC Marc Weinberg, University of Wisconsin

12. Compact Muon Solenoid Marc Weinberg, University of Wisconsin

13. Size of CMS Marc Weinberg, University of Wisconsin

14. CMS magnet • High magnetic field in tracker: axial magnetic field of 4T • Field bends charged particles • Tracking resolution depends on • Put EM calorimeter and hadronic calorimeter inside solenoid • Largest solenoid on Earth: 6 m diameter, stores 2.5 GJ energy Marc Weinberg, University of Wisconsin

15. CMS tracker • Precise measurement of trajectories of charged particles • Coverage extends out to |η| < 2.5 • Resolution: • Silicon pixel detectors used closest to interaction region • Silicon strip detectors used in barrel and endcaps Marc Weinberg, University of Wisconsin

16. Electromagnetic calorimeter • Measures e/γ energy and position out to |η| < 3 • ~ 76,000 lead tungstate (PbWO4) crystals • Resolution: Marc Weinberg, University of Wisconsin

17. Hadronic calorimeter • Samples showers to measure their energy and position • Barrel / endcap region Resolution: • Brass / scintillator layers • |η| < 3 • Forward region Resolution: • Steel plates / quartz fibers • 3 < |η| < 5 Marc Weinberg, University of Wisconsin

18. Muon system • Identify muons, provide position information for track matching • Drift tube chambers in barrel out to |η| < 1.3 • Cathode strip chambers in endcaps • Wires / strips measure r / φ respectively • Coverage: 0.9 < |η| < 2.4 • Resistive plate chambers • Capture avalanche charge on metal strips • Coverage: |η| < 2.1 Marc Weinberg, University of Wisconsin

19. Particle identification at CMS Marc Weinberg, University of Wisconsin

20. CMS trigger system • Reduces 40 MHz beam crossing with 1 GHz QCD events • Level 1 (L1) trigger • Analyzes calorimeter and muon information within 3 μs • Finds leptons, photons, jets, and missing ET • Reduces rate to 100 kHz • High level trigger (HLT) • Offline-like algorithms of progressive complexity • Better identify / measure leptons, photons and jets • Reduces rate to 100 Hz Trigger rejection ~ 4 x 105 Marc Weinberg, University of Wisconsin

21. Level 1 trigger • Information from calorimeters and muon detectors • Electron / photon identification • Muon identification • Jet identification • ET, global sums • Highly complex • Trigger primitives: ~ 5,000 electronics boards of 7 types • Regional / global: 45 crates, 630 boards, 32 board types • Flexible • Most algorithms implemented in reprogrammable FPGAs Marc Weinberg, University of Wisconsin

22. e/γ identification in L1 trigger Marc Weinberg, University of Wisconsin

23. Muon identification in L1 trigger • Link local track segments into distinct 3D muon tracks • Reconstruction in η suppresses accelerator muons • Measure pT, η and φ of muon candidates from reconstructed tracks • Provides independent measurement of muon momentum Drift tubes Cathode strip chambers Marc Weinberg, University of Wisconsin

24. Simulation of SUSY • Specific SUSY model studied in analysis: minimal supergravity (mSUGRA) • Five free parameters in mSUGRA • Too many points in parameter space; need to pick one. For LM1: • m0 = 60 GeV • m1/2 = 250 GeV • tan β = 10 • A0 = 0 • sign(μ): + • High cross section: Starting where the Tevatron leaves off mSUGRA cross section Marc Weinberg, University of Wisconsin

25. SUSY study work flow Decay simulation (SDECAY 1.2) SUSY spectrum (ISASUGRA 7.75) Hadronization (PYTHIA 6.409) Detector simulation (CMSSW GEANT) Reconstruction (CMSSW RECO) Analysis (CMSSW ANALYSIS) = steps performed for this analysis = next steps Marc Weinberg, University of Wisconsin

26. Selecting same sign dileptons • mSUGRA assumptions: • Same mass: • Same couplings to other particles • Four different cases to consider: • Ordered by pTfor: • Previous studies only looked at • CMS Internal Note 2006/087 • Methodology: • Leptons ordered by decreasing pT • If exists, choose same-sign pair with largest pT • If two same-sign pairs found with opposite signs, choose lepton pair with largest scalar sum of pT Marc Weinberg, University of Wisconsin

27. Signal identification strategy Marc Weinberg, University of Wisconsin

28. Generator lepton pT Require both ss leptons pT > 10 GeV Events shown require both ss leptons pT > 5 GeV Marc Weinberg, University of Wisconsin

29. Generator lepton isolation Require next-to-leading ss lepton Iso < 6 GeV Isolation parameter: pT sum of particles within ΔR < 0.3 of lepton Require leading ss lepton Iso < 10 GeV Marc Weinberg, University of Wisconsin

30. Generator jet multiplicity All jets shown: ET > 50 GeV Reduce EW boson backgrounds Require ≥ 3 jets Marc Weinberg, University of Wisconsin

31. Generator jet ET Require next-to-next-to-leading jet ET > 55 GeV Require next-to-leading jet ET > 130 GeV Require leading jet ET > 175 GeV Marc Weinberg, University of Wisconsin

32. Generator missing ET e.g. prev. study ss 2μ QCD Require missing ET > 200 GeV Marc Weinberg, University of Wisconsin

33. Signal identification Marc Weinberg, University of Wisconsin

34. Previous study of ss dimuons only • Study: CMS Internal Note 2006/087 • Same sign dimuons only • Plot: missing ET • SM events (shaded area) and LM1 (solid line) • Done for integrated luminosity 10 fb-1 • Table: number of events passing requirements • SM backgrounds and LM1 • Note: missing ET powerful discriminator Marc Weinberg, University of Wisconsin

35. Results from previousss dimuon study CMS reach for same sign dimuons at different luminosities Marc Weinberg, University of Wisconsin

36. Summary / future plans • Same sign dilepton signal • Likely discovery channel for new physics • Significant reduction of SM backgrounds • Wide range of mSUGRA parameter points can be detected with 100 pb-1 luminosity • Future Monte Carlo studies • Produce fully reconstructed signal data • Compare with MC backgrounds • Refine selection criteria for dilepton pairs • Optimize requirements for low luminosities • Work on regional calorimeter trigger: lepton/jet identification vital to analysis • Prepare to take real data at CMS Marc Weinberg, University of Wisconsin

37. Extra slides Marc Weinberg, University of Wisconsin

38. Jet identification in L1 trigger • Jet ET • 12 x 12 trigger tower ET sums in 4 x 4 region steps with central region > others • Larger trigger towers in HF but ~ same jet region size, 1.5 η x 1.0 φ • Output • Top 4 jets in central rapidity and top 4 jets in forward rapidity max 4 x 4 region Marc Weinberg, University of Wisconsin