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High pt Physics

Discover the study of high-PT SM processes like top quark, W/Z production, and new physics searches involving high-mass objects. Learn about the direct observation and significance of top quark in the Standard Model. Explore experimental techniques, results of top searches, and mass measurements at the Tevatron. Current advancements and prospects for Run II data are also discussed.

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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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