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Higgs Searches at LHC

Higgs Searches at LHC. Marco Pieri, UCSD – San Diego Hadron Collider Physics Symposium 2005 4-9 July 2005, Les Diablerets, Switzerland. SM Higgs boson MSSM Higgs bosons Higgs bosons and SUSY particles Measurement of Higgs boson parameters. Current status of Higgs Searches.

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Higgs Searches at LHC

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  1. Higgs Searches at LHC Marco Pieri, UCSD – San Diego Hadron Collider Physics Symposium 2005 4-9 July 2005, Les Diablerets, Switzerland • SM Higgs boson • MSSM Higgs bosons • Higgs bosons and SUSY particles • Measurement of Higgs boson parameters

  2. Current status of Higgs Searches INDIRECT CONSTRAINTS ON THE SM HIGGS BOSON • Electroweak fits to all high Q2 measurements give: • MH=129+74-49 GeV • MH<285 GeV @ 95% CL • The central value and the upper limit have increased during the last few years DIRECT SEARCHES AT LEP GAVE NEGATIVE RESULTS • SM Higgs • MH>114.1 GeV @95% CL • MSSM neutral Higgs bosons • Mh, MA>92.9, 93.3 GeV @95% CL • Charged Higgs Bosons • MH± >89.6 GeV @95% CL for BR(MH±→ τν) =1 • MH± >78.6 GeV @95% CL for any BR Marco Pieri - UCSD San Diego

  3. Introduction • ATLAS and CMS are preparing for the search for the Higgs bosons of different models • Most of the studies presented are still carried out with fast simulation for the background. Full simulation has been used for the signal and for the estimation of the crucial aspects of the detectors • Studies with full simulation of signal and background are in progress • Most analyses shown in the following are optimized for the low luminosity phase LHC operation • Low luminosity phase: • ℒ~ 2 x 1033 cm-2s-1 • Int ℒ~ 30 fb-1 • High luminosity phase: • ℒ~ 1 x 1034 cm-2s-1 • Int ℒ~ 300 fb-1 Marco Pieri - UCSD San Diego

  4. SM Higgs production NLO Cross sections M. Spira et al. gg fusion IVB fusion Marco Pieri - UCSD San Diego

  5. SM Higgs decays When WW channel opens up pronounced dip in the ZZ BR For very large mass the width of the Higgs boson becomes very large (ΓH >200 GeV for MH≳700 GeV) Marco Pieri - UCSD San Diego

  6. Most important SM search channels Low mass MH≲160 GeV Intermediate mass (160 GeV ≲MH≲700 GeV) High mass (MH≳700 GeV) • IVB fusion qqH →ZZ →ℓℓνν • IVB fusion qqH →WW →ℓνjj • inclusive H → WW • inclusive H → ZZ H →γγ and H →ZZ* → 4ℓ are the only channels with a very good mass resolution ~1% Marco Pieri - UCSD San Diego

  7. H→γγ • Sigma x BR ~90 fb for MH = 110-130 GeV • Irreducible backgrounds from gg→γγ, qq →γγ, pp →γ jet →γγ jet • Reducible background from fake photons from jets and isolated π0 (isolation requirements) • Very good mass resolution ~1% • Vertex estimated from the underlying event and recoiling jet H →γγMH = 115 GeV Marco Pieri - UCSD San Diego

  8. H → ZZ* → 4ℓ H → ZZ* →ℓ+ℓ-ℓ+ℓ-ℓ=e,μ • Irreducible background: • ZZ production • Reducible backgrounds • tt and Zbb • Very good mass resolution ~1% Branching ratio dip due to opening of WW channel • In this channel (and in the H→γγ) background can be easily estimated from data by fitting the sidebands • Above MH ~ 2MZ the two Z bosons are real and σxBR is larger • Golden channel for Higgs discovery at LHC Marco Pieri - UCSD San Diego

  9. H->bb, ttH production channel • Allows the measurement of the Higgs coupling to fermions • ttH →ℓνqqbbbb • Most useful for very light Higgs: MH≲ 130 GeV • Fully reconstruct the top decays and estimate the right bb combination for the Higgs boson Mass resolution not excellent > 10 % b-tagging performances for CMS and ATLAS similar Marco Pieri - UCSD San Diego

  10. Higgs decay products forward jets IVB Fusion qqH (low mass H) qqH → qqγγMH = 120 GeV Proposed by Dockshitzer, Khoze, Trojan and Rainwater, Zeppenfeld et al. qqH → qqWW*, qqττ(also qqγγ) • Much worse mass resolution or only transverse mass measurable • Background estimation from data much more difficult Tagging jets from qq are at high rapidity and large Δη ATLAS • Background is highly reduced by • tagging the two forward jets • requiring low activity in the central detector • Signal to BG ratio is increased reducing the effect of BG uncertainty Marco Pieri - UCSD San Diego

  11. IVB fusion: qqH → qqττ xτi=fraction of τ energy carried by visible decay products • ATLAS carried out a recent study in the qqH channels: • qqWW* → qq ℓννℓνν • qqWW* → qq ℓννjet jet • qqττ→ qq ℓννℓνν • qqττ→ qq ℓνhad ν • with ℓ=e,μ • Main backgrounds • Z+jets, tt and WW+jets signal Wjj background ATLAS 30 fb-1 τ reconstruction: • τ decay products are highly boosted, assume that they are collinear • From module and direction of the measured missing Et derive the neutrinos momenta • Mass resolution ~10% at MH=120 GeV Marco Pieri - UCSD San Diego

  12. IVB fusion: qqH → qqWW* • H →WW →ℓνℓν or ℓνqq • Trigger on the lepton(s) and on missing Et • Main backgrounds tt and tW • Higgs mass cannot be reconstructed, only transverse mass • Difficult to estimate the BG from the sidebands (syst BG ~ 10%) ATLAS MH=120 GeV One way to estimate the background: release lepton cuts use shape from MC ATLAS ATLAS MH=160 GeV Plots for WW→eμνν channel Marco Pieri - UCSD San Diego

  13. Results for low mass ATLAS • VBF channels improve a lot discovery potential compared to previous results • No K-factors used, LO cross sections • With 30 fb-1 more than 5 sigma significance for MH>100 GeV • Higgs boson can be discovered in more than one channel, possible to measure its couplings Marco Pieri - UCSD San Diego

  14. Results for the whole mass range • For mass larger than ~200 GeV use ZZ and WW leptonic decays • For mass larger than ~700 GeV use qqH, H →ZZ →ℓℓννand H →WW →ℓνqq For low mass results updated with IVB fusion All mass range accessible at 5σ significance with 10 fb-1 With a few fb-1 possible to discover the Higgs boson with mass between ~150 and ~500 GeV in the WW and ZZ channels Marco Pieri - UCSD San Diego

  15. MSSM Higgs Searches • Two Higgs doublets model • 5 Higgs bosons: • 2 Neutral scalars h,H • 1 Neutral pseudo-scalar A • 2 Charged scalars H± • In the Higgs sector all masses and couplings are determined by two independent parameters • Most common choice: • tanβ – ratio of vacuum expectation values of the two doublets • MA – mass of pseudo-scalar Higgs boson In the MSSM: Mh≲ 135 GeV Marco Pieri - UCSD San Diego

  16. Neutral MSSM Higgs bosons • Decoupling limit (MA≳200 GeV) • h behaves like HSM • Standard model searches directly apply • MH~MA~MH± • MA=O(MZ) and large tanβ • H behaves similarly to SM Higgs (SM searches apply) • In other cases for large tanβ and MA<200 GeV • A →WW,ZZ never allowed at tree level, • h,H→WW,ZZ highly suppressed • h,H,A almost exclusively decay into bb and ττ • Large MA small tanβ • H,A decays almost 100% into tt • for lower masses (200-300 GeV) also H → hh and A → Zh • If SUSY particles are light the Higgs bosons may decay into s-particles Marco Pieri - UCSD San Diego

  17. h,H production and decay tanβ = 30 h,H decays h,H production Decoupling region Large tanβ mainly bb, ττdecays Large tanβ hbb, Hbb (and Abb) production dominates Marco Pieri - UCSD San Diego

  18. Results from SM Higgs Searches • In a large part of the MSSM parameter space SM Higgs searches are effective to find the MSSM h boson CMS 5σ discovery contours • In the decoupling region if h observed hard to distinguish SM from MSSM • Search for H, A and H± • For large tanβ exploit the large cross section of Higgs boson production in association with a bb pair • bbH,A → bbττ • bbH,A → bbμμ • bbH,A → bbbb (very difficult) • B-tagging (+ τ id and missing Et for the τ channel) are the key issues Marco Pieri - UCSD San Diego

  19. bbH,A → bbττ bbH,A →bbττ • for MH ≲ 400 GeV • ττ → ℓννℓνν • ττ → ℓνν hadν • Higher mass also add • ττ→ had ν had ν • b-tagging, τ id and missing Et are the basic ingredients • From the cross section measurement it is possible to extract the value of tanβ • tanβ uncertainty due to variation of SUSY parameters (MHMAX scenario considered) in a range ±20% is 6% Marco Pieri - UCSD San Diego

  20. bbH,A → bbμμ H,A→μμ • low rate, BR(H→μμ) ~10-3 • high efficiency • precise mass measurement (μμ mass resolution ~1%) • Main backgrounds: • Z/γ* →μμ • tt →μμ X • Selection requires 2 muons, b-tagging and central jet veto CMS 20 fb-1 CMS 5σ discovery contours Marco Pieri - UCSD San Diego

  21. Results on H,A 5σ discovery regions in the MHMAX scenario Marco Pieri - UCSD San Diego

  22. Charged Higgs bosons H± MH±<mt-mb • Mainly produced in top decays tt→tH±b • in the MSSM BR(H±→τν)~100% MH±>mt+mb • Mainly produced in association with a t quark (gb→tH±) • BR(H±→tb)~100% for small tanβ • H±→tb decay dominates but BR (H±→τν) still sizeable for large tanβ includes top decays T. Plehn et al. Analyses are in progress for the mass region MH± ~mtop Marco Pieri - UCSD San Diego

  23. Main search channel H±→τν MH±<mt-mb • Main channel tt →bH±bW → bτνbℓν • ATLAS also considers tt → bτνbqq • Use transverse MT mass built with τ jet + missing Et • tt background has MT < MW MH±>mt+mb • gb → tH± with H± →τν and t → bqq • Exploit helicity correlations • Similar endpoint of MT at MW for the background • MT can also be used for Higgs mass measurement (likelihood fit) ATLAS 10 fb-1 30 fb-1 Marco Pieri - UCSD San Diego

  24. Discovery regions for Charged Higgs Bosons 5σ discovery regions in the MSSM tanβ – MA plane for MHMAX scenario ATLAS search in tt →bH±bW → bτνbqq improves the sensitivity in this region Marco Pieri - UCSD San Diego

  25. MSSM scans • MHMAX scenario • MSUSY = 1 TeV • maximal mh < 133 GeV • No-mixing scenario • MSUSY = 1 TeV • mh < 116 GeV • Gluophobic scenario • suppressed coupling to gluons (cancellation of top+stop loops) • Small rate for : gg H • MSUSY = 350 GeV, mh < 119 GeV • Small αscenario • coupling to b and τsuppressed for large tanβ, MA 150-500 GeV • MSUSY = 800 GeV • mh < 123 GeV 4 CP conserving benchmarks suggested by Carena et al. ATLAS studied the 4 benchmarks • With 30 fb-1 h or H can be seen in VBF channels in almost all parameter space ATLAS preliminary Marco Pieri - UCSD San Diego

  26. Results of MSSM scans • Smaller region covered by the ττ channel in the small α scenario (reduced coupling) • Covered by increased coupling to gauge bosons covered by h→WW h→γγ(enhanced branching ratio to gauge bosons) ATLAS preliminary 5σ discovery regions Marco Pieri - UCSD San Diego

  27. Higgs Bosons visibility in the MSSM 5σ discovery regions in the MSSM tanβ – MA plane for MHMAX scenario 4 Higgs observable 3 Higgs observable 2 Higgs observable 1 Higgs observable All the plane is covered but there is a large area where only h can be seen Marco Pieri - UCSD San Diego

  28. MSSM Higgs bosons and SUSY particles • If SUSY particles are heavier than the Higgs bosons Higgs bosons could be produced in gauginos decays: • χ2→h,H,A χ1 • χ1±→H±χ1 • Different cascades possible involving heavier gauginos • Search for h,H → bb • Neutralinos and charginos would be copiously produced in the decays of squarks and gluinos Possible to observe SUSY → h,H,A with h,H,A→bb • If SUSY particles are lighter than Higgs bosons we could have a rich variety of decays, some scenarios have been investigated: • H,A →χ2χ2 • using χ2→ℓ+ℓ-χ1 decay (4ℓ + missing Et events) • h →χ1χ1 • invisible Higgs decays Marco Pieri - UCSD San Diego

  29. Invisible decays of the Higgs Boson • IVB fusion is the most sensitive process • Trigger on forward jets + missing ET • Selection: • forward jet tagging, central jet-veto, M(jet jet) • lepton veto, missing Et • Δφ jet-jet small • If we do not require gaugino mass unification and M1<<M2 Mχ can be rather small and BR(h →χχ) can be very large ATLAS - Accessible region for 95% CL exclusion from chargino searches Marco Pieri - UCSD San Diego

  30. Measurement of Higgs bosons parameters • After discovering the Higgs bosons we should measure their parameters • Studies for high luminosity (Int L = 300 fb-1) SM Higgs boson mass • direct reconstruction: 4ℓ, γγ, bb • likelyhood fit WW SM Higgs boson width • from ZZ →4ℓ ATLAS INT L = 300 fb-1 Marco Pieri - UCSD San Diego

  31. Measurement of Higgs couplings • From σ x BR measurements in all channels where the Higgs boson can be observed: D. Zeppenfeld et al. SM framework Production cross section Decay BR Marco Pieri - UCSD San Diego

  32. Other studies Many other scenarios have been studied • CP Violating MSSM • Investigated by ATLAS, reduced discovery potential for small Higgs boson masses • Strongly interacting Higgs Sector: VLVL scattering • If no Higgs boson is found at LHC • Radions (Randall Sundrum model) • φ →hh • ... • See Atlas Physics TDR and the soon coming CMS Physics TDR for details Marco Pieri - UCSD San Diego

  33. Conclusions • ATLAS and CMS have studied the prospects of Higgs boson discovery for SM and MSSM • SM Higgs boson can be discovered with 5 sigma with 10 fb-1 at low luminosity in the whole mass range • At least 1 MSSM Higgs boson can be found for all investigated benchmarks • In some regions difficult to discriminate between SM and MSSM • WW and ZZ fusion process is very important both for SM and MSSM • Two years from the beginning of LHC, must continue to prepare the actual analyses • based on data • with minimal use of MC information • study of all the possible control samples needed to verify the performances of the detector • Studies with full simulation of signal and all the backgrounds are in progress • We are getting ready to find the Higgs boson(s) at LHC Marco Pieri - UCSD San Diego

  34. EXTRA Marco Pieri - UCSD San Diego

  35. H → ZZ → 4ℓ Marco Pieri - UCSD San Diego

  36. Higgs boson width Marco Pieri - UCSD San Diego

  37. High mass search • Above MH~700 GeV the width of the Higgs boson becomes very large (>200 GeV), need higher rate • Use IVB fusion • H →ZZ →ℓℓννand • H →WW →ℓνjj ATLAS 100 fb-1 Marco Pieri - UCSD San Diego

  38. MSSM h,H decays Small tanβ H decays into tt when allowed Decoupling region Large tanβ bb, ττdecays Marco Pieri - UCSD San Diego

  39. MSSM Production processes Large tanβ hbb, Hbb and Abb production dominates Marco Pieri - UCSD San Diego

  40. bbH,A->bbbb CMS new analysis • Investigation of feasibility of the 4b channel • S/B ~5%, large effects of systematic error on BG estimation Seems extremely difficult to control the BG with the needed precision Marco Pieri - UCSD San Diego

  41. Hadronic cannel: H±→tb • CMS repeated the study with NLO cross section calculation • Old results showed some sensitivity • New results by indicate that with the current analysis the very small expected signal is washed out by systematic errors on the background estimation Effect of systematic error on BG Marco Pieri - UCSD San Diego

  42. Discovery regions for MSSM Higgs bosons 5σ discovery regions ATLAS MHMAX scenario 300 fb-1 1 boson h,H,A,H+- 2 bosons 3 bosons h,H,A All 4 bosons h only Similar results in the other 3 benchmarks Excluded by LEP h,H,A Marco Pieri - UCSD San Diego

  43. Measurement of Higgs boson couplings Marco Pieri - UCSD San Diego

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