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Low mass SM Higgs boson Introduction

Low mass SM Higgs boson Introduction. J.-F. Grivaz LAL-Orsay. Ph.D students. Why “low mass” ?. LEP direct searches: M_H >114 GeV. Precision EW measurements: M_H < 154 GeV at 95% C.L (K. M önig). High mass getting “under control” (G. Bernardi). Preferred by SUSY (M. Carena). 0.2 0.1.

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Low mass SM Higgs boson Introduction

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  1. Low mass SM Higgs bosonIntroduction J.-F. Grivaz LAL-Orsay Ph.D students D0-France October 2008

  2. Why “low mass” ? LEP direct searches: M_H >114 GeV Precision EW measurements: M_H < 154 GeV at 95% C.L (K. Mönig) High mass getting “under control” (G. Bernardi) Preferred by SUSY (M. Carena) D0-France October 2008

  3. 0.2 0.1 pb Excluded at LEP Profile of a low mass (mH < 135 GeV) Higgs boson • Hbb • Dominant • (~73% at 115 GeV) To disentangle Higgs final states from the QCD background, one needs leptons and/or missing ET in the production or in the decay. • HWW* • Increasingly useful with • increasing mass • For Hbb, this means • no gluon-gluon fusion • Higgstrahlung with W/Z leptonic decays • H • may add some sensitivity • (not yet fully exploited) D0-France October 2008

  4. Putting it all together: • Three main channels: • WHebb 14.1 fb • WHbb “” • ZH bb 15.6 fb • In spite of its lower cross section: • ZH (ee/)bb 22.6 fb • is also powerful because • of its very distinct signature • The French groups have launched a three-year coordinated effort • (the ANR “HiggsTeV” project) on all these channels: • WH at LPNHE and IPHC • ZH in bb at LAL and CPPM • ZH in llbb at CPPM • and also on common tools: • triggering, b-tagging, jet energy resolution, advanced analysis techniques • (cf. A. Duperrin, S. Greder) D0-France October 2008

  5. On the way to the Higgs: 30 Done (F. Déliot) 0.1 times the branching fractions Similar final state Lower cross section, but Mbbconstraint + more channels + 2 expts D0-France October 2008

  6. The main backgrounds: • QCD multijets (specifically with b-jets: 108times the signal) • lepton misidentification, fake missing ET • estimated from data at DØ, analysis dependent • W/Z + jets • the main background before b-tagging • generic studies in the “V+jets” group at DØ • W/Z + bb • Non-peaking irreducible background (103 times the signal) • generic studies in the “V+jets” group at DØ • t-tbar, single top • Also a source of W bosons and of b-jets • Dibosons, specifically (W/Z)(Zbb) • Neighboring peaking background D0-France October 2008

  7. V+jets studies • Selections: • Well understood triggers • single EM, single muon, diEM, di-muon • Clean, high pT, isolated leptons • Jets within a well understood acceptance • Energy scale corrected • For the simulation: • Trigger turn-ons • Lepton identification scale factors • Energy scale and resolution adjustments for jets The main goal is to validate the simulation D0-France October 2008

  8. Generators for V+jets: • Mostly ALPGEN with MLM matching • interfaced with PYTHIAfor parton showering and hadronization • Alternative generators for comparisons: • PYTHIA • ALPGEN interfaced with HERWIG • SHERPA with CKKW matching ALPGEN generations: (W/Z)+(0n)-lp (all exclusive except n-lp inclusive) Heavy flavors are generated separately: (W/Z)cc and (W/Z)bb + (0n)-lp HF appropriately removed from W/Z + n-lp Real zero bias events overlaid according to the data luminosity profile D0-France October 2008

  9. “A priori” corrections to ALPGEN: • The inclusive W and Z production cross sections are normalized to theory at NNLO • The HF contributions are further increased based on NLO K-factors from MCFM • The Z pT distribution is reweighted to match the DØ measurement (unfolded) Good agreement between data and RESBOS at low Z pT “Rescaled” NNLO OK at high pT ALPGEN reweighted to data at high Z pT ALPGEN reweighted to RESBOS at low Z pT D0-France October 2008

  10. Basic checks in Z events • After Z pT reweighting: • No additional scale factor • needed in the inclusive sample • Jet multiplicity improved • No additional scale factor needed for  2 jets ( 1.2 before reweighting) 1st and 2nd jet pT well modeled (muon channel) Z pT  OK (muon channel) C. Ochando D0-France October 2008

  11. Underway: W pT reweighting No measurement with similar precision as for Z  ee  Rely on theory for the W pT / Z pT ratio Verified that OK at NNLO for a few pT values (Melnikov-Petriello) At the moment, an additional scale factor is needed for W + 2 jets (~ 1.25) Hopefully improved once W pT reweighting is available D0-France October 2008

  12. W/Z + heavy flavors: • Theoretical expectations for HF predictions are on a less solid ground, • e.g., no massive quark calculation with MCFM at NLO. • Determine an additional scale factor from data. This is done, for a given process, e.g., W+2-jets, and within some analysis cuts by comparing the numbers of events with 0-tag, 1-tag, 2-tags, given the known tag rates for b and c jets, and the mistag rate. The results are consistent with unity, but with still large statistical and systematic uncertainties. In the end, the W/Z+HF cross sections are affected by uncertainties of ~ 30%. D0-France October 2008

  13. Further data/MC comparisons: Z  ee / Jet pT > 15 GeV / Detector level PYTHIA PYTHIA v6.314 SHERPA v1.0.6 PYTHIA PYTHIA too soft (As expected) SHERPA SHERPA SHERPA a bit too hard D0-France October 2008

  14. Z  /  1 jet / Jet pT > 20 GeV / Unfolded PYTHIA v6.418 ALPGEN v2.13 +PYTHIA v6.323 ALPGEN v2.13 +HERWIG v6.510 SHERPA v1.1.1 (native showering) Ratios to ALPGEN  + PYTHIA +HERWIG  Z pT: Data in between ALPGEN+PYTHIA and ALPGEN+HERWIG… (No Z pT reweighting applied here) D0-France October 2008

  15. Z  /  1 jet / Jet pT > 20 GeV / Unfolded Ratios to ALPGEN +HERWIG Jet-1 pT and : ALPGEN+HERWIG in fairly good agreement with data D0-France October 2008

  16. W  e /  2 jets / Jet-1(2) pT > 30(20) GeV / Detector level ALPGEN +PYTHIA Work in progress Jet ’s broader than MC (better with SHERPA) Good description of jet pT’s D0-France October 2008

  17. W + b-jets: W (e/)  + 1 or 2 jets with pT > 20 GeV “Ultratight” secondary vertex tagging for high b purity. Fit the vertex mass distribution to b + charm + light templates Measured cross section 3.5 times bigger than ALPGEN prediction… (Investigations are underway) Similar, but less dramatic result in Z+b-jets. D0-France October 2008

  18. The next step: (We/ )(W/Zqq) Similar topology to WH  e/  bb including a mass peak (can’t separate W and Z) except for the presence of b jets Small S/B :4 pb vs 500 pb for W+2 jets 28 fb for WH vs 20 pb for Wbb Similar need for multivariate discriminants • CDF analysis in 1.2 fb1: • One central lepton with pT>20 GeV • MET > 25 GeV • MT > 30 GeV • 2 jets with pT>15 GeV • (j1,j2) < 2.5 • NN trained with 6 angular and shape variables • (little correlation with the dijet mass) • Cut on 6v-NN to improve the significance • Signal fraction enhanced by 50% D0-France October 2008

  19. 3-parameter fit to the dijet mass: • 2 for the background • 1 for the signal fraction • Main systematics: • Background shape, • Jet energy scale and resolution Background subtracted signal Significance: 1.7 • The next steps: • achieve a 3-5 significance in this channel • move to (We/ )(Zbb) • and possibly (Z ee, , ) (Zbb) • The ultimate benchmarks D0-France October 2008

  20. Search for (Zee/)(Hbb) 2 high pT, isolated, same flavor, opposite sign leptons consistent with the Z mass 2 high pT jets, b-tagged B. Calpas • Very clean signal, but low cross section: 2  2.6 fb • Maximize lepton acceptance (loose lepton ID criteria) • Maximize b-tagging efficiency (single and double tag) • Maximize the information used (multivariate discriminants) Backgrounds: • Multijetswith two misidentified leptons and with real or fake b jets • ends up to be very small in spite of huge initial cross section • Z/* + jets, more specificallyZ/*+ bb (2  1.4 pb  B/S ~ 500) • top pairs, mostly(t  b l )(t  b l )(2  70 fb  B/S ~ 30) • dibosons, more specifically(Zll)(Zbb)(2  14 fb  B/S ~ 5) D0-France October 2008

  21. The CDF search in 1 fb1(arXiv:0807.4493) 1st e/:pT > 18 GeV and || < 1 2nd e: pT > 10 GeV if || < 1 or >18 GeV if 1<|| < 2.4 2nd : pT > 10 GeV Isolated within R<0.4 76 < Mee or M < 106 GeV Opposite sign (for electrons: only in CC) Jet1 ET > 25 GeV Jet2 ET > 15 GeV Both with || < 2 One or both b-tagged Fake lepton background from data Fake b-tags from udsg from data Acceptance for mH = 120 GeV: 10.8% D0-France October 2008

  22. No real missing ET expected Correct the jet energies with a NN which uses the MET projections onto those jets The dijet mass resolution is improved from 18% to 11%. Separation of signal from backgrounds with a 2D-NN: ZH vs Zbb and ZH vs tt 8 input variables:  (j1,j2,l1,l2)pT, MET, Mjj, R(j1,Z), R(j2,Z), R(j1,j2), Sph., j2  • Main systematic uncertainties: • Background cross sections (40% for Zbb, 20% for tt) • Zbb shape from PYTHIA vs. ALPGEN • Signal shape: ISR/FSR, PDF, JES • Luminosity, b-tagging (8% for b-jets) 0 1 Anti Z+jets NN before b-tag after a cut on Anti tt NN D0-France October 2008

  23. Results of the search in 1 fb1 Systematic uncertainties increase the expected limit by 14% (12% from b-tagging) D0-France October 2008

  24. Improvements with 2.4 fb1 • Larger integrated luminosity • Looser diEM trigger • (no track requirement) • Looser lepton identification • (isolated tracks for electrons in the gap) • Improved NN for jet energy correction • Loose b-tag in addition to tight • Better Z+jets simulation • 2D-NN re-optimization Most signal-like event D0-France October 2008

  25. The DØ search in 2.3 fb1 • Some differences wrt CDF: • Lepton pT > 15 GeV and || < 2 • Jet pT > 15 GeV in || < 2.5 • 2-loose or 1-tight b-tags Distributions at pre-tag level Electron channel:10v-NN Muon channel: 23v-BDT ICHEP08 results D0-France October 2008

  26. For the other low-mass Higgs search channels, see: • A. Duperrin(Z)(Hbb) • G. Bernardi (We/ )(Hbb) + other less important channels D0-France October 2008

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