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Higgs Physics at the LHC

Higgs Physics at the LHC. Bruce Mellado University of Wisconsin-Madison HEP Seminar, UC San Diego, 02/07/06. Outline. Introduction Quest for the Higgs Boson The Large Hadron Collider (LHC) The ATLAS and CMS detectors The Higgs Analysis (ATLAS) Low Mass (H,)

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Higgs Physics at the LHC

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  1. Higgs Physics at the LHC Bruce Mellado University of Wisconsin-Madison HEP Seminar, UC San Diego, 02/07/06

  2. Outline • Introduction • Quest for the Higgs Boson • The Large Hadron Collider (LHC) • The ATLAS and CMS detectors • The Higgs Analysis (ATLAS) • Low Mass (H,) • Heavier Higgs (HWW(*),ZZ(*)) • Outlook and Conclusions

  3. Macroscopic Matter elements atoms Nucleus Electrons Protons & Neutrons hadrons Leptons Quarks up down strange charm bottom top electrons muons taus neutrinos Building blocks for Matter: Quarks and Leptons

  4. Standard Model of Particle Physics • Quarks and Leptons interact via the exchange of force carriers quark, lepton force carrier quark, lepton A Higgs boson is predicted and required to give mass to particles The Higgs boson has yet to be found!

  5. Higgs Discovery at LHC Higgs hunters

  6. Cross-section Particle production rate • The Large Hadron Collider, a p-p collider • Official schedule: • First collisions, summer 07 • About 100 pb-1 by end 2007 • 0(1) fb-1 by end of 2008 • 0(10) fb-1 by end of 2009 The LHC will produce heavy particles at rates orders of magnitude greater than in predecessor accelerators Start to understand accelerator & detector Almost enough data to calibrate detector Limits on SM Higgs, SUSY discovery Higgs discovery Need to reach installation rate of 25 dipoles/week

  7. 22 m Weight: 7000 t 44 m The ATLAS Detector

  8. Pixel + strip silicon tracker PbWO4 crystal ECAL Copper + scintillator sandwich HCAL -chambers 4 Tesla solenoid Very-Forward-CAL(Steel + quartz fibre) The CMS Detector

  9. ATLAS versus CMS ? ATLAS & CMS have very similar performance with some differences … • ATLAS2 X bigger due to complex muon system • ATLAS m resolution better in forward region (toroidal B-field) • CMS has better ECAL and inside solenoid  Hwidth factor of two better • ATLAS jet energy resolution 40% better(ECAL+HCAL combination better). • CMS B-field only 4 Tesla (2T in ATLAS)  Pt resolution doubles in ATLAS • ATLAS Transition Radiation Tracker  Additional electron-pion separation • CMS can do topological cuts at Level 1 trigger Very similar sensitivity to Higgs

  10. How are we going to search for the Higgs Boson?

  11. Indirect evidence is driven by radiative corrections Direct searches at LEP, e+e- collisions, (1989-2000) First Hint of Higgs boson with mass 115 GeV observed by ALEPH. LEP experiments together see about 2 effect CDF+D0 Top Quark Mass = 172.7 ± 2.9 GeV MH>114.1 GeV @ 95% C.L. MH=914532 <186 GeV @ 95% C.L.

  12. Higgs Production Cross-sections Leading Process (gg fusion) Sub-leading Process (VBF)

  13. Jet Jet SM Higgs + 2jets at the LHC • D.Zeppenfeld, D.Rainwater, et al. proposed to search for a Low Mass Higgs in association with two jets with jet veto • Central jet veto initially suggested in V.Barger, K.Cheung and T.Han in PRD 42 3052 (1990) Tagging Jets   Central Jet Veto Higgs Decay Products =-ln(tan(/2))

  14. SM Higgs + 1jet at the LHC S.Abdullinetal PL B431 (1998) for H B.Mellado, W.Quayle and Sau Lan Wu Phys.Lett.B611:60-65,2005  for H and HWW(*) • Large invariant mass of leading jet and Higgs candidate • Large PT of Higgs candidate • Leading jet is more forward than in QCD background Higgs Decay Products Tag jet  MHJ Not Tagged Tag jet  Loose Central Jet Veto (“top killer”) Quasi-central Tagging Jet =-ln(tan(/2))

  15. Main Decay Modes Close to LEP limit: H,,bb For MH>140 GeV: HWW(*),ZZ(*)

  16. H HZZ HWW H • Combination of strongest channels in terms of luminosity required for 5 observation (ATLAS) Working plots, not ATLAS official (yet) Systematic errors included Combination Low Mass Higgs Intermediate and heavy Higgs

  17. Working plots, not ATLAS official (yet) ~30 fb-1 For same detector performance TDR (1999) • Enhancement of sensitivity w.r.t. ATLAS physics TDR (1999). Need about 4 times less luminosity for discovery in the low mass region ~7 fb-1 2009 2008 Systematic errors included 2006 2007 Based on full MC simulation studies. Made possible due to huge computing effort (10M events, 10-15 cpu minutes/event): collaboration with UW Computer Science Department

  18. Strong enhancement of sensitivity w.r.t. ATLAS physics TDR (1999) due to a number of factors • Inclusion of H+1jet and H+2jet analyses in H,,WW(*) searches • Strong improvement in the HWW(*) analysis • Better understanding of electron-pion and photon-pion separation • Introduction of Object-Based method in Missing ET reconstruction  expect strong improvement in Missing ET resolution for Higgs physics • More realistic implementation of QCD Higher Order corrections in MC’s These improvements are equally applicable to CMS

  19. Low Mass Higgs: H Outstanding issues Photon resolution Photon-jet separation Splitting of phase space according to jet multiplicity Fully reconstruct Higgs kinematics

  20. Photon Resolution Converted photons are harder to reconstruct (and identify) • Aim at resolution: a constant term c<0.7% • Make use of ppZee() • Special care with converted  Unconverted  Fraction of photons converting to e+e- before reaching calorimeter for ATLAS CMS has about less conversions but more bending (4T) With converted  

  21. B A C Photon-Jet Separation • Need to achieve >103(PT>25 GeV) rejection against light jets • Make use of ppZee() and multi-jet events to optimize  identification and isolation. Optimization is very important ATLAS A jet can be observed in the detector as a single photon p   Hadronization K,0 Path C enhances signal significance by 10-20%

  22. Combined +0j/1j/2j Analysis Pre-selection Pick event if PT1>40 GeV and PT2>25 GeV +2j Analysis Pick event if JJ,MJJ>thresholds Increase of signal to background ratio +1j Analysis Pick event if PTJ,MJ>thresholds +0j Analysis Pick rest of the events

  23. SM Higgs (+ 0,1,2 Jets) • Narrow peak on top of smooth background. Use side bands to extract background under signal peak • Separation of events according to jet multiplicity maximizes sensitivity H() + 1 jet H() + 2 jets H() +0 jet 10 fb-1 30 fb-1 30 fb-1 30 fb-1 30 fb-1 Increase of signal to background ratio

  24. Combined H+0,1,2jet analyses gives very strong enhancement of the sensitivity with respect to inclusive search 5

  25. Low Mass Higgs: H Missing Energy Outstanding issues Missing ET reconstruction Lepton Identification Splitting of phase space according to jet multiplicity Missing Energy Hadronic 

  26. Collinear Approximation • In order to reconstruct the Higgs mass need to use the collinear approximation Tau decay products are collinear to tau direction Fraction of  momentum carried by lepton • x1 and x2 can be calculated if the missing ET is known • Good missing ET reconstruction is essential

  27. Object-Based Missing ET • Successfully demonstrated in ATLAS and implemented in the software the Object-based method in Missing ET reconstruction This is also crucial for SUSY searches!

  28. H(ll) TDR (1999) • Due to the Object-Based method in Missing ET reconstruction we were able to improve the Higgs mass resolution w.r.t. to Physics ATLAS TDR (1999) Object-Based Method =11.4 GeV RMS 19.8 GeV =9.6 GeV RMS 18.8 GeV M (GeV)

  29. Low Mass H()+1,2jets • Slicing of phase space enhances sensitivity • Main background: Z+jets and tt • Use Zee, and b-tagged tt as control samples H(ll) +2jets H(ll) +1jets MH=120 GeV 30 fb-1 Background shape and comes from control sample

  30. (e+) (e-) Intermediate and Heavy Higgs: (MH>140 GeV) HZZ(*)4l MH>140 GeV: HZZ(*)4l Fully reconstruct Higgs kinematics Outstanding issues Lepton Identification and Isolation Suppression of backgrounds coming from tt and Zbb

  31. l+ W+ bl-+X p t p l- t W-  bl++X l+ l- p Z0 p bl-+X bl++X pptt4l+X • Suppress reducible backgrounds using combined information from calorimeter and tracking • Left out with irreducible background (non-resonant ppZZ(*) ) Reducible Backgrounds  ppZbb4l+X

  32. HZZ(*)4l event rates using for 30 fb-1 using NLO rates for signal and backgrounds. ppZbb4l (2 isolated leptons) + X Reducible background ppttWWbb4l (2 isolated leptons)+ X ppZZ4l (4 isolated leptons) + X Irreducible background MH=300 GeV 30 fb-1 MH=130 GeV 30 fb-1

  33. l+  H W+ W- l+  Intermediate mass Higgs: (140<MH<200 GeV) HWW(*)2l2 Missing Energy Outstanding issues Extraction of tt and WW backgrounds Splitting of phase space according to jet multiplicity Lepton Identification and Isolation, Missing ET Missing Energy

  34. SM Higgs HWW(*)2l2 • Strong potential due to large signal yield, but no narrow resonance. Left with broad transverse mass spectrum • Combined H+0,1,2jet analysis strongly improves sensitivity Backgrounds: ppWW+X MH=160 GeV e H+2jets Double top Single top

  35. Control Samples for HWW(*) • Since Higgs is a spin-0 particle, decay leptons tend to be close to each other. Exploit it to define control samples for background extraction Background-like region Signal-like region ll (rad) ll (rad)

  36. SM HWW +0,1,2 jets • Defined three independent analysis, depending on the number of tagged jets • Systematic errors added in significance calculation

  37. Outlook and Conclusions • The Standard Model (SM) a successfully describes the world of particle physics • However, the particle responsible to giving mass to particles has not been discovered yet! • The LHC will be the energy frontier accelerator: expert first proton-proton collisions in summer 2007 • The LHC will produce heavy particles (such as the Higgs boson) at rates orders of magnitude greater than in predecessor accelerators • The LHC era may be a revolution in particle physics! • ATLAS and CMS are multi-purpose detectors with great and similar capabilities. If the SM Higgs exists it will be observed with less than 10 fb-1 of understood data

  38. Additional Slides

  39. u c t up charm top Quarks s d b down bottom strange n ne n t m Leptons m-neutrino t-neutrino e-neutrino e t m muon tau electron Generations of matter I II III Building Blocks of Matter in the Standard Model • Quarks and leptons are organized in families or generations of matter • So far we observe three generations (I, II ,II) • Second and third generations are copies of the first, only much heavier • All have intrinsic angular momentum (spin) of ½ (fermions) • All particles have anti-particles • Display same mass and spin • Opposite electric charge

  40. Forces in Nature • We believe Nature displays three levels of interactions 1 Strength 10-3 - 10-5 10-36

  41. New particles are being discovered as predicted in the Standard Model Force Carriers • The Standard Model is very successful BUT: The Higgs boson has yet to be found! We need to explain the masses!

  42. ATLAS has excellent calorimeters • Excellent resolution and linearity for electrons, photons, hadrons • Powerful particle identification and isolation Fine segmentation (specially in the first layer) is a very powerful tool to identify and isolate electrons and photons

  43. Particle Detection • In order to observe the Higgs boson or any other new particle we need to detect their decay products Exploit the fact that different particles interact with matter differently Measure momentum/energy of particles + Identify electrons, photons, muons, taus and hadrons

  44. Partons (quark and gluons) in proton collide at high energies and produce heavy particles Proton Proton Remnants E=mc2 Proton Parton Parton Parton-Parton Interaction The LHC will be the energy frontier. We will be able to observe the Higgs and other new heavy particles Proton Remnants

  45. Level-1 Hardware trigger 75 kHz 2 μs Target processing time ~ 2 kHz ~ 10 ms High Level Triggers (HLT) Level-2 + Event Filter Software trigger ~ 200 Hz ~ 2 s Rate The ATLAS Trigger System • Trigger is crucial: reduce 1 GHz interaction rate (~2 Pb/sec) to ~200 Hz (~400 Mb/sec) which can be handled by today’s computing technology

  46. Tag jet Tag jet Tag jet Tag jet Low Mass Higgs Associated with Jets • A lot of progress since ATLAS Physics Technical Design Report (TDR 1999), mostly from the addition of H+jets channels • Slicing phase space in regions with different S/B is more optimal when inclusive analysis has little S/B H+2jet H+0jet H+1jet Tag jet Not tagged Tag jet Not tagged Not Tagged

  47. Analysis Strategy Higgs Boson Search • Concentrate on the most powerful analyses 114<MH<140 GeV (low mass) H (+0,1,2 jets) H (+1,2 jets) MH>140 GeV (intermediate and heavy) HWW(*)ll (+0,1,2 jets) HZZ(*)4l (inclusive)

  48. Complex final state: ttH(bb)lepton++bbbb+jj Signal Background ppttbb ppttjj • Analysis very sensitive to b-tagging efficiency (b4) • Parton/Hadron level studies b60% needed • Need ~100 times rejection against light jets and ~10 times against charm to suppress ttjj

  49. May achieve 3-5 effect for MH=120 GeV and 30 fb-1 • Need to address issues related to background shapes and differences in hadronic scales for light and b-jets 30 fb-1

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