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Searches for Higgs Bosons at CDF

Searches for Higgs Bosons at CDF. Thomas Wright University of Michigan SLAC Experimental Seminar February 13, 2007. Run 2 at the Tevatron. World’s highest-energy collider Record luminosity 278E30 cm -2 s -1 Will have 2 fb -1 on tape in ~1 week! Integrating 40-45 pb -1 /week.

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Searches for Higgs Bosons at CDF

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  1. Searches for Higgs Bosons at CDF Thomas Wright University of Michigan SLAC Experimental Seminar February 13, 2007

  2. Run 2 at the Tevatron World’s highest-energy collider Record luminosity 278E30 cm-2s-1 Will have 2 fb-1 on tape in ~1 week! Integrating 40-45 pb-1/week Results shown here use data samples of ~1 fb-1 Look for updates on expanded samples this summer

  3. The CDF II Detector

  4. The CDF II Detector Azimuthally symmetric “barrel” geometry Central detectors cover ||<1 Plug calorimeter extends to ||<3.6 Silicon extends to z =  50 cm (luminous region z ~ 25 cm) Tracking out to ||<2

  5. Particle Identification • Charged leptons identified by characteristic energy deposition patterns • Presence of neutrinos is inferred from energy imbalance – “missing energy” • Because net pz of the scattering partons is not known, mostly work in the transverse plane (i.e pT, ET, missing-ET) • B-jet identification uses the silicon tracker • 8 layers, 704 ladders, 722432 channels • Total sensor area = 6 m2 • SVX II – 5 double-sided layers (r + rz) • L00 r only – mounted directly to beampipe (R = 1.4 cm)

  6. The CDF II Trigger System • Interaction rate very high, but most not “interesting” • Limited bandwidth to mass storage – must be choosy • Level1 system • Synchronous – no deadtime • Single CAL towers (photons and jets), COT tracks (with pair correlations), track-tower matches (electrons and taus), muons, missing energy • Level2 system • Asynchronous - ~5% deadtime • All Level1 objects, plus CAL clusters (jets) and silicon tracking • Level2 accept triggers full detector readout (few % deadtime) • Level3 runs a version of the offline reconstruction – final rate reduction before writing to tape • Always tuning the system to accommodate higher luminosity 2.5 MHz crossing rate (396 ns) Output 20-30 kHz Synchronous Latency 25-30 s Output ~700 Hz Readout latency ~650 s Output 70-90 Hz

  7. The Higgs Boson of the Standard Model • Electroweak symmetry can be broken using the “Higgs mechanism” • 4 new scalar fields • Three give mass to the W’s and Z • Other manifested as a single scalar – the “Higgs boson” • If there is such a particle, precision electroweak measurements favor a low mass • LEP2 searches excludemH < 114.4 GeV/c2 @ 95% CL • SM fit requires mH < 166 GeV/c2 @ 95% CL (199 if including LEP2 direct searches) as of last year New CDF W mass (most precise single measurement) moves best fit to 80+36-26 GeV/c2(was 85+39-28)

  8. Higgs Production and Decay Ideally, use gg  H  bb, WW But, QCD bb background too high (pb-1) For low mH, use WH+ZH, H  bb (associated production) At high mH the WW decay mode opens up – can use gg  H production

  9. The H  WW* l l Channel • Largest BR for mH > 135 GeV/c2 • Uses gg  H production • Now including VBF (6-10% extra cross section) • Event selection • Two isolated leptons with pT > 20 and 10 GeV/c • Opposite charge • Missing-ET > mH/4 • If missing-ET aligned with lepton, > 50 GeV • mll > (mH/2)-5 GeV/c2 • pT,1+pT,2+missing-ET < mH • Jet veto • Including W BR’s, acceptance is 0.3-0.7% depending on mH W  l BR not included

  10. H  WW* Backgrounds W+  e+  e- W- • Predominantly WW • Also Drell-Yan and other diboson channels, and from fake leptons • Not possible to reconstruct Higgs mass due to multiple neutrinos • Can exploit scalar nature of Higgs • Leptons from H  WW* are closer in  • Treat each bin of  as a separate counting experiment

  11. H  WW* Cross Section Limits • Sensitivity about 6x SM level at 160 GeV • For 6 fb-1 and combined with DØ, within factor 2 of SM • Get more events • WZ search almost doubled acceptance by adding new lepton types • Get more out of events • More than just Df • Multivariate (NN) • Matrix elements excluded at 95% C.L.

  12. Higgs Production and Decay Ideally, use gg  H  bb, WW But, QCD bb background too high (pb-1) For low mH, use WH+ZH, H  bb (associated production) At high mH the WW decay mode opens up – can use gg  H production

  13. B-Jet Identification (b-tagging) • B-hadrons are long-lived – search for displaced vertices • Construct event-by-event primary within beamspot (10-32 m) • Fit displaced tracks and cut on Lxy significance ( ~ 200 m) • Calibrate performance from data (low-pT lepton samples) b-fraction ~80% measure tag efficiency in data and MC Tag this jet

  14. Fake B-Tags (mistags) • Fake tags are (almost) symmetric in displacement Lxy • Rate of tags with Lxy<0 is a good estimate for the mistag rate • Parametrize mistag rate which can be applied to any sample • ~30% correction for tags from /KS and interactions with detector material Lxy > 0 Lxy < 0

  15. The WH  lbb Channel • Event selection • Isolated e or  with pT>20 GeV/c • Missing-ET > 20 GeV • Exactly two jets with ET>15 GeV • At least one b-tagged jet • Acceptance is 1.8-2.1% • Backgrounds include • Non-W events (fake lepton, fake missing-ET, b decays) • W + mistagged jets • W + heavy flavor jets • Diboson production (WW, WZ, ZZ) • Z • Top quark production (including single top) This channel uses a neural net filter on the b-tags to reject half of the background (~10% signal loss)

  16. WH Backgrounds Use W+1-jet bin to test W+HF bkgd Top pair cross section Measured from the W+3,4-jets events SM Higgs would be about two events

  17. WH Dijet Mass At least one jet b-tagged with NN Both jets b-tagged

  18. WH Cross Section Limits excluded at 95% C.L.

  19. The ZH bb Channel b-jet Missing ET A di-jet QCD event: 2nd jet 180o y Fake Missing ET x 1st jet b-jet • Distinctive final state of b-jets recoiling against missing-ET • Event selection • Missing-ET > 75 GeV • Lepton veto • Exactly two jets with ET > 60 and 20 GeV • Missing-ET not aligned with either jet • Acceptance is 0.5-0.8% (for ZH) • About half the events would be WH with lost lepton • Backgrounds include • QCD with fake missing-ET • QCD bb production • W/Z + jets • Top production • Diboson production

  20. ZH Dijet Mass

  21. ZH Cross Section Limits

  22. The ZH  llbb Channel • Look for ZH also in Z decays to charged leptons (e or m) • Lose in BR, gain in background • Z+jets and top pairs • Instead of dijet mass, enhance S/B using neural networks

  23. ZH Cross Section Limits

  24. Combined Limits (Relative to SM) Not including new WW* Factor of ~8 away from SM prediction at 115 GeV Expect factor 2-3 from more luminosity Another factor of √2 from combination with DØ As with WW*, need to improve the analyses in order to reach SM sensitivity

  25. Multivariate B-Tagger Multivariate b-tag algorithm in the pipeline Much like the one used by SLD Plots are for simulation only, performance characterization on data is in progress • Up to 30% higher b-tag efficiency compared to the NN tagger already used in the WH search channel • More double-tagged events • Better S/B • Better dijet mass resolution

  26. Double-Tagging in WH Similar limits for exclusive 1-tag and 2-tag samples 20% improvement over inclusive ≥1-tag result

  27. The W/Z+H qqbb Channel • Was competitive with the leptonic W channel in Run I (but a little lucky) • 70% of W/Z decays are into hadrons • Base selection is four jets with two b-tagged • Search in tagged dijet mass • Can use the four-jet trigger designed for all-hadronic top pairs – also working on a new dedicated trigger • Background dominated by QCD multijet production (with real tags) • Data-driven estimates in progress

  28. Higgs in the MSSM 0 b b 0 b • Minimum of five scalars: h, H, A, H+, H- • Separate couplings for up-type and down-type fermions • Properties of the Higgs sector largely determined by mZ and by two other parameters: • mA : mass of pseudoscalar • tanb : ratio of down-type to up-type couplings • If tanb is small, then h looks a lot like a standard model Higgs • If tanb is large • Production via b quarks can be greatly enhanced (factor ~tan2) • Decays to bb (~90%) and  (~10%) dominate • LEP-II searches have excluded mA<93 GeV/c2 and tanb < 5-10

  29. MSSM Higgs Masses • At high tanb, the A becomes nearly degenerate with h or H • Can search for single resonance, double the cross section • The other neutral Higgs is like SM Higgs (no production enhancement)

  30. Scenario Dependence Higgs properties are largely, but not completely, determined by mA and tanb Loop corrections introduce dependence on other SUSY parameters M. Carena et al., Eur.Phys.J. C45 (2006) 797-814 (hep-ph/0511023) Db is a function of the other SUSY parameters and depends on the “benchmark” scenario Db ~ mtanb (sign of m critical) For tanb = 50, m = -200 mhmax: Db = -0.21 no-mixing: Db = -0.11 Need both channels to get the full story

  31. The gg+bb Channel • High cross section and unique final state (not QCD) • Best signature is one  decay into e or  and the other hadronically (46% BR) • Now also e+m channel (+6%) • Event selection • One e or  with pT > 10 GeV/c • One hadronic  with pT > 15 GeV/c, mass < 1.8 GeV/c2 (including p0’s) • Or, e+m with pT > 6 GeV/c • Opposite charge • Missing-ET not recoiling against leptons (rejects W  l) • Acceptance is 1.2-2.1% • Backgrounds include • Z  • W  l +jet  fake had • QCD multijet (both  fake)

  32. Higgs Discriminant • Because of multiple neutrinos cannot reconstruct tt invariant mass • In cases where taus are not back-to-back can use missing-ET projections • Low efficiency • Instead, use “visible mass” • Mass of the visible parts of the two taus and the missing-ET

  33. Sample Fit • Signal is normalized to 95% CL exclusion limit

  34. MSSM Di-Tau Limits Background-only pseudoexperiments indicate <2s significance when considering the entire mvis range

  35. MSSM Interpretation

  36. The gg  bb bbbb Channel • bb final state not unique enough • Require one of the additional b’sto have high pT and b-tag it • Results are for a different cross sectionthan the tt case (factor ~4) • Basic event selection is three b-tagged jets • Search in dijet mass of two leading jets • Background expected to be ~100% QCD multijet production • Almost all bbq, bbc, bbb • Data-driven estimates exist • At high tanb the Higgs can developsignificant width – fit templates arefunction of cross section • We expect CDF result on 1 fb-1 outwithin a few weeks *editorial comment belongs to me, not DØ Dawson, Jackson, Reina, Wackeroth hep-ph/0603112 ?*

  37. SM Higgs at the LHC • Discovery prospects are excellent • Strategies for low-mass region are different – more focused on backgrounds than cross section • Experience gained at the Tevatron will be very useful • tt event reconstruction • dijet mass reconstruction • W/Z + jets background estimation techniques • b-tagging and  ID at hadron colliders • Starting from a bump at the Tevatron gets us there that much faster!

  38. Tevatron Prospects

  39. Summary • CDF is searching for the Higgs in a variety of production and decay scenarios • Tools are in place to combine results from different channels • Lots of effort going into adding new channels and improving the existing ones • MSSM Higgs searches looking quite interesting

  40. Backup Material

  41. The Standard Model • Matter is made out of fermions: • quarks and leptons • 3 generations • Forces are carried by Bosons: • Electroweak: ,W,Z • Strong: gluons • Higgs boson: • Gives mass to particles • Much is still unknown • Is the EW symmetry really broken by a Higgs? What kind of Higgs(es)? • Are there any other particles? New gauge bosons (Z’, W’)? Extra generations of quarks? Extra dimensions? Superpartners? • Let’s have a look! H

  42. What about Neutrinos? Not nearly enough material to stop them in the detector Instead, infer their presence by energy imbalance i.e. add up everything you see, then reverse it Commonly called “missing-ET” Only get net neutrino momentum if >1 Only works in transverse plane

  43. PDFs

  44. Non-W Background to WH Channel Signal region D predicted by Model event kinematics from sideband • Use missing-ET and isolation ratio (assumed uncorrelated) in sidebands to extrapolate into signal region • Isolation ratio = (lepton pT)/(non-lepton energy in cone with - radius 0.4 around the lepton)

  45. B-Tag Efficiency Measurement • Large b-hadron mass gives a wide pT,rel distribution relative to non-b contributions • Fit untagged and tagged jets with b and one of four non-b templates to get b-tag efficiency • Spread of results using the four non-b used as a systematic error

  46. Dijet Mass Resolution Raw: what we use now H1: track + CAL energy flow MTL: correct for soft leptons Hyperball: multivariate nearest-neighbor algorithm, pick the most likely “true” dijet mass

  47. W + jets Simulation • Lots of activity in recent years • We use the ALPGEN generator • Tree-level W + N partons • Also W+c+Np, W+cc+Np, W+bb+Np • HERWIG parton shower adds soft gluon radiation • Monte Carlo prediction normalized to observed number of W+jets • Fraction of events containing heavy quarks calibrated from data • b-tag rates in data and ALPGEN multijet samples • Scale ALPGEN prediction by 1.5  0.4

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