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Discovery Physics at the LHC

Discovery Physics at the LHC. Antonella De Santo Royal Holloway, University of London YETI08 Durham, January 9, 2008. Foreword – LHC’s Main Goals. Elucidate mechanism for EW symmetry breaking Search or Higgs boson in O(100 GeV)-O(1 TeV) range

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Discovery Physics at the LHC

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  1. Discovery Physics at the LHC Antonella De Santo Royal Holloway, University of London YETI08 Durham, January 9, 2008

  2. Foreword – LHC’s Main Goals • Elucidate mechanism for EW symmetry breaking • Search or Higgs boson in O(100 GeV)-O(1 TeV) range • If no light Higgs is found, study WW scattering at high mass • Look for evidence of new physics at TeV-scale • Deviations from SM predictions • SM only low-energy “effective theory” of more fundamental theory valid at higher energies Antonella De Santo, RHUL

  3. Outline • Introduction • The LHC • ATLAS and CMS • EW Symmetry Breaking • Higgs boson • Supersymmetry • mSUGRA • Beyond SUSY • Extra Dimensions Not an exhaustive list! Antonella De Santo, RHUL

  4. THE LHC

  5. Antonella De Santo, RHUL

  6. The LHC – some basic facts • LHC housed in former LEP tunnel • 27 km circumference • Proton-proton machine • Superconducting magnets • B = 8.4 T • Limiting factor for total energy • (Only 60% of circumference is magnetised) Design luminosity: “Low” = 1033 cm-2 s-1 (O(10 fb-1) / yr ) “High” = 1034 cm-2 s-1 (O(100 fb-1) / yr) 25 ns bunch crossing  40 MHz crossing frequency 2835 / 3564 “full” bunches Antonella De Santo, RHUL

  7. Cross Sections Minimum Bias rate: O(109 Hz) !! 14 TeV “Needle in haystack” Antonella De Santo, RHUL

  8. Minimum Bias sinel(pp)~70 mb Non-single diffractive interactions (soft partons) “Operational” definition of “minimum bias” depends on experimental trigger • Measured at lower energies (SppS and Tevatron) • Large uncertainties on extrapolation to LHC energies LHC predictions CDF UA5 Antonella De Santo, RHUL

  9. Minimum Bias and Pile-up • At nominal high luminosity (L=1034cm-2s-1=107mb-1 Hz), on average 23 minimum bias events superimposed on any rare discovery signal • And ~1000 low-pt tracks per event ! • Moreover, due to finite detector response time, out-of-time pile-upfrom different bunch crossings • Event must be “time stamped” • Important impact on detector design • Fast electronics, high granularity, radiation hardness Antonella De Santo, RHUL

  10. H 4m The Challenge Rec. trks w/ pt>25 GeV Antonella De Santo, RHUL

  11. H 4m H 4m +30 Minimum Bias The Challenge Rec. trks w/ pt>25 GeV Antonella De Santo, RHUL

  12. x1p x2p p p p p u u u u d d Collisions at a hadron collider Etot=? Ptot=? No constraint on total initial energy Broad range of √s  good for discovery (All possible processes are “on” simultaneously) Typically x1≠x2 hard scatter boosted along beam direction PDFs Antonella De Santo, RHUL

  13. h = 0 h = -1.0 h = +1.0 h = -2.5 h = +2.5 beam axis beam axis Event kinematics – pT and h • Equivalent to rapidity for massless particles • Invariant under boost Instead of polar angle q : q Pseudo-rapidity Antonella De Santo, RHUL

  14. Event kinematics – Etmiss • Assuming that the partons in the initial state move parallel to the proton beams, and that the total pT of the unseen proton remnants (lost along the beam pipe) is ~0 : Total missing pT approximated by vector sum of all energy deposits in calorimeters (Etmiss) Antonella De Santo, RHUL

  15. Search Strategy for Rare Processes • High-pT physics largely dominated by QCD processes, while interesting physics has EW cross sections • Purely hadronic channels essentially hopeless • Must rely on distinctive final state signatures • leptons (e, mu and taus) • photons • b-jets • missing ET Further signal suppression from branching ratios Antonella De Santo, RHUL

  16. Detector Requirements • Excellent position and momentum resolution in central tracker • b-jets, taus • Excellent ECAL performance • electrons, photons • v. good granularity (energy and position measurements) • Good HCAL performance • jets, Etmiss (neutrinos, SUSY stable LSP, etc) • good granularity (energy and position measurements) • good h coverage (hermeticity for Etmiss measurements) • Excellent muon identification and momentum resolution • from “combined” muons in external spectrometer + central tracker Antonella De Santo, RHUL

  17. The Detectors CMS Muon System Calorimetry Magnet system Tracking ATLAS Antonella De Santo, RHUL

  18. ATLAS vs CMS Antonella De Santo, RHUL

  19. Detectors Detectors Lvl-1 Front end pipelines Lvl-1 Front end pipelines Readout buffers Readout buffers Lvl-2 Switching network Switching network Lvl-3 HLT Processor farms Processor farms Overview of the CMS and ATLAS Triggers 40 MHz input rate (bunch crossing) L1 L1 O(100 kHz) L1 rate L2 HLT O(1 kHz) L2 rate (ATLAS only) HLT EF O(100 Hz) output rate (on tape) CMS – 2 levels ATLAS – 3 levels ( L2 + EF = HLT ) L1: Hardware based (calo+m’s) HLT: Software based Event size 1-2 MBytes (HLT = High-Level Trigger) Antonella De Santo, RHUL

  20. Detectors Detectors Lvl-1 Front end pipelines Lvl-1 Front end pipelines Readout buffers Readout buffers Lvl-2 Switching network Switching network Lvl-3 HLT Processor farms Processor farms Overview of the CMS and ATLAS Triggers 40 MHz input rate (bunch crossing) L1 L1 Regions of Interest (RoIs) O(100 kHz) L1 rate L2 HLT O(1 kHz) L2 rate (ATLAS only) HLT EF O(100 Hz) output rate (on tape) CMS – 2 levels ATLAS – 3 levels ( L2 + EF = HLT ) L1: Hardware based (calo+m’s) HLT: Software based Event size 1-2 MBytes (HLT = High-Level Trigger) Antonella De Santo, RHUL

  21. ATLAS Antonella De Santo, RHUL

  22. CMS Antonella De Santo, RHUL

  23. ATLAS CMS CERN, Building 40 Antonella De Santo, RHUL

  24. EW SYMMETRY BREAKING AND HIGGS BOSON

  25. Constraints on Higgs Mass Direct LEP limit : MH >114.4 GeV (95% C.L.) mlimit = 144 GeV no perturbative unitarity unstable vacuum From http://lepewwg.web.cern.ch/LEPEWWG/ Antonella De Santo, RHUL

  26. SM Higgs Couplings • Different Feynman rules for fermions and gauge bosons, but in all cases • Higgs couplings proportional to mass Antonella De Santo, RHUL

  27. SM Higgs Production at the LHC Gluon Fusion Vector Boson Fusion Higgs-strahlung ttbar H (“associated” production) Antonella De Santo, RHUL

  28. SM Higgs Production Cross Sections Antonella De Santo, RHUL

  29. Standard Model Higgs Decays • Heaviest quark (b-bbar) BR dominates at low masses, until there is enough energy to produce gauge boson pairs (WW, ZZ) • But b-bbar very difficult due to huge QCD background and limited b-jet resolution (O(15%)) • Despite much lower BR, Hgg via intermediate ttbar loop has better Signal/Bkgd ratio for low Higgs mass Antonella De Santo, RHUL

  30. 100 fb-1 CMS Physics TDR, 2006 Low Mass Higgs (MH<140 GeV) – H gg • Direct Higgs coupling to gg forbidden, as photon is mass less • Low branching ratio (~10-3), but nice mass peak thanks to excellent ECAL energy resolution (both ATLAS and CMS) S/B 1:20 Resolution ~1 GeV at 100 GeV Antonella De Santo, RHUL

  31. _ q q γ γ π0 gluon H gg – Backgrounds • Irreducible (from physics) • – Intrinsically v. similar to signal Reducible (can be suppressed in principle) – e.g. increased calo segmentation Antonella De Santo, RHUL

  32. e+ e- Z q _ H q e+ Z e- High Mass Higgs (MH>2MZ) – H  4 lep HZZ*l+l– l+l– (l=e,m) Very little background (ZZ, Zbb, t-ttbar) CMS Resolution <1 GeV at 100 GeV Antonella De Santo, RHUL

  33. Vector Boson Fusion Tagging Forward Jets Antonella De Santo, RHUL

  34. 4s for 30 fb-1 CMS, Htt Vector Boson Fusion • Two tagging forward jets • Higgs decay products in central region • Lep-lep or lep-had signature for Htt • High-pT jet veto m- J2 J1 e- H tt  emnn Antonella De Santo, RHUL

  35. SM Higgs – Discovery Potential Significance vs. MH CMS, 30 fb-1 ATLAS, 30 fb-1 Antonella De Santo, RHUL

  36. What If No Higgs?... • If no Higgs boson is found at the TeV scale, new mechanism is required to avoid divergencies at high energy • Study of the WW cross section may hold the key • in this case, as WW scattering becomes strong • at the TeV scale in the absence of a light Higgs Antonella De Santo, RHUL

  37. What If No Higgs?... • If no Higgs boson is found at the TeV scale, new mechanism is required to avoid divergencies at high energy • Study of the WW cross section may hold the key • in this case, as WW scattering becomes strong • at the TeV scale in the absence of a light Higgs Not Today Antonella De Santo, RHUL

  38. SUPERSYMMETRY

  39. Why go Beyond the Standard Model? • Despite its many successes, Standard Model is widely believed to be only an effective theory, valid up to a scale L << MPlanck • Gravity not included in SM • Hierarchy/naturalness problem: • MEW << MPlanck • Fine-tuning • Unification of couplings • Need a more fundamental theory of which SM is only a low-energy approximation Antonella De Santo, RHUL

  40. Supersymmetry (SUSY) • Space-time symmetry that relates fermions (matter) and bosons (interactions) • Further doubling of the particle spectrum • Every SM field has a “superpartner” with same mass • Spin differs by 1/2 between SUSY and SM partners • Identical gauge numbers • Identical couplings • Superpartners have not been observed • SUSY must be a broken symmetry • But SUSY-breaking terms in Lagrangian must not re-introduce quadratic divergences in theory ! Antonella De Santo, RHUL

  41. Minimal Supersymmetric Standard Model Antonella De Santo, RHUL

  42. R-parity • This can be prevented by introducing a new symmetry in the theory, called R-parity: MSSM contains L- ad B-violating terms, which could in principle allow proton decay via sparticle diagrams: • R=+1 (SM particles), R=-1 (SUSY particles) • Two important consequences: • LSP (=Lightest SUSY Particle) is stable – typically neutralino • Sparticles can only be produced in pairs (in scattering of SM particles) Antonella De Santo, RHUL

  43. R-parity In the following, assume R-parity conservation Stable LSP (neutralino) Antonella De Santo, RHUL

  44. WMAP Neutralino as Dark Matter Constituent • Neutralino LSP is a good DM candidate • stable • electrically neutral • weakly and gravitationally interacting 0.094 < WCDMh2 < 0.136 (95% CL) Antonella De Santo, RHUL

  45. mSUGRA (or CMSSM) • Soft SUSY breaking mediated by gravitational interaction at GUT scale • Only five parameters: • m0—universal scalar mass • m1/2—universal gaugino mass • A0—trilinear soft breaking parameter at GUT scale • tanb—ratio of Higgs vevs • sgn(m)—sign of SUSY Higgs mass term • (|m|determined by EW symmetry breaking) • Useful framework to provide benchmark scenarios Antonella De Santo, RHUL

  46. gluino, squarks charginos, neutralinos, sleptons RGE Evolution • Universal boundary conditions at some high scale (GUT) • Evolution down to EW scale through Renormalisation Group Equations (RGE) Universal gaugino mass Universal scalar mass Antonella De Santo, RHUL

  47. Four regions compatible with WMAP value for Wh2, different mechanisms for neutralino annihilation: 3000 (GeV) 100 250 350 (GeV) mSUGRA Parameter Space Antonella De Santo, RHUL

  48. Four regions compatible with WMAP value for Wh2, different mechanisms for neutralino annihilation: 3000 (GeV) 100 250 350 (GeV) mSUGRA Parameter Space • bulk • neutralino mostly bino, annihilation to ff via sfermion exchange Antonella De Santo, RHUL

  49. Four regions compatible with WMAP value for Wh2, different mechanisms for neutralino annihilation: 3000 (GeV) 100 250 350 (GeV) mSUGRA Parameter Space • bulk • neutralino mostly bino, annihilation to ff via sfermion exchange • focus point • neutralino has strong higgsino component, annihilation to WW, ZZ Antonella De Santo, RHUL

  50. Four regions compatible with WMAP value for Wh2, different mechanisms for neutralino annihilation: 3000 (GeV) 100 250 350 (GeV) mSUGRA Parameter Space • bulk • neutralino mostly bino, annihilation to ff via sfermion exchange • focus point • neutralino has strong higgsino component, annihilation to WW, ZZ • co-annihilation • pure bino, small NLSP-LSP mass difference, typically coannihilation with stau Antonella De Santo, RHUL

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