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Higgs boson searches at LHC

Higgs boson searches at LHC. Domenico Giordano INFN - Università di Bari. Incontri di Fisica delle Alte Energie Catania, 30/3 – 01/04/2005. The Large Hadron Collider. p-p collider Beam Energy 7 TeV Bunch Crossing Rate 40 MHz Luminosity

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Higgs boson searches at LHC

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  1. Higgs boson searches at LHC Domenico Giordano INFN - Università di Bari Incontri di Fisica delle Alte Energie Catania, 30/3 – 01/04/2005

  2. The Large Hadron Collider p-p collider Beam Energy 7 TeV Bunch Crossing Rate 40 MHz Luminosity Low 2x1033 cm-2s-1 = 2x106 mb-1Hz High 1034 cm-2s-1 = 107 mb-1Hz Interaction Rate ~1GHz Interactions/Crossing ~23 (@ High Lumi.) basically minimum bias events Physics goals: • SM Higgs boson discovery • Supersimmetry discovery • B-physics, Top quark physics, Standard physics (QCD, EW) • Heavy Ion physics tracks with pt > 2 GeV tracks with pt > 25 GeV

  3. LHC Physics • Cross-sections of physics processes vary over many orders of magnitude • Inelastic s(pp) = 55 mb; • heavy-flavor factory: s(bb)= 500 mb; s(tt)= 1 nb; • vector-bosons factory; • s(H) = O(10 pb) (mH=200 GeV) • Low cross sections for discovery physics (Higgs production) Rejection power O(1013) (H-> 120 GeV) • Huge event rate  Highly Selective Trigger System • Extreme demands on detectors: • high granularity • high radiation environment • high data-taking rate

  4. ATLAS & CMS General pourpose detectors optimized for Higgs boson and New Physics search • Variety of signatures • , e//, ETmiss, b, t, jets • Experimental requirements • Large acceptance in h coverage • Precise muon detection system (trigger & pT meas. - e.g. H ZZ 4m) • Very good em calorimetry (excellent e/g identif., good E resol. - e.g. Hgg) • Good hermetic jet and ETmiss calorimetry (e.g. Htt) • Efficient tracking, vertex reconst. (good IP resol. - e.g. Hbb)

  5. SM Higgs production @ LHC Four main production mechanisms g g fusion • Largest rate • k = 1.5÷1.8 WW, ZZ fusion • Second largest rate • k 1.1 • two forward jets with high inv. mass W, Z bremsstrahlung ttH/bbH associate production • k 1.3 • Same coupling as in WW, ZZ fusion, different partonic luminosity • Trigger on 1 or 2 leptons @ high pT • k 1.2 • Reconstruction of tt pair allows bkg discrimination

  6. 114.4 SM Higgs decays Higgs couples to the heaviest possible particles: H bb dominates ... ... until WW, ZZ thresholds open Large QCD backgrounds:  (H  bb)  20 pb;  (bb)  500 b no hope to trigger/extract fully had. final states  look for final states with e(m),  Low mass region: mH < 2 mZ : H  bb : good BR, poor resolution  ttH, WH H   : small BR, but best resolution H   : via VBF H  ZZ*  4l H  WW*  ll or ljj : via VBF mH > 2 mZ : H  ZZ  4l“golden channel” qqH  ZZ  ll  * qqH  ZZ  ll jj * qqH  WW ljj * *forward jet tagging

  7. Low mass Higgs: H  gg • Rare decay mode (BR~10-3) accessible for mH < 150 GeV • Look for mass peak • achieve 1% resolution on mH • severe requirements on ECAL: • acceptance, energy and angle resolution, g/jet and g/p0 separation • motivation for LAr/PbWO4 calorimeters • Inclusive production mode pp  H  ggX: • sNLO*BR= 91.1 fb (mH = 120 GeV) • Large background (S/B  1:20): • smooth continuum of  pairs • Dominant, irreducible • can be estimated from sidebands • j and QCD jets with mis-identified ’s 100 fb-1 preliminary CMS AN 2003-009 ATLAS study: S/B ~6 for 30 fb-1, mH= 120 GeV

  8. Low mass Higgs: ttH  ttbb • Complementary to H for mH ≤ 130 GeV • sLO(ttH)*BR(Hbb)= 1.09 - 0.32 pb • Complex final state topology: • 1 high pT isolated lepton (trigger) and ETmiss • ≥ 6 jets out of which ≥ 4 b-tagged jets • b-tagging and jet energy performance crucial ! • Backgrounds • ttbb, Ztt, tt+jets, W+jets • Systematic uncertainties: mainly from ttjj bkg Significant discrepancy between ATLAS and CMS sLO sensitive to factoriz. scale, PDF and parton level cuts • Likelihood based study • b-tagging of 4 jets, anti-b-tagging of 2 jets • mass reconstruction of W± and 2 top quarks ATLAS(CMS) L=30 fb-1, no K-factors ATLAS: results of a new analysis respect to TDR ttbb from AcerMC, new PDF, new QCD-scale mh (GeV) = 120 130 140 S/B 2.8(3.5) 1.9(2.8) 1.0

  9. Jet Jet Higgs searches via VBF Forward jets Motivation • Strong discovery potential for mH< 150 GeV • Determine Higgs coupling to W/Z • Useful for Invisible Higgs Production  = 4 pb @ 120 GeV = 20% of total Signature • Two high pT jets at large  and large  Tag jets = highest pT jet in each h-hemisphere • Lack of colour exchange in initial state small jet activity in central region  central jet veto (pT>20 GeV LowLumi) (ATLAS Full simulation: fake jet rate <2%) Decays • HWWllnn and lnjj • Httlnnlnnlnnj • H gg f h Higgs Decay  distribution of tag jets (all by ATLAS, red by CMS)

  10. VBF: HWW* 10 fb-1 WWe s = 500 to 2000 fb for MH = 120 to 190 GeV Background • tt, WWjj, W+4jets Selection • VBF cuts, mjj, • 2 isolated high pT leptons, • mT(lln) (against DY) • t-jet veto (against ttjj) • lepton angular correlation (against tt, WW) (anti-correlation of W spins from H decay) Bkg estimation at level of 10% from data + MC shape relaxing lepton cuts Significance • >5 @ 10 fb-1 (mH=140 ÷190 GeV) (combined WWllnnand lnjj, DBG = 10%) MH=160 GeV ATLAS 60 fb-1 CMS

  11. VBS: H, Hgg Httem H • Main Backgrounds • ll : tt, QCD Zjj • l h : EW & QCD Z(tt)jj • Selection: VBF cuts, t and M reconstruction using collinear approximation (resolution ~10%) • Background estimate: • From sidebands (mH > 125 GeV) • From Z(tt) peak (mH < 125 GeV) Hgg • Main Background: ggjj • Clear improvement respect to inclusive production (S/B ~ 1 mH = 115-140 GeV) s = 300(64) fb @ mH= 120(150) GeV ATLAS 30 fb-1 s M ~11 GeV CMS 30 fb-1 CMS 60 fb-1

  12. (fb-1) The “golden channel”: H ZZ(*) 4l Very clean signatureH  ZZ (*)  l+l-l’+l’- (l=e,m) Four isolated high pT leptons Three topologies 4m, 2e2m, 4e Valid for the mass range 120 < mH < 600 GeV Background • Irreducible: ZZ Reducible: Zbb, tt Selection • Cut on pT of four leptons • Isolation of four leptons (Zbb, tt) • Impact parameter, vertex (Zbb, tt) • Mass of two lepton pairs and 4 lepton state Significance > 5 • With <30 fb-1: mH ∈[120; 600] GeV • Already at 5 fb-1: mH ∈[190; 450] GeV Higgs properties Mass, spin CP quantum numbers H  ZZ (*)  2e2m CMS full simulation Preliminary 30 fb-1 5 fb-1 Old CMS studies

  13. H WW  2m2n Interm. Mass Higgs: H WW(*) 2l2n This channel can contribute in the mass region 160÷180 GeV where the BR (H  ZZ (*) ) is smallest due to opening WW channel Backgrounds: WW, WZ, tt, Wt Signature: • 2 isolated high pT leptons, and 2 neutrinos • MT(ll) (No narrow mass peak) • central jet veto, b-jet veto • strong lepton angular correlations 30 fb-1 bkg. Systematic (ATLAS/CMS) need more justification; learn systematic from Tevatron analysis

  14. ATLAS 30 fb-1 MH = 1 TeV Very High Mass Higgs • Cross-section O(pb) for mH>600 GeV • Higgs width increases dramatically with MH (GH  MH3) • Need more abundant channels final states: llnn, lljj,lnjj • VBF distinctive signature of two very forward jets: • qqH  qqZZ  qqllnn • qqH  qqWW  qqlnjj • qqH  qqZZ  qqlljj

  15. LHC can probe entire set of ”allowed” Higgs mass values (100 GeV – 1 TeV) Several channels available over the full mass range Good coverage also at low mass after inclusion of VBF channels Discovery over full mass range possible already at 10 fb-1 ( < one year at 1033 cm-2 s-1) However, it will take time to operate, understand and calibrate detectors ... SM Higgs discovery potential Under update for CMS P-TDR (end 2005)

  16. Higgs Mass & Width Measurements • Direct where Higgs mass can be reconstructed: Hgg , Hbb, HZZ4l • “Indirect”from Likelihood fit to transverse mass spectrum: HWWlnln ,WHWWW lnlnlnln Uncertainties • statistical • absolute energy scale (0.1% for l/g, 1% for jets) • 5% on BG and signal rates for H  WW channels Resolution: • For gg & 4l ≈ 1.5 GeV/c2 • For bb ≈ 15 GeV/c2 • At large masses decreasing precision due to large GH Direct width measurement: Mass peak width of H  ZZ  4l channel for MH > 200 GeV (GH > Gexp) ATLAS 300 fb-1 DM/M: 0.1% to 1%

  17. The Higgs sector of MSSM • anomaly-free theory • generate mass for “up” and “down” type quarks (and charged leptons) Minimal Higgs sector structure: • 2 Higgs doublets • 5 Higgs bosons: h, H, A, H; • 2 free parameters defining Higgs sector (tree-level): mA, tan b = vu/vd Mass limits: mh<mZ cosb, mA< mH, mW < mH+ • Large radiative corrections to masses & couplings: • depends on SUSY parameters • top mass, stop mixing • mh < 135 GeV

  18. MSSM: benchmark scenarios • SUSY particles are heavy: no contribution to Higgs production/decay • SUSY particles contribute in production/decays - H/A  c20 c20  4l + Emiss ; h prod. in cascade decays (c20  hc10) - Impact on Higgs decay to SM particles generally small h  gg 10% smaller, A/H  SM at most 40% smaller Other branches • stop mixing: Maximal – No mixing • tan b value: low – high

  19. MSSM Higgses Couplings MSSM/SM • High tanb  Large BR(h,A,H bb,tt) • Small a  small BR(hbb,tt) • A does not couple to W/Z  No VBF prod. • HVV suppressed for large tanb a = mixing bw h/H • h/Hand A Decay modes: Production: - direct gg->h/H/A and ass. prod. ggbb h/H/A MA>300 GeV, tanb>10: >90% from ass. prod - h bb (90%) tt (8%) - H/Amm and H/Att enhanced with tanb h is SM-like for mA > mhmax • H+- Production: Decay modes: - H+-tn -H+-tb (MH+->Mt) - MH+-<Mt:tt events with decay tb H+- - gbt H+-; gg(qq)tbH+- ; qqH+-

  20. bbhmm VBF, htt VBF,htt+WW tthbb WWhlnbb VBF,hWW combined Light Higgs boson h VBF channels very useful: “cover large part of MSSM plane“ Search for SM-like channels ATLAS new, updated analysis 30 fb-1 Excluded by LEP 5s contours

  21. pp  bbH/A Full simulation & reconstruction 20 fb-1 • H/A  mm Clean signature and good mass resolution (1-2%) BR(H/A2m) 4x10-3 but x-sec enhanced by tanb Background • Z/g* 2m(dominant) rejected using b-tagging (vertex + i.p. e~40%) • tt (W mn) rejected using central jet veto Signal • in this param space is superposition of H and A (Dm~2GeV) • Two-muon mass resol. not enough to resolve 2 GeV • H/A  tt • Channels:ll, l+jet, jet+jet • Measure tanb from event rate with uncertainty from17.2% (jet+jet) to10.8% (l+jet) • Background • Z/g*, QCD rejected using b-tagging • ttrejected using central jet veto • W+jet, QCD rejected using t-tagging

  22. Discovery potential for H,A (SUSY particles heavy) 5s contours

  23. Discovery potential for H± • mH± < mt pptt H±(tn) b Wb , thadr. • Leptonic channel H±(tn)b W(ln) b Excess of t’s in tt event • Hadronic channel H±(tn)b W(qq) b (ATLAS) Closes “hole” at tanb~7 • mH± > mt • assoc. prod. with t:gb→tH± (tn, tb) bkg: tt, Wt • direct prod. qq→ H±(tn) bkg: W(tn) • In H±(tn) : (tpn) use helicity correlations to suppress bkg. ATLAS 30fb-1

  24. 5s contours 300 fb-1 SUSY particles heavy 4 Higgs observable 2 Higgs observable 3 Higgs observable 1 Higgs observable Overall discovery potential • Plane fully coveredwith 30 fb-1 • 2 or more Higgses observable in large fraction of plane • disentangle SM / MSSM But… • significant area where only lightest Higgs bosonh is observable • can SM be discriminated from extended Higgs sector by parameter determination?

  25. Conclusions • SM Higgs boson can be discovered (5σ) at LHC over the full mass range with <30 fb-1 • Vector Boson Fusion significantly enhances sensitivity for low and medium MH • Forward jet tagging crucial • MSSM MA-tanb space will be almost completely accessible with 30 fb-1 • for MA < 500 GeV, several Higgs bosons observable • “weak region” for MA > 500 GeV: ~ only h observable unless A/H/H SUSY particles BUT… • The discovery is only the first step: need to determine Higgs properties in order to distinguish bw different models (and make sure that it is really a Higgs boson) • mass, spin, CP properties, couplings to different particles, …. • Still a lot of work to be done • Detector understanding (calibration, alignment, …) • Improvement, validation and tuning of MC tools (use of Tevatron data) • Full simulation analyses (CMS P-TDR, end 2005)

  26. Backup slides

  27. Trigger requirements • Cover all SM topologies and those expected from new physics • Inclusive selection (to discover unexpected new physics) • Keep safety margin against uncertainties • Knowledge of (background) cross-sections • Real detector behavior, beam-related (and other) backgrounds • Performance of the selection software (Efficiency must be measurable from data) ATLAS CMS HLT @ 2x1033 cm-2 s-1

  28. H  gg : Vtx reco The vertex of the Higgs boson can be found with the help of additional tracks in the same event: • Higgs boson pT is balanced by the rest of the particles in the event • Vertex can be identified by the hardest tracks of the bunch crossing. Vertex correction significantly improves the mH resolution: • sH/mH 0.7% (mH = 110-150 GeV) • efficiency in 2.5 GeV mass window is improved by 31%.

  29. Higgs partial widths and coupling ratios • With 30 fb-1 of data • Accuracy of relative branching ratios and relative couplings vary from 20% to 60% depending on coupling and mass • Worst channel: H bb H WW chosen as reference as best measured for MH>120 GeV ATLAS ATLAS

  30. MSSM Higgses decay

  31. Jet Jet Invisible Higgs h  LSP VBF most promising channel • Trigger on forward jets + ETmiss • Backgrounds: • Events that originate ETmiss: Wjj, Zjj, QCD multi-jets • Bkg estimate from data (Zll, W ln) to level of 3% • Selection • VBF cuts (forward jets, central jet veto) • Lepton veto • Dfjj Model independent, 95% CL upper limit of h invisible x-sec No sensitivity is expected for heavy scalar H due to suppression of the VBF x-sec

  32. Higgs decays to SUSY particles If SUSY particles kinematically accessibles • Higgses can decay directly to or come from decays of SUSY particles • possible complementary coverage respect to SM • A/H  20 204l+ETmiss (20 10ll) Selection: Lepton isolation, ETmiss, jet veto M1=60 GeV, M2=120/180 GeV m=-500GeV, Msleptons=250 GeV Msquark,gluino=1TeV • gbtH+, H 2,30 1,2 3l+ETmiss Accesses only a limited fraction of parameter space • M2 = 210 GeV, • = 135 GeV, Msleptons = 110 GeV, Msquark, gluino = 1TeV

  33. Light Higgs boson h: 30 fb -1 bbhmm VBF, htt Excluded by LEP VBF,htt+WW tthbb WWhlnbb VBF,hWW combined

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