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Standard Model Physics at the Tevatron

Quark Confinement and the Hadron Spectrum VII Ponta Delgada, Azores - September 4 th , 2006. Standard Model Physics at the Tevatron. Tommaso Dorigo University of Padova and INFN. Introduction: the Tevatron, CDF and D0 in Run II Higgs boson searches and prospects Top quark physics

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Standard Model Physics at the Tevatron

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  1. Quark Confinement and the Hadron Spectrum VII Ponta Delgada, Azores - September 4th, 2006 Standard Model Physics at the Tevatron Tommaso Dorigo University of Padova and INFN • Introduction: • the Tevatron, CDF and D0 in Run II • Higgs boson searches and prospects • Top quark physics • measurements and prospects • Selected precision electroweak • measurements • Concluding remarks

  2. The Tevatron in Run II • The Tevatron: a proton-antiproton collider at 2 TeV energy in the CM • Massively upgraded with respect to Run I, to increase L by 1.5 orders of magnitude • After bumpy start, the machine is now working excellently • So far delivered more than 1.5 fb-1 /exp • Last week: new lumi record 2.3 E32! • Plan is to run until 2009 and collect 4-8 fb-1 • of data • search for the Higgs boson • measure top mass with 1 GeV error • improve W mass measurement • measure single top production • and a lot more B, QCD, EXO physics

  3. 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…

  4. 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

  5. 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 next year). 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. l

  6. 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 40 GeV accuracy, and the direct search limit already cuts away a large part of the allowed mass region • Latest LEPEWWG results (summer ‘06): MH=85+39-28 GeV, MH<166 GeV @ 95% CL

  7. Higgs Sensitivity WG Predictions Idea: with available data and operating detectors, CDF and D0 could better assess their reach Surprisingly, the new results met or exceeded 1998 Susy/Higgs WG ones. In 2003 the Tevatron chances for Higgs discovery were re-evaluated • Identified keys to success: • mass resolution • improvements; • optimized b-tagging; • merging all search • channels. DESIGN Lum /exp (fb-1) BASE

  8. Can we see dijet resonances ? • A low mass Higgs search entails believing that we can: • - appropriately reconstruct • hadronically- decaying objects • - accurately understand our • background shapes to identify • small S/N signals • All of that can be proven by reconstructing the Zbb decay • Both CDF and D0 now see the Zbb signal • CDF now using it to extract b-JES measurement at <2% • D0 designing new trigger for it

  9. WHlnbb searches To search for WHlnbb decays, events with a leptonic W decay signature are collected. B-tagging suppresses the QCD backgrounds, and Mjj spectra are fit for a signal excess as a function of Higgs mass. D0 extracts a limit of 2.4 pb (95%CL) for MH=115 GeV with 380 pb-1 of data; CDF excludes cross sections above 3.4 pb with 1 fb-1 These limits are above SM cross sections  no exclusion region in Mh yet D0 CDF

  10. ZH llbb and nnbb searches ZH production has a striking signature, both when the Z decays to electrons or muons, and when it produces large missing transverse energy. After b-tagging QCD backgrounds become manageable. A signal can be sought in the dijet mass spectrum, or through a neural network trained to distinguish it from QCD and top 95% CL limits range from 2.2 pb (CDF, nnbb) to 3.4 pb (D0, llbb) for MH=115 GeV

  11. 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 is seen. To search for Higgs decays to the same signature, events with two high-Pt leptons (e,m) and large missing Et are selected; the tt background is rejected with a jet veto. ee and mm channels also get a Z veto Then both experiments use the helicity-preferred alignment of charged leptons in F to discriminate HWW from SM backgrounds

  12. D0 searches for HWW D0 searches for HWW events by selecting two high-Pt, isolated leptons (ee,em,mm), significant missing Et , and little jet activity ee and mm channels also demand an explicit rejection of Z decays with a mass cut (M<80 GeV) The final cut is on the azimuthal angle between the charged leptons, DF<2.0 ee: see 11, expect 11.4 events em: see 18, expect 28.1 events mm: see 10, expect 10.5 events Expected SM H signal is small – and limits still dominated by systs. Will improve soon!

  13. Putting it all together…

  14. Production of top at Tevatron • At the Tevatron, production of top pairs occurs by qq annihilation (85%) or gluon fusion (15%) • NNLO cross section is 6.1 pb 1/1010 collisions 4 events per hour • Single top production is not irrelevant (3 pb), • but its signature is way less discriminant •  Fewer jets, higher QCD backgrounds • main interest: natural determination of Vtb, SM checks (st well predicted)

  15. Top Quark Decays Since Vtb=1andMt>Mb+MW, tWb is dominant mode. Each b quark yields a jet. Final states of top pairs are classified according to the decay of the two W bosons • Measurements exploit mainly three decay channels involving electrons, muons, and jets: • Dilepton (ee,em,mm): B=4/81, S/N ~ 3-10 • Single lepton (e,m): B=8/27, S/N ~2-5 • All hadronic: B=4/9, S/N ~1/6 Top quark decays also constitute an excellent laboratory to study weak interactions of quarks free from QCD effects: • Mt large  Gt =f(Mt3) ~ 1.5 GeV >> LQCD and thus: • t is produced and decays free; • polarization can be studied in the • decay, since the depolarization time • td ~ Mt/L2 is much longer • SM tests have just begun in this sector!

  16. CDF analysis of b-tagged lepton+jets The most precise single measurement of top pair production cross section uses events with a lepton (e, m), missing Et>20 GeV, 3 or 4 jets with Et>15 GeV, and one or two b-tags. In 695 pb-1 of data, 314 events are observed with 1 b-tag (70±7 expected from background sources); 79 with 2 b-tags (9±1 expected from backgrounds) The cross section is measured at 8.2±0.6±1.0 pb. Systematics are now the dominant source Of uncertainty: - b-tag efficiency: 6.5% - luminosity: 6.0% - PDF: 5.8% - JES: 3.0% - ISR/FSR modeling: 2.6% Total: 11.5% limiting factor in comparisons to theory (15% uncertainty) Double tags

  17. Summary of measurements of stt Top pair production is studied at the Tevatron in all significant final states and with different methodologies. The experimental error is by now no larger than the theoretical one (NNLO, Cacciari, Kidonakis – 15%).

  18. Top mass measurements Mt has been measured at CDF and D0 with a variety of different techniques In Run II the goal is now to obtain a precision below 1% per experiment That would mean getting very close to LHC reach and not far to the physical limit of precision with direct reconstruction techniques These days, LHC worry about their jet energy scale a lot – but they should not: the Tevatron is providing a great calibration point with Mt !! To reach the goal of 1 GeV uncertainty further improvements are needed in the treatment of systematic uncertainties, or a luminosity of at least 4 fb-1

  19. Example: CDF l+jets measurement • 166 Candidates with a lepton, four jets (>=1 b-tag) and missing ET are selected from 940 pb-1 of data • A likelihood is calculated for each event using the leading-order ttbar and W+jets differential cross-sections convoluted with parameterized transfer functions that absorb the detector smearing of jet energy measurements: • The jet energy scale (JES) systematic uncertainty is convoluted with the statistical error using an in-situ measurement of the hadronic W boson mass. • The final measured top quark mass and JES systematic is extracted from a joint likelihood of the product of the individual event likelihoods:

  20. The calibration of JES with the W signal allows to reduce the • impact of the scale uncertainty. Other systematics include: • initial state radiation modeling ( 0.72 GeV) • final state radiation modeling (0.76 GeV) • b-jet scale uncertainty (0.6 GeV) • for a total systematics of 1.36 GeV. • Result: • Mt = 170.9 ± 2.2 (stat+JES) ± 1.4 (syst) GeV

  21. Other top physics results Besides measuring top mass and cross section, CDF and D0 have begun to study in detail production and decay characteristics of top quarks Simply impossible to cover the many nice results here Bottomline: all is well in the top sector… These exercises will become powerful investigations in LHC

  22. Production of W and Z bosons • At the Tevatron W and Z bosons are • fundamental tools for calibrations and checks • EM energy scale • Momentum scale in tracker • Studies of the resolution in missing • transverse energy • Studies of the calorimeter response to • low Et hadronic activity (boson recoil) • input to PDF at low x from W charge • asymmetry measurements • Their cross section, known at NNLO (2% precision), may provide an important normalization point to sidestep the luminosity error

  23. Assorted signals W bosons collected by CDF and D0 are now more than a million, and several hundred thousand Z bosons • Besides being a invaluable tool to understand our detectors, W and Z samples are used for several measurements: • W mass (of course!) • Direct measurement of W width • Z PT distribution • Forward-backward W and Z asymmetry • Gauge boson couplings (Wg, Zg, WZ) No time to discuss these! Only quoting a couple

  24. Z boson PT measurement D0 recently issued a new measurement of the Z transverse momentum spectrum, using 960 pb-1 of data with 63,901 Zee decays Z PT not calculable at low PT in perturbation theory, but soft-gluon resummation techniques get finite result  multiple soft gluon effect well testable  model checks allow reduction of systematics in W mass Measurement still dominated by systematics – currently 8% (largest source: lepton ID efficiency dep. on Z PT), will be reduced soon

  25. sW with high rapidity electrons and W production asymmetry • CDF measured the Wen process with forward electrons (1.1<h<2.8) using the ISL and the new plug calorimeter • Important measurement for the determination of the PDF’s (d/u at small x) thanks to the asymmetry in production and decay (PDF, V-A) • Important also to detect effect from soft gluon resummation in low-x production  impact in all LHC cross section measurements Result: sW = 2.874 ± 0.034(stat) ± 0.167(syst) ± 0.172 (lum.) nb. These new W asymmetry data points are an important input to MRST fits

  26. Summary of Results Cross sections for Z (left) and W (right) production have been determined in Run II with all the leptonic final states

  27. Zooming out: 22 years of EW measurements

  28. Conclusions and perspectives • What I showed today: • Run II at the Tevatron has entered a mature stage, and precision measurements are being produced • Mt= 171.4±1.2 (stat) ±1.8(syst) GeV (CDF+D0) • First improvements in MW determinations expected soon • Higgs mass limits above 114 GeV foreseen for end 2007 • EW measurements have a wide variety of implications • The Higgs boson might be at reach of CDF and D0 before LHC experiments start playing the game!! • Other hot topics from the Tevatron: • Many precision QCD measurements and new limits to exotics processes have been obtained with 1 fb-1 statistics • Dedicated triggers and careful analyses have allowed CDF and D0 to produce the first measurement of Bs mixing, as well as a huge variety of other interesting B physics results • Regardless of the Higgs, Run II @ Tevatron has already proven a success – but more is coming!

  29. BACKUP SLIDES

  30. 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

  31. 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 is seen. To search for Higgs decays to the same signature, events with two high-Pt leptons (e,m) and large missing Et are selected; the tt background is rejected with a jet veto. ee and mm channels also get a Z veto Then both experiments use the helicity-preferred alignment of charged leptons in F to discriminate HWW from SM backgrounds

  32. Secondary vertex tagging This event display shows how charged tracks are used to fit for secondary vertices in jets from a ttbar candidate (single lepton decay) Decay lengths for 50 GeV b-jets are typically of the order of a few millimeters and they can be easily reconstructed with tracks having at least 3 associated hits in the silicon detectors (sd is around 20 microns)

  33. Identification of High Pt Leptons Most high Pt final states studied at the Tevatron involve the detection of leptons - easy to trigger on - high signal purity - easy to calibrate using standard candles (W,Z bosons) Tevatron experiments are exploiting to the fullest these signatures, producing lots of precision Electroweak physics measurements with them Tau leptons are also beginning to contribute appreciably, especially to new physics searches which may be generation-dependent CDF D0

  34. Top properties pot-pourri • ggtt/all = 0.25±0.24±0.10 (CDF) • F0 = -0.03±0.06 (CDF), 0.08+0.08-0.05 (D0) • B(Wb)/B(Wq)=1.06+0.27-0.24 (CDF), 1.03+0.19-0.17 (D0) • Z’tt excluded up to 730 GeV (CDF), 680 GeV (D0)

  35. Implications for LHC LHC starts collecting physics data in April 2008 if everything works as it should, the Higgs is discovered by CMS and ATLAS in 2009 (a few fb-1 should suffice) However, fits prefer a Higgs mass in the region favoring Tevatron and hampering LHC…. Let’s hypothesize MH=115 GeV. Three possible scenarios: • Scenario A: Tevatron design, LHC delays  firs hints from CDF e D0 (3s, early 2008) allow LHC to put their chips in the right place  confirmation, common discovery (as did Adone for J/y? Seems improbable… • Scenario B: Tevatron design, LHC in time  Tevatron “confirms” the first signal from LHC • Scenario C: Tevatron base plan (or killed), LHC whatever  you know the story.

  36. A bit of history of Top Quark Quest

  37. Top mass measurements The top mass has been measured by CDF and D0 in all dominant final states with many different techniques Some of the technologies developed to squeeze the most out of the statistics, and lately to address the largest systematics directly, are quite refined. CDF and D0 are consistently doing better than they expected themselves to do in the top mass accuracy From direct kinematical fits, use of LO matrix element and response functions, dynamical likelihoods, internal jet energy scale fits, neutrino weighting, correlations with b-quark flight distance… Everything is tried

  38. Run II: where we are right now Have 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

  39. The Top Quark at the Tevatron • The top quark is a teenager by now! Run I results: • s(tt) =5.7±1.6 pb (D0), 6.5±1.4 pb (CDF) @ 1.8TeV Mt = 178.0±2.7±3.3 GeV (D0+CDF) • many other measurements – but still imprecise – of Vtb, BR, spin; limits to single production, non-SM production and decays. • From the “discovery” mode the Tevatron soon adapted to using top quarks as a perfect pQCD laboratory • As new data pours in, the plan is the same: first, cross section measurements are performed; then the mass, then the kinematics and the search of anomalies, and lastly, the measurement of intrinsic phhysical properties • That modus operandi allows to optimize the output of physics results as analysis tools get perfected and more sophisticated: • high- Pt lepton identification • b-tagging • precise measurement of jet energy scale

  40. Cross section measurements • Top quark pairs are now extracted by CDF and D0 with a multitude of methodologies from their datasets, in the three dominant final states (dileptonic, single lepton, all hadronic) • Tens of different measurements of production cross section • Sensitive to different experimental systematics – common: luminosity error • The different datasets provide a view to top quark properties (discussed later) • The most precise stt measurements come from the analysis of single lepton decays  compromise between S/N and yield • Secondary vertex tagging of b-quark jets still most powerful handle • Both CDF and D0 use MC to estimate physical backgrounds and data to handle false b-tags • “W+1 jet” and “W+2 jet” events are used to verify sample composition and yield

  41. Jetclu and Midpoint

  42. The CDF Kt algorithm

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

  44. Improving the dijet mass resolution • One of the keys to a successful extraction of the H signal is to increase the dijet mass resolution for pairs of b-jets • Standard CDF/D0 jet correction algorithms tuned for best scale determination, not for best resolution • H1 algorithm: use tracker to measure Pt of charged component • in HSWG studied prototype of b-specific correction using identified muons, Et dependence of had response on top of H1 • Also developed advanced algorithm to account for subtle correlation among satellite observables and jet Et measurement • Global result is that sM/M=10% is achievable in central calorimeter

  45. Study of V-A in the decay of quarks free from strong interactions

  46. W decays and lepton Pt

  47. Measuring Jets In hadronic interactions, jets of hadrons are the most common things one can observe They are common, but are they obvious to define ? “Obvious: something you may think about for 20 years and maybe understand” After 20 years of studies of pQCD, we think we understand what is going on… What we measure in our detectors is the combination of a multitude of effects Disentangling them is the key to understanding each of them better

  48. Identification and measurement of hadronic jets Both CDF and D0 mainly use a cone algorithm (R=0.4 or 0.5) to identify localized depositions of energy in their calorimeters and measure hard partons • When faced with the measurement of the kinematics of hard • parton emissions, one has to deal with two distinct issues: • SCALE: to calibrate the energy response, to minimize the average • measurement error on a sample of jets • RESOLUTION: to improve the precision of the energy measurement, decreasing the measurement error on an individual jet • The first issue is fundamental for precision mass measurements • of hadronically decaying objects (e.g. top quarks) • The second issue is critical for the successful identification of • low S/N signals (e.g. Higgs bosons)

  49. Calibration of Jet Energy • To calibrate the energy measurement in CDF we use a detector-dependent correction, a scale correction, and a treatment of additional small physical effects • eta-dependent correction  dijet balancing • multiple interaction correction  f(Nvtx) • absolute scale correction: E/p of single tracks are used to tune the MC, which is then used to derive “calhad” corrections. • last, out of cone and underlying event corrections are made • Systematic errors reduced to 3% (data/MC comparisons, g-jet balancing) • Calorimeter stability, MC (fragmentation, simulation of single particle resp.) • Understanding of out-of-cone radiation and UE • Simulation of response function versus jet rapidity • D0 has an almost-compensated calorimeter (e/p <1.05, linear with energy ); disuniformities and gaps among cryostats need to be corrected • EM part is calibrated with Zee decays • U noise measured in situ; other offset corrections address pile-up (energy from previous interactions) and underlying event • Response is measured as a function of rapidity and Et with gamma-jet events • showering correction: Et flux vs DR off jet cones

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