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Joe Incandela University of California, Santa Barbara

An alternative method for measuring the top quark mass: (mining the possibilities provided by a new detector). Joe Incandela University of California, Santa Barbara. B decays in top events. For top the average B decay length is large

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Joe Incandela University of California, Santa Barbara

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  1. An alternative method for measuring the top quark mass: (mining the possibilities provided by a new detector) Joe Incandela University of California, Santa Barbara

  2. B decays in top events • For top the average B decay length is large • cto~0.5 mm and typical relativistic boost factor ~10 expect around ~0.5 cm • Can one get the top mass from the decay length? • Short Answer: Yes if you have lots and lots of events - CDF Note 1921, Dec. 1992 • CDF Run 1 • During and just after construction of the CDF silicon detector for Run 1b, a small group of us had a lot of fun thinking about novel ways it could be used for important physics • Mainly R. Culbertson, D. Glenzinski, JI, R. Snider and D. Stuart • Measuring the top mass with the b decay length was an idea from that period that I later completely forgot about • It came back to mind last year, and C. Hill led the push to see if it might still be interesting • For fun, I will rewind and also cover some of the other ideas from that time, and show where they led. J.R.Incandela - Brookhaven HEP Seminar

  3. SVX The first identified top event • Run 1b CDF • 4 layers 2.9 to 8 cm • 60 mm pitch • Constructed 1991-93 J.R.Incandela - Brookhaven HEP Seminar

  4. Silicon and B tagging • SVX (Run 1a) and SVX’ (1b) had great Impact Parameter (IP) resolution. • We spent a couple of years learning how to tag b jets • From the death of the SVX (Run 1a) we learned that vertex b tagging could even tolerate low S/N on the critical inner layer J.R.Incandela - Brookhaven HEP Seminar

  5. “Standalone” Si Tracking in SVX c. 1993 J.R.Incandela - Brookhaven HEP Seminar

  6. Standalone Tracking With SVX • Worked surprisingly well! • Most tracks matched to regular tracking • Those that did not were exactly as expected given the limited coverage of the central tracker (CTC) • Prompt fraction as expected What more could we do with this new detector? J.R.Incandela - Brookhaven HEP Seminar

  7. Delta Rays in Cosmic Rays J.R.Incandela - Brookhaven HEP Seminar

  8. Delta’s in pp collisions J.R.Incandela - Brookhaven HEP Seminar

  9. -dE/dx and CHAMPS p 200 GeV CHAMP(MC) d Sensitive to charged particles with bg ≲ 0.85 data J.R.Incandela - Brookhaven HEP Seminar SEARCH FOR LONG LIVED CHAMPS D. Stuart et al. Phys.Rev.Lett.90:131801,2003

  10. What about forward lepton id and b tagging? • 5 cm lever arm • is not enough • pT resolution poor extrapolation of tracks not precise • impact parameter resolution not good enough to help in b tagging • Not a complete loss • Actually got ~5% gain in b tagging • In combination with EM calorimetry we could do a limited amount of forward electron id • What we needed was one more layer at ~20 cm radius! J.R.Incandela - Brookhaven HEP Seminar

  11. A large radius Silicon layer • So we suggested a 5th layer be added to the SVX II (Run 2 upgrade) at large radius (20 cm) • The committee report supported a 5th layer … At 10 cm! We lost this round… J.R.Incandela - Brookhaven HEP Seminar

  12. ISL • CDF planned to build a fiber tracker to bridge from the silicon to the outer straw tubes • Both the fibers and the straws got cancelled ! • Reviews in 1996: many concerns • JI chaired straw tube review – recommended the COT • D. Stuart chaired the fiber tracker review • He came up with a way to optimize the readout of a silicon system in this region - made it affordable!! • JI led an engineering design team and provided all standalone tracking tools to A. Yagil to study the performance in simulation. We completed a full technical design report in three weeks. • The IFT was replaced by the Intermediate Radius Silicon (IRS) - later renamed the ISL • Main purpose of ISL, at least for D. Stuart and I, was increased coverage for leptons and b tagging to go after new physics, top, and higgs. J.R.Incandela - Brookhaven HEP Seminar

  13. J.R.Incandela - Brookhaven HEP Seminar D. Stuart in the ISL during final testing in late 2000

  14. B tagging 50 % of b daughter tracks in top decays have pT 3 GeV Mtop=175 • We learned that b jets from top get tagged efficiently by finding displaced tracks below a few GeV • But… SVX II had lots of material (front end electronics) on the innermost layer causing multiple scattering • low momentum tracks would be poorly resolved • Needed a fix… proposed ANOTHER silicon layer at ~1.5 cm • “How do you fix a problem of too much material by adding more material?” – L. Nodulman at the very first presentation of the idea of putting a layer of silicon right on the beam pipe (Dec. 9, 1997) J.R.Incandela - Brookhaven HEP Seminar

  15. IP Resolution* The cure - a beampipe Layer ! • A non-Functioning Layer 00 increases rms by only ~1-2 mm at 1 GeV. J.R.Incandela - Brookhaven HEP Seminar *Assumes layer at 1.5 cm and  8 mm hit resolution.

  16. Si on the beam pipe J.R.Incandela - Brookhaven HEP Seminar

  17. Layer 00 The CDF II Si Tracker J.R.Incandela - Brookhaven HEP Seminar • All silicon is p/n • All layers except Layer 1 are double-sided silicon • 90 degree: Double Metal

  18. What are we up to now... • We expect to find new physics at the LHC which is now under construction. • Meanwhile, • We’re preparing for the LHC era in our work at the Tevatron • Direct searches • Indirect limits • Experimental lessons • New Methods • And we are building lots of silicon for the LHC as well… • Top quarks are a good basis • Fertile ground for all of above • I’ll present some recent work we’ve done at UCSB • top quark studies • new tools and ideas • relevance for new physics J.R.Incandela - Brookhaven HEP Seminar

  19. Run II of the Tevatron • Substantial upgrade to accelerator • s = 1.8 ! 1.96 TeV • 6 £ 6 ! 36 £ 36 bunches • Peak luminosity increased (currently »1.0 x 1032 cm-2sec-1) • Corresponding overhauls of CDF and DZero experiments • Cope with higher collision rate • To improve physics capability J.R.Incandela - Brookhaven HEP Seminar

  20. Problems with the SM Hierarchy Problem: the fundamental scale is the Planck scale ~ 1019 GeV What is the underlying reason for EWK symmetry breaking and why at such low energy ? Fermion and Higgs Masses ? What determines them? Gravity ? How to reconcile with Quantum Mechanics? Fundamental Scalar Theories are Fundamentally pathological Quadratic divergences Replacing the SM: SUSY has a following (Requires  2 Higgs doublets) Superpartners cancel divergent terms in MH As a local symmetry  spin-2 graviton appears Appears in string theories Yields gauge coupling unification at ~1016 GeV if there are exactly 2 higgs doublets (+ singlets) But there are other possibilities like Large Extra Dimensions None of the above? All of the Above? Beyond Standard Model J.R.Incandela - Brookhaven HEP Seminar

  21. Possible New Physics in Dileptons? • Dilepton events with large missing energy • Highly visible if they are there: First place to see Supersymmetry? e.g. Pair production of stop the superpartner of top • Large due to (LSP) • Massive • Non-interacting if R-parity is conserved J.R.Incandela - Brookhaven HEP Seminar

  22. Slightly Provocative Run 1 Result: Dileptons • 9 events observed on a background of 2.4 • 7 were e • 4 had > 100 GeV • tt = 8.2 +4.4-3.4 • SM expectation • tt = 5.2 at √s= 1.8 TeV • A Simple Counting experiment • Assumed excess above background = top • Valid assumption or could there be new physics? • Dilepton signature mimicked by many processes in extensions to the SM J.R.Incandela - Brookhaven HEP Seminar

  23. Calorimeter Seeded Silicon Tracking • Start with EM calorimeter cluster centroid + primary vertex – Use energy to get curvature 2 seed tracks • Project into silicon and attach hits using standard pattern recognition • Select best 2match and refit • When  an ISL hit, results are excellent J.R.Incandela - Brookhaven HEP Seminar

  24. Si tracking of forward electrons • Drell-Yan sample with both EM clusters forward (no drift chamber tracking) • One leg with Silicon tracking • Two legs with Silicon tracking J.R.Incandela - Brookhaven HEP Seminar Forward lepton id vastly increases the acceptance for multi-leptonic final states. (D. Stuart et al. using this to search for massive Z’)

  25. Dilepton Results – PRL 93 142001 (2004) • C. Hill, JI, C. Mills, et al. used them to increase the acceptance for top dileptons • Run II Result Consistent with SM: 6.7 § 0.5 pb • Kinematic distributions also consistent with SM • No early evidence of new physics…what about the Higgs? J.R.Incandela - Brookhaven HEP Seminar

  26. LEP What about the Higgs? • Tevatron best-case luminosity scenario =8.5 fb-1 • only a light Higgs is potentially observable • mH . 120 GeV • LEP searches already exclude much of this region • mH >114 @ 95% C.L. • And it won’t be easy • Must combine results from all modes • Must combine results from both experiments • i.e. will not be so convincing J.R.Incandela - Brookhaven HEP Seminar

  27. Can begin with Tevatron Data LHC t H W W W W b W We can constrain its mass … Back to Top • Indirect experimental bounds on the mass of a SM Higgs boson obtained from • Precision electroweak data • Measurements of the mass of the W boson • Measurements of the mass of the top quark • These measurements are sensitive to mH through radiative corrections J.R.Incandela - Brookhaven HEP Seminar • Becomes significant at LHC • Indicates where to look for the Higgs before its found • Could say something about whether Higgs is SM or not once it has been found

  28. E T b p  mt = 5.3 GeV @ CDF Top Mass Experimental Challenges • Mass reconstruction done at parton level but … • We detect missing transverse energy not neutrinos • unknown • Ambiguity in mtop • We measure jets not quarks • Energy is “corrected” back to parton level • Limited jet energy resolution • Imperfect jet energy scale • Many possible jet- parton assignments • Can be reduced if one or more b jets are tagged jet lepton  p b jet jet jet J.R.Incandela - Brookhaven HEP Seminar

  29. The “Most Important” High PT Measurement • Huge effort underway to measure mtop as precisely as possible • Goal mtop ~ 2 GeV by end of Run II  Many techniques • All are limited by jet energy scale uncertainty J.R.Incandela - Brookhaven HEP Seminar

  30. Top Mass at the LHC • Various Improvements • Double b tags • 87k tt with S/B~80 in 10 fb-1 • In-situ light jet calibration using hadronic W decays • Assumes b-jet scale know to ~1% via Z+b events (?) • Precision of ≳ 1 GeV per experiment • JES still a common and dominant systematic • ATLAS hep-ex/0403021 Two b tags Lepton + jets J.R.Incandela - Brookhaven HEP Seminar

  31. Average b decay length • Top quarks are so heavy they are produced “near threshold” • Much of top’s mass ends up in boost imparted to b-quark • b¼ 0.4(mt/mb) • mtop can be measured by measuring mean b decay length • At hadron colliders, use transverse decay length ´ Lxy J.R.Incandela - Brookhaven HEP Seminar

  32. Principle Advantages • Only need tracking • Insensitive to jet energy scale • A simple statistical measure of one parameter • Many systematic uncertainties depend on measured quantities that continue improve. J.R.Incandela - Brookhaven HEP Seminar

  33. Principle Disadvantages • A simple statistical measure of one parameter • Need lots of b decays! • No jet energy scale uncertainty, but a whole new set of systematics to consider • Factors that affect the top quarks momenta • QCD radiation • Parton distributions • Factors that affect the b hadron momentum • Fragmentation • Factors that affect the b hadron decay length • Lifetime measurements • Backgrounds to tt • with real or false lifetime On tape ~600 pb-1 ¼ 175 tagged b’s J.R.Incandela - Brookhaven HEP Seminar

  34. Characterize expected performance of mean decay length method At the Tevatron At the LHC No specific detector was simulated General experimental considerations were incorporated Statistical uncertainties were estimated for current and future datasets Systematic uncertainties Studied all that might be significant Where large, we generally found “easy” ways to mitigate them Our Studies J.R.Incandela - Brookhaven HEP Seminar Hill, Incandela, Lamb: PRD 71, 054029 (2005)

  35. Tevatron Study • Large Monte Carlo samples of signal & background events, mtop range 130 – 230 GeV • Selection based on CDF lepton+jets+b-tag cross-section analysis • 1 lepton with PT > 20 GeV • 1 neutrino with PT > 20 GeV •  3 partons with PT > 15 GeV • at least one must be a b-quark • HT > 200 GeV • Simulate secondary vertex tagging • Project trajectories of b-hadron daughters to get impact parameter • Smear IP resolution as a function of pT using parameterization of CDF data with  = 36 m and  = 25 m-GeV/c • Apply IP significance cut (2.5) • Smear 2D vertices to match resolution obtained in CDF simulation • Include non-Gaussian tails • Extract dependence of mtop on mean Lxy • Call this the “mass estimator” J.R.Incandela - Brookhaven HEP Seminar

  36. Backgrounds at the Tevatron • Backgrounds considered • Wbb, Wcc, and Wc • QCD • Mistagged light quark jets • Single-top ignored • Contributes ~ 2% • Not exactly a background • s-channel has identical Lxy distribution to tt • t-channel is different • Simple mistag model • Use negative side of LXY distribution in data • Fit Gaussian+exponential • Add a positive flat component to account for Kaons, etc. J.R.Incandela - Brookhaven HEP Seminar

  37. Sample Signal + Background Shape (mt= 175 GeV) • Lxy shapes normalized to background estimates in CDF lepton+jets cross-section analysis (hep-ex/0410041) • S/B = 3:1 J.R.Incandela - Brookhaven HEP Seminar

  38. Tevatron mtop Estimator • From each signal+background distribution compute resulting mean Lxy as a funtion of mtop • Fit to third degree polynomial J.R.Incandela - Brookhaven HEP Seminar Full simulation gives similar slope in the vicinity of 175 GeV

  39. Tevatron Statistical Uncertainty • Construct confidence intervals using ensembles of Poisson fluctuated MC “data” samples • Two integrated luminosity scenarios • Run II now • 500 pb-1 • mtop  20 GeV • End of Run II • 8.5 fb-1 • mtop  5 GeV • N.B. Study did not include b’s from tt dilepton • They reduce m to  4.4 GeV J.R.Incandela - Brookhaven HEP Seminar

  40. Fragmentation Uncertainty • Measurement is sensitive to the mean fractional energy carried by b-hadron after fragmentation ´E • Best measurements from LEP/SLD in Z!bb • Single most precise value from OPAL has an uncertainty of »0.5% • We assume universality of fragmentation J.R.Incandela - Brookhaven HEP Seminar

  41. Tagging Uncertainties • B tag efficiency not uniform • Varies with ET and  • Uncertainties in єtag(ET,) translate into ET distribution uncertainties that affect the mean Lxy • Also b-baryons have shorter lifetimes • b¼ 0.8 £B§ • If not taken into account, b-fractions sample biased causing error in mtop J.R.Incandela - Brookhaven HEP Seminar

  42. Lxy Simulation vs Data – Positive Tags J.R.Incandela - Brookhaven HEP Seminar Compare the MC to b enriched di-jet data containing a well identified electron or muon

  43. Lxy Simulation vs Data – Negative Tags J.R.Incandela - Brookhaven HEP Seminar

  44. Other Tevatron Systematic Uncertainties • Compared §1 variant mass estimators with “default” to quantify uncertainties • Total:2.9 GeV • b-hadron fragmentation: 0.9 GeV • b-hadron lifetimes: 1.0 GeV • b = 1.574 § 0.008 £ 10-12 sec • ISR: 0.7 GeV • Based on CDF studies of DY • FSR: 1.2 GeV • b-tagging: 0.8 GeV • PDFs: 0.8 GeV • Jet Energy Scale: 2.0 GeV • Results from HT cut which depends on calorimeter • This cut was included for historical reasons only and can be dropped or reformulated (e.g.  track pT) • Background Normalization = 0.3 GeV • Statistics limited even after 8.5 fb-1 J.R.Incandela - Brookhaven HEP Seminar

  45. Tevatron Conclusion & Outlook • Worth pursuing as a complementary measurement • Including lepton+jet & dilepton top events, estimate a combined stat+sys uncertainty of 5 GeV • By end of Run II, gives a decent contribution to precision when combined with calorimetric methods • A reduction in uncertainty of > 10% mtop = 2.5 GeV ! 2.2 GeV • Our first analysis of CDF data is underway • Expect a result soon J.R.Incandela - Brookhaven HEP Seminar

  46. Top quarks @ the LHC • tt production predominantly via gluon fusion • Tops are still mostly produced at threshold • Expect decay length technique can be used 90% J.R.Incandela - Brookhaven HEP Seminar

  47. LHC Study of Decay Length Technique • LHC is a Top Factory • tt¼ 833 pb !8,000,000 tt per year (low luminosity operation) • Allow aggressive event selection to mitigate systematics • We considered only double b-tagged dilepton events • Background free  no corresponding background uncertainty • No hadronic W less FSR  reduced radiation uncertainty (N. Kidonakis – DPF 2004) J.R.Incandela - Brookhaven HEP Seminar

  48. LHC Study Continued • Basic kinematic selection • 2 leptons • pT > 35, 25 GeV/c • > 40 GeV • Cut QCD radiation • Exactly two tagged b-jets with ET > 15 GeV • No additional clustered energy above 10 GeV J.R.Incandela - Brookhaven HEP Seminar

  49. LHC mtop Estimator • After all cuts, find correlation comparable to that found for the Tevatron Note: Since statistical uncertainty is much smaller at the LHC, we can now focus on a much smaller mass range. J.R.Incandela - Brookhaven HEP Seminar

  50. LHC Statistical Uncertainty • One year at low luminosity: • statistical uncertainty < 1 GeV J.R.Incandela - Brookhaven HEP Seminar

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