Searches for the SM Higgs Boson at the Tevatron Collider

1. Moriond QCD – March 16th, 2005 Searches for the SM Higgs Boson at the Tevatron Collider Contents: Introduction to Run 2 at the Tevatron Higgs Sensitivity WG predictions Hbb decay searches HWW decay searches Combined limits & conclusions Tommaso Dorigo, University of Padova and INFN Representing the CDF and D0 Collaborations

2. Tevatron Run 2 – Quick Overview Tevatron is performing well – delivered 800 pb-1 so far. Lstart above 1032 now common. L has been following design curve! Upgrades continuing – electron cooling of antiprotons is critical. As L increases, CDF and D0 catching up by modifying trigger tables, improving DAQ Design curve means 8 fb-1 by 2009! WE ARE HERE

3. e n q W W* H q b b SM Higgs: Production and Decay At the Tevatron, about five 120 GeV Higgs bosons are produced in a typical day of running (will be 15/day in two years). Direct production occurs mostly via gluon-gluon fusion diagrams. Associated production through a virtual W or Z boson provides sensitivity in the region where LHC will have more trouble. At higher mass, the WW(*) final state becomes dominant. Even the WHWWW(*) process is promising despite the low yield, due to the striking signature of missing Et plus three leptons, two of which may be of the same charge but different flavor. l

4. What we know about the Higgs • Although they did not directly observe it, the LEP experiments have collected a wealth of information on the Higgs boson through comparisons of EW observables to EW theory + radiative corrections • From theory we know its couplings, its decay modes, and how its mass impacts the W and top masses. • If it exists, then we know its mass with about 60 GeV accuracy, and the direct search limit already cuts away a large part of the allowed mass region • Latest LEP results: MH=126+73-48 GeV, MH<280 GeV @ 95% CL (Winter ‘05).

5. Higgs Sensitivity WG Predictions In 2003 the Tevatron chances for Higgs discovery were re-evaluated CDF Idea: with available data and operating detectors, can better assess Tevatron reach Surprisingly, the new results meet or exceed 1998 Susy/Higgs WG ones. Lum (fb-1) DESIGN • Keys to success: • mass resolution improvements; • - optimized b-tagging; • - shape information vs counting. BASE

6. Identification of High Pt Leptons Most final states produced by Higgs decay involve high-Pt leptons. CDF and D0 have efficient lepton triggers and high purity ID selections A host of electroweak measurements is being produced at the Tevatron. Tau leptonsare also starting to contribute appreciably to precision measurements (but no results for SM H searches with these yet). CDF D0 CDF

7. Tagging b-jets D0 Identifying b-jets is of paramount importance for low-mass Higgs boson searches. Three methods are well-tested and used: • Soft lepton tagging • Secondary vertex tagging • Jet Probability tagging For double tag searches, efficiency factors get squared! To retain signal, both CDF/D0 have loose and tight tagging options Efficiency drops at low jet Et and high rapidity but is 45-50% for central b-jets from Higgs decay Mistag rates are kept typically at 0.5% Tight/loose SV tag eff. SV tagging: tracks with significant IP are used in a iterative fit to identify the secondary vertex inside the jet CDF I.P. B

8. Can we see dijet resonances if they are there? • A low mass Higgs search entails believing that we can: • - appropriately reconstruct hadronically-decaying objects • - accurately understand our background shapes • All of that can be proven if we see the Zbb decay in our data. The S/N is not higher than 1/5 at the most in the signal region • good testing ground for H! • can use to test/improve dijet mass resolution with advanced algorithms We barely saw it in Run 1… Can we use it in Run 2 ??

9. CDF sees Zbb decays in Run 2! Double b-tagged events with no extra jets and a back-to-back topology are the signal-enriched sample: Et3<10 GeV, DF12>3 Among 85,784 selected events CDF finds3400±500 Zbb decays - signal size ok - resolution as expected - jet energy scale ok! This is a proof that we are in business with small S/N jet resonances! CDF expects to stringently constrain the b-jet energy scale with this dataset

10. Low Mass H Searches The only chance to see Hbb at the Tevatron is through associated production with bosons ZHllbb is the cleanest signature, but it yields too few events W/ZHjjbb has the lowest S/N but the high BR helps at larger Higgs mass WHlnbb is next-to-best, but CDF was “unlucky” in Run I The best channel is ZHnnbb CDF has anew combination of Run 1 resultswith ZHllbb, nnbb channels. They search events with two jets with DF<2.6, missing Et>40 GeV, no isolated track with Pt>10 GeV. The limit is obtained by a fit to the mass distribution of b-tagged events. The Run 1 CDF limit is now at 7.2 to 6.6 pb for MH=110 to 130 GeV.

11. New search for WH in Run 2 To search for WHlnbb events a detailed understanding of the composition of the W+jets sample is mandatory. In the 2-jet bin CDF finds62 eventswith a b-tag, where66.5±9.0 are expected, mostly from Wbb production and mistags. A fit to the dijet mass distribution allows to extract a95% CL limit of 5 pbto SM WH production. The obtained limit is consistent both with a priori predictions and with expectations based on HSWG results.

12. WH Search in D0 D0 also study their W+2jet bin with b-tagging in 174 pb-1 of high-Pt leptons from Run 2 data. They find 76 events with one b-tag (exp. 72.6±20.0), 6 with two b-tags (exp. 4.4±1.2). The dijet mass distributions show no anomaly with 1 b-tag. The 2-tag distribution is divided in search windows to set limits to Higgs production. 95%CL limits on sWH*B(Hbb) are set at 9-12 pb for MH=115-135 GeV By-product:a95% CL limitis set toWbb production(DR>0.75, Pt>20 GeV)at 6.6 pb.

13. e+ n W+ n W- e- High Mass Searches: HWW(*) The SM production of WW pairs has been measured by CDF in Run 1 and by both CDF and D0 in Run 2: excellent agreement with NLO. To search for Higgs boson decays, events with two high-Pt leptons (e,m) and large missing Et are selected; the tt background is rejected with a jet veto. Then both experiments use the helicity-preferred alignment of charged leptons in F to discriminate known backgrounds.

14. CDF results on HWW CDF searches for HWW events by selecting two tight leptons (ee,em,mm) with Ete(Ptm)>20 GeV and missing Et>25 GeV (50 GeV if DFll<20°). A strict jet veto (Et<15 GeV if |h|<2.5) rejects top candidates. Finally, a small dilepton mass is required (Mll<55-80 GeV for MH=140-180 GeV). 8 eventsare observed in 184 pb-1 of Run 2 datawith the Mll <80 GeV cut,with an expectedbackground of 8.9±1.0. A likelihood fit to the DFll distribution is performed to extract a limit on the HWW cross section as a function of its mass. The result is sHWW*B(WWllnn)>5.6 pb for MH=160 GeV.

15. D0 Results on HWW D0 also searches for HWW decays by selecting events with two oppositely charged leptons (ee,em: Pt>12,8 GeV; mm: Pt>20,10 GeV), missing Et>20 GeV (30 for mm), and imposing a loose jet veto (Et<90 GeV, or Et1,Et2 <50,30 GeV). The azimuthal angle DFll between the two leptons is then required to be less than 1.5 for electron pairs (2.0 for the em,mm combinations). Combining the three channels they find 9 events, when 11.2±3.2 are expected from background sources in 177 pb-1 of Run 2 data. They can thus excludes*B>5.7 pb at 95% C.L. for MH=160 GeV.

16. WHWWW(*) Search CDF also searches for the striking signature of three W bosons in 193.5 pb-1 of Run 2 data. First, the dataset with a lepton with Pt>20 GeV and a second with Pt>6 GeV of same charge is analyzed and found in agreement with expectations. Then, optimized cuts are applied to the second lepton (e.g. Pt>18 GeV for MH>160 GeV) and on the vector sum of leptons transverse momenta (Ptll>35 GeV). Zero events are observed, when 0.95±0.61± 0.18 are expected from known sources. 95% CL limits are thus set at 12 (8) pb for MH=110 (160) GeV.

17. Putting it all together…

18. Summary and Outlook The Higgs boson is being hunted at the Tevatron in all advantageous search channels. D0 and CDF are competing – that’s good! – but will soon start to also combine their results. No surprises with the analyzed 200 pb-1 samples, but we have already three times more data on tape to look at! We are on track to supersede the LEP2 lower limit on MH in time for Moriond QCD 2007! By the end of 2009, the Tevatron might be able to see a MH=115 GeV Higgs at 5s, or exclude it all the way to 180 GeV. …but that will require both cunning and the Tevatron delivering according to the design plan! What I feel I can promise at 95% CL: exclusion up to 135 GeV, 3s evidence at 115 GeV.

19. CDF and D0 in Run 2

20. The CDF Detector CDF significantly upgraded from Run 1: • New L00+SVX+ISL silicon detector • New central tracker • Extended muon coverage to |h|<1.5 • New end-plug calorimeters • SVT measures IP to 45 mm at Level 2! The challenge is now a smooth operation for many years of running…

21. The D0 Detector • Massively upgraded from Run 1 to include: • 77,000 ch scintillating fiber tracking • 2.0 Tesla solenoid • 800,000 channel silicon detector • (4 barrel layers, 2-sided disks) • Extended muon coverage (MDT) • Tracker working well despite low • volume (R=1/3 RCDF) • High performance b-tag to |h|<2.0

24. Design for 2005 Base for 2005 2005 : so far along Design 2004 2003 2002

25. CDF: a WWeenn candidate CDF: a WWemnn candidate