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High pt Physics

High pt Physics. Study of high pt SM processes: Top quark, W and Z production and decay features Direct searches for new physics: High mass objects  high pt EWK symmetry breaking couples to vector bosons and highest mass leptons/quarks Most promising signatures involve studying: Top quark

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High pt Physics

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  1. High pt Physics • Study of high pt SM processes: • Top quark, W and Z production and decay features • Direct searches for new physics: • High mass objects  high pt • EWK symmetry breaking couples to vector bosons and highest mass leptons/quarks • Most promising signatures involve studying: • Top quark • Tau leptons • W and Z bosons • Final states with high pt leptons, jets and missing transverse energy F. Bedeschi, INFN-Pisa

  2. Top-quark Top quark production and decay F. Bedeschi, INFN-Pisa

  3. Top-quark • t lepton discovery at SLAC in 1975 •  third generation of quark and leptons! • Much indirect evidence for the existence of a nt • Direct observation only in 2000 at Fermilab • b-quark discovered at Fermilab in 1977 • top-quark needed by SM to complete the third generation • At the end of the 1970’s the hunt for the top quark had started • Discovered at Fermilab in 1995 by the CDF and D0 collaborations after 25 years F. Bedeschi, INFN-Pisa

  4. Top quark • Summary of top searches • Direct • Indirect mass (LEP) vs time Discovery 1995 F. Bedeschi, INFN-Pisa

  5. W q b t W g q t b Top quark • How do we recognize a top quark? • Pair production of very heavy object • Decays into a W and a b-quark (b-jet) • Topology of final state (e.g. number of high energy leptons and jets) depends on W decay • Wln • Wq1q2 F. Bedeschi, INFN-Pisa

  6. Top quark Both W decay e or m leptons Easy • Top decay configurations One of the W decays to t Very difficult One W decays to e or m leptons and one to jets Medium Both W decay into jets Difficult F. Bedeschi, INFN-Pisa

  7. Top quark • Most results from lepton + jet channels • Select events as follows: • Look for semileptonic W decays: • High Et lepton • Missing transverse energy • Request at least 3 additional energetic jets • At least 1 of the jets is b-tagged (i.e. appears to contain a B hadron) B hadrons from top decay travel for a few mm before decaying. These decays can be observed with an accurate tracking system. Other b-tagging techniques use leptons, their S/N is much worse. F. Bedeschi, INFN-Pisa

  8. Top quark • An impressive top candidate event from Run 1 F. Bedeschi, INFN-Pisa

  9. Top quark • Our biggest nightmare • Will we observe the top again with the new Run II data …. ? F. Bedeschi, INFN-Pisa

  10. Top quark • Yes !!! • 15 b-tagged l+jet events (57 pb-1) • 5 di-lepton events (72 pb-1) F. Bedeschi, INFN-Pisa

  11. Top quark q l Background W n q b • Top x-sections: • Key issue: • How many candidates are from top and how many are background? • Rather accurate NNL theoretical calculation • Data confirm SM predictions b F. Bedeschi, INFN-Pisa

  12. Top quark • Run II x-section: • CDF: l+jets • 5.3 ± 1.9 ± 0.8 ± 0.3 (lum) pb • D0: di-lepton, l+jet combined • 8.5 ± 4.0 ± 4.9 ± 0.8 (lum) pb • Good consistency with run 1 and theory expectations CDF-II F. Bedeschi, INFN-Pisa

  13. q b q p W t n p W l t b Top quark • Measuring the top mass • Use lepton+jet sample • All kinematics known, but: • Pzn • 3 constraints: • M(ln) = M(qq) = MW • M(lnb) = M(qqb) • 2C kinematic fit • Combinatoric ambiguity • 2 combination if double b-tag • 6 combination if single b-tag • Gluon radiation can give extra jets! • MC to check/correct for systematic effects F. Bedeschi, INFN-Pisa

  14. Top quark • Systematics mostly from: • Jet energy scale 4.4 GeV • background 1.3 GeV • Gluon radiation 2.6 GeV • Top mass measurement Run 1 CDF+D0: Mtop = 174.3±5.1 GeV CDF W + jets Mtop = 175.9±4.8±5.3 GeV F. Bedeschi, INFN-Pisa

  15. 168.4  12.8 GeV D0 ll PRL 80, 2063 (1998) 173.3  7.8 GeV D0 lj PRD 58, 52001 (1998) 172.1  7.1 GeV D0 combined 167.4  11.4 GeV CDF ll PRL 82, 271 (1999) 176.1  7.2 GeV CDF lj PRL 80 2767 (1998) 186.0  11.5 GeV CDF jj PRL 79, 1992 (1997) 176.1  6.6 GeV CDF combined 174.3  5.1 GeV Tevatron FERMILAB-TM-2084 150 160 170 180 190 (GeV) m t Top quark • Summary of mass measurements at the Tevatron Tevatron indirect F. Bedeschi, INFN-Pisa

  16. Top quark Invariant mass from untagged quarks calibrates light q energy scale and gluon radiation (FS) • How much better can we do in Run II? Per experiment Similar for di-leptons Use Zbb to calibrate b-jet energy scale dmH/dmt~ 50 GeV/4 GeV F. Bedeschi, INFN-Pisa

  17. Top quark • Run II mass measurements • Still understanding energy scale (largest systematics) • Consistent with run 1 • Expect: x ± 9 ± 7 very soon Pretag sample F. Bedeschi, INFN-Pisa

  18. Top quark • Summary: • Top quark is an essential element of the SM • Discovered at the Tevatron after a long search in many labs • Production x-section and mass are measured and are consistent with SM • First run II results are consistent with previous findings and will improve on accuracy once enough luminosity is acquired and work on systematics completed. F. Bedeschi, INFN-Pisa

  19. Weak bosons WEAK BOSONS F. Bedeschi, INFN-Pisa

  20. q W g q’ q p q W g q’ Weak bosons p q l • Large W and Z bosons production at Tevatron • In 100 pb-1 (Run 1) • NW ~ 50,000 observed • NZ ~ 5,000 observed • Use leptonic decays to overcome large di-jet background W(Z) n (l) Can be produced with additional jets F. Bedeschi, INFN-Pisa

  21. Weak bosons • Z selection: • 2 high momentum electrons (or muons) • Leptons are same type and opposite charge • Leptons are isolated (small isolation) • Isolation = energy contained in a cone around lepton direction • Cut on di-lepton invariant mass Run 1 • Additional Z bosons (Z’) would appear as additional peaks in plot above • Deviations from SM could indicate quark/lepton compositeness or the presence of other new physics F. Bedeschi, INFN-Pisa

  22. Z’ 95% CL mass limits (GeV) • Run II searches consistent with SM • Need more data to improve over run 1 D0 50 pb-1 F. Bedeschi, INFN-Pisa

  23. Using t’s • Better t in run II • Tune t ID methods with W and Z’s • Tracks point to narrow jets • Isolation • Use later in searches (e.g. SUSY) F. Bedeschi, INFN-Pisa

  24. q q l+ */Z l– e- p q* p e+ Weak bosons • Z’s interfere with g* and cause asymmetry in electron angular distribution • Asymmetry defined in di-lepton CM • AFB = (electrons in proton hemisphere – electrons in p-bar hemisphere)/total • Asymmetry is very sensitive to the presence of additional Z’ • Run 1 measurements consistent with SM, but interesting fluctuation at high mass • No effect observed in Run II Run 1 F. Bedeschi, INFN-Pisa

  25. Weak bosons • W selection: • High energy electron (or muon) • Lepton is isolated • Large missing ET • MET = |vector sum of all calorimeter energy in transverse plane|  ~ ETn • Jacobian peaks: • Both lepton ET and MET peak at about ½ the W mass • Expect correlation between ET of lepton and neutrino F. Bedeschi, INFN-Pisa

  26. Weak bosons • W and Z cross sections can be calculated to high accuracy in SM (~5 %) W NB: 6% difference in CDF-D0 relative luminosity normalization Z F. Bedeschi, INFN-Pisa

  27. W/Z x-sections • Grand summary: great SM consistency! F. Bedeschi, INFN-Pisa

  28. Weak bosons t H W W W W b • W mass • Fundamental SM parameter • Related to Mtop and MHiggs • Good measurement of W and top masses tells us about the Higgs mass! • Measurement • Fit ETl (D0) or MT (CDF) distributions • Shape depends on: • Energy scale • W production model • Gluon radiation • Many other subtle effects! M2T = 2pTlpTn(1-cos(Dfl,n)) F. Bedeschi, INFN-Pisa

  29. Weak bosons • Summary of Run 1 results and comparison with LEP (July 2003 update based on old results) F. Bedeschi, INFN-Pisa

  30. Weak bosons • Indirect vs direct measurements • Lower Higgs mass favored F. Bedeschi, INFN-Pisa

  31. Weak bosons • Where is the Higgs? • Global fits assuming SM yield: • MH = 85 +54/-34 GeV • MH < 199 GeV @ 95% F. Bedeschi, INFN-Pisa

  32. Weak bosons Run II W mass expectations for the W e n channel • W mass • Most systematics scale with luminosity • E.g. size of Z control sample • s(MW) ~15 MeV/c2/experiment expected using all channels Run II Linear colliders dmH/dmW~ 50 GeV/25 MeV F. Bedeschi, INFN-Pisa

  33. Weak bosons • Summary • W and Z bosons can be studied with high statistics at Tevatron • Production x-section measurement are consistent with the SM expectations to ~5% • Drell-Yan spectrum and FB asymmetry does not indicate any higher mass Z bosons • W mass measurement consistent with e+e- direct and indirect results • Higgs mass is expected to be small: • Tevatron has a serious chance for discovery if it manages to get enough luminosity F. Bedeschi, INFN-Pisa

  34. Higgs boson Higgs boson search F. Bedeschi, INFN-Pisa

  35. Higgs search • The Higgs boson is the last SM particle still to be found • It has a fundamental role in the SM to generate the masses of the W and Z bosons, and of the fermions • However one could imagine more complex mechanisms than the basic SM Higgs, pointing toward new physics Prof. Peter Higgs F. Bedeschi, INFN-Pisa

  36. g t H t g t Higgs search • Light (100 – 200 GeV) Higgs production: • Higgs couplings prefer higher masses • Main production mechanisms: • Virtual top quark loops • Associated W/Z production s ~ 1.0 – 0.1 pb H • ~ 0.5 – 0.02 pb sW ~2xsZ q W*/Z* q W/Z Cfr. Top quark s ~ 5 pb F. Bedeschi, INFN-Pisa

  37. Higgs search • Light Higgs decay: • Higgs prefers the heaviest particles kinematically available • bb dominant mode up to MHiggs ~ 135 GeV • WW pair production takes over beyond that F. Bedeschi, INFN-Pisa

  38. Higgs search V = W, Z • Light Higgs experimental signatures: • MH < 140 GeV: • Use associate HV production (too much bck to ggHbb from gggbb): • assume H bb • Different signatures depending on W/Z decay modes • Signatures: • ZH  l+l- bb cleanest, but low rate • ZH  nn bb • WH  l±n bb • V H  qq’ bb very bad S/N • Higgs appears as bump in bb invariant mass F. Bedeschi, INFN-Pisa

  39. Higgs search • An example of how a 120 GeV Higgs discovery plot could look like with 2 experiments and 15 fb-1 of data Excess over bck. due to Higgs Good jet energy resolution is critical Sum of all channels F. Bedeschi, INFN-Pisa

  40. Higgs search V = W, Z • Light Higgs experimental signatures: • 130 GeV < MH < 200 GeV: • Use associate HV production and direct H production • assume H WW* • Associate production signatures: • VH  V WW*  l+ l+ l- Cleanest, but low rate • VH  V WW*  l±l± jj • Direct production signatures: • HWW*l+l- nn Highest rate F. Bedeschi, INFN-Pisa

  41. Higgs search • We have looked in our Run 1 data! • Far from required sensitivity CDF Run 1 data F. Bedeschi, INFN-Pisa

  42. Higgs search • Sensitivity re-evaluated recently (June 2003) 8.56 fb-1 design plan CAUTION! 4.41 fb-1 base plan F. Bedeschi, INFN-Pisa

  43. Higgs search • Summary • If Higgs is really light there is a chance of discovering it at the Tevatron • It is going to be a lot of difficult work • The quality of the final result will depend much on the total integrated luminosity delivered by the Tevatron F. Bedeschi, INFN-Pisa

  44. Other searches Our example • We think the SM is an effective theory and many models could extend it: • SM on larger gauge groups, SUSY, technicolor, extra dimensions, quark/lepton compositness…. • Many new particles could be discovered: • Additional vector and Higgs bosons, SUSY particles, technicolored particles, various species of gravitons, leptoquarks, excited fermions, prions, …… • Huge parameter space to explore • Look for deviations from SM • Use theoretical models as guide for exploration • More than ever observations can guide the theory!!! F. Bedeschi, INFN-Pisa

  45. SUSY • Supersymmetry is needed in many theoretical contexts: • Supersymmetric bosons (fermions) associated to SM fermions (bosons) • In most models new conserved R-parity quantum number • SUSY particles are pair produced • Lightest SUSY particle (LSP) is stable and very weakly interacting • Tevatron could discover or place serious bounds on SUSY Expectations based on 2 fb-1 F. Bedeschi, INFN-Pisa

  46. SUSY • SUSY searches (1) • Look for hadronic decays: • Charginos & heavier neutralinos eventually decay to quarks and neutral LPS • Signature is  3 jets +MET • Requires accurate study of SM backgrounds • Run 1 results • Run 2 work in progress F. Bedeschi, INFN-Pisa

  47. SUSY • SUSY searches (2) • Search for 3 high pt leptons + MET • No events observed by CDF & D0 in Run 1 and 2 • Use of t’s can significantly increase sensitivity if tan b is large An ee candidate from run II F. Bedeschi, INFN-Pisa

  48. SUSY - Higgs • Could be easier than SM: • 4 b final states are very strong signature • Do no need associated W/Z if tan b is large Run 1: Excluded regions at the 95% C.L. Mh vs tan g2 ~ 1/cos2(b)=1+tg2b  = h, A, H F. Bedeschi, INFN-Pisa

  49. Conclusions • Physics at the Tevatron is extremely varied and can test SM from many points of view • Precision measurements and direct searches • New data collected is of high quality • Building up statistics and basic analysis tools for the more sophisticated analyses • The new run is expected to deliver up to 100 the integrated luminosity of Run 1 with greatly improved detectors • Chances of discoveries are high even excluding the Higgs • Many interesting measurements are guaranteed • Highest energy hadronic collider for several years before LHC turns on … great place to do exciting physics now and be prepared for the future when it comes F. Bedeschi, INFN-Pisa

  50. Backup Slides F. Bedeschi, INFN-Pisa

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