1 / 55

The Top Quark: 2006 and Beyond

The Top Quark: 2006 and Beyond. An (updated) summary of TOP2006: International Workshop on Top Quark Physics Coimbra, Portugal, January 2006 John Womersley Director of Particle Physics CCLRC – Rutherford Appleton Laboratory. If this was all we were talking about, I don’t think we’d be here.

afra
Download Presentation

The Top Quark: 2006 and Beyond

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The Top Quark: 2006 and Beyond An (updated) summary of TOP2006: International Workshop on Top Quark PhysicsCoimbra, Portugal, January 2006 John Womersley Director of Particle Physics CCLRC – Rutherford Appleton Laboratory

  2. If this was all we were talking about, I don’t think we’d be here The Standard Model • Decades of experimentation with accelerators, and theoretical synthesis, have culminated in what we call the “Standard Model” • A theory of matter and forces • A quantum field theory describing point-like fermions (quarks and leptons) • matter particles • which interact byexchanging vector bosons (photons, W±and Z, gluons) • Force carriers

  3. a revolução está vindo! *

  4. But (like capitalism!) it contains the seeds of its own destruction * revolution is coming • The standard model makes precise and accurate predictions • It provides an understanding of what nucleons, atoms, stars, you and me are made of • Its spectacular success in describing phenomena at energy scales below 1 TeV is based on • At least one unobserved ingredient • the SM Higgs • Whose mass is unstable to loop corrections • requires something like supersymmetry to fix • And which has an energy density 1060 times too great to exist in the universe we live in • The way forward is through experiment (and only experiment) • tantalizing – we know the answers are accessible • and also a bit frustrating – we have known this for 20 years…

  5. Quarks and leptons 4% Meanwhile, back in the universe … • What shapes the cosmos? • Old answer: the mass it contains, through gravity • But we now know • There is much more mass than we’d expect from the stars we see, or from the amount of helium formed in the early universe • Dark matter • The velocity of distant galaxies shows there is some kind of energy driving the expansion of the universe, as well as mass slowing it down • Dark Energy • We do not know what 96% of the universe is made of!

  6. These questions seem to come together at the TeV scale: With TeV scale accelerators we are exploring what the universe contained ~ 1ps after the big bang! EW symmetry breaking WIMP Mass and cross section

  7. Electroweakphysics QCD Higgsor new physics What does any of this have to do with top? • We know there’s new physics at the electroweak scale • We really don’t know what it is • Right now, the top quark is our only window on this physics • Couples strongly to the Higgs field: what is this telling us? • A window on fermion mass generation: does it really happen through Yukawa couplings? • Offers a unique physics laboratory: top

  8. Top and new physics Solutions to EWSB • Supersymmetry • Top Yukawa coupling is modified w.r.t. its SM value • Mass scale of top partners must be low (not true of other superpartners) • New physics associated with top may be first to be seen • Little Higgs models • New vector-like top-partner T, m ~ 1–2 TeV • mixes with top, decays to th, tZ, bW • Strongly-coupled models: Technicolor and its descendents • If mass dynamically generated, top is special because of its large mass: extra interactions (topcolor…) • Resonances in tt, tb (single top in s-channel) • Modified spacetime: Extra dimensions • Not such a special role for top, but can havett production through KK resonances

  9. t W+ l+ qq l+ qq b b b b b t W- l- l- qq qq b b b b b tt final states • Standard Model: t  Wb dominates tau+X 21% mu+jets 44% e+jets 15% e+e 15% e+mu mu+mu all hadronic 1% 30% e/ + jets 5% ee/e/ 3% 1%

  10. Neutrino Muon W b t W t b How to catch a Top quark

  11. So… • Top requires an excellent understanding of the whole detector and of QCD • Triggering, tracking, b-tags, electrons, muons, jets, missing ET • Performance must be understood and modelled In particular: • Early effort to understand Jet Energy Scale • for event kinematics and top quark mass • b-tagging • To reduce backgrounds • To reduce combinatorics in measurements of top quark properties • Sophisticated techniques • To maximise sensitivity to rare processes • To maximise sensitivity to deviations from SM • Team work and efficient tools

  12. Jet energy scale • The jet energy scale is the dominant uncertainty in many measurements of the top quark. • CDF and DØ use different approaches to determine the jet energy scale and uncertainty: • CDF: Scale mainly from single particle response + jet fragmentation model. Cross-checked with photon/Z-jet pT balance etc. • ~3% uncertainty in Run II. Further improvements in progress. • DØ: Scale mainly from photon-jet pT balance. Cross-checked with the closure tests in photon/Z+jet events etc. • new Run II calibration (uncertainty ~ 2%) will come out soon. • In-situ mWcalibration has been successfully used to improve JES by both CDF and DØ in top mass measurements. • Expect result on the b-jet energy scale from photon/b-jet pT balance & Z bbsoon.

  13. B-tagging • A significant body of experience has been gained at the Tevatron experiments • both have developed multiple b-tagging tools • Many issues deserve attention for the LHC: • Alignment of the silicon tracking detector • Understanding charge deposition • Understanding material in the tracking volume • Tracking simulation and its relation to reality • Monte Carlo scale factors • Determination of efficiencies from data – calibration data must be collected at appropriate ET and η

  14. Event Generators Significant progress on event generators: • top-quark production with spin correlations • single top production including 2  2 + 2  3 with proper matching • tree level generators with additional multi-jets in the final state • prescriptions to match tree-level + showering without double counting • generators with full NLO corrections to top production processes • event generators for top production and decays due to interactions beyond SM Pythia Full 2  6 process NLO calculation MC@NLO Additional final state jets Polarised top decays TopReX Alpgen + work on b-quark fragmentation in top decays

  15. Top production • If the top is “just” a very heavy quark, its production cross section can be calculated in QCD Dileptons: Cleanest channel Lepton + jets, b-tagged: Higher yields PLB 626, 35 (2005) L=230 pb-1 1 secondary vertex tag

  16. Cross section measurements • All channels consistent with each other and with QCD • Ongoing effort to combine measurements within and among experiments.

  17. Extracting the top mass Two basic techniques • Template method: • extract a quantity from each event, e.g. a reconstructed top mass • find the best fit for the distribution of this quantity to “templates” • Matrix element (or dynamic likelihood) method • Calculate a likelihood distribution from each event as a function of hypothesised top mass • Multiply these distributions to get the overall likelihood

  18. Top mass • Both experiments are now simultaneously calibrating the jet energy scale in situ using the W  jj decay within top events Combined fit to top mass … … and shift in overall jet scale from nominal value But … no information on ET or  dependence, or on b-jet scale DØ lepton + jetsmatrix element

  19. Mass in dilepton events • Reduced statistics, but less sensitivity to JES • On the other hand, can’t incorporate W  jets calibration in the same way • With the full statistics, these channels are starting to become competitive with lepton + jets channel Feb 2006 Update 750 pb-1

  20. Top mass status January 2006 Most precise measurements come from lepton + jets Use of W  jets calibration is an important recent improvement

  21. hep-ex/0510048  = ~1.1 GeV Prospects • With plausible (but not easy to achieve) assumptions about evolution of systematic errors

  22. Improvements in progress • The Tevatron experiments have quantified the improvements in sensitivity needed to reach their Higgs projections • EM coverage, efficiency; dijet mass resolution; b-tagging... • Will also improve performance for top Improvement in b-taggingusing neural networks Improvement in dijet mass resolutionusing track momentum as well as calorimetry

  23. W mass • Tevatron goal: improve on LEP2 • will require ~ 1fb-1 or more • Strategy: extract mass from kinematic quantities • Transverse mass • (Lepton pT) • (Missing ET) • Overall scale is set by Z (using LEP’s mass measurement) • CDF analysis with ~ 200pb-1mW = 76 MeV mW still blinded

  24. W mass prospects

  25. What this would mean • A 25-30 % measurement of the Higgs mass

  26. How does top decay? • In the SM, top decays almost exclusively to a W and a b-quark, but in principle it could decay to other down-type quarks too • Can test by measuring R = B(t  b)/B(t  q) • Compare number of double b-tagged to single b-tagged events All consistent with R = 1 (SM)i.e. 100% top  b Lepton+jets and dilepton (~160 pb-1) DØ Run II Preliminary Lepton+jets (~230 pb-1)

  27. Top  charged Higgs • If MH <mt - mb then t  H+b competes with t  W+b • Sizeable B(t H+b) expected at • low tan: H cs, Wbb dominate • high tan : H   dominates • different effect on cross section measurements in various channels. • CDF used tt measurements in dileptons, lepton+jets and lepton+tau channels • allowed for losses to t H+b decays • Simultaneous fit to all channels assuming same tt Still room for substantial BR t  H+b (as high as 50%?)

  28. Top charge • Using 21 double-tagged events, find 17 with convergent kinematic fit • Apply jet-charge algorithm to the b-tagged jets • Expect b (q = 1/3) to fragment to a jet with leading negative hadrons, butb (q = +1/3) to fragment to leading positive hadrons • Jet charge is a pT weighted sum of track charges • Allows to separate hypothesis of top  W+b from Q  W-b • Data are consistent with q = ±2/3 and exclude q = ±4/3 (94%CL)

  29. L=230 pb-1 Spin in Top decays Left-handed Right-handed • Because its mass is so large, the top quark is expected to decay very rapidly (~ yoctoseconds) • No time to form a top meson • Top  Wb decay then preserves the spin information • reflected in decay angle and momentum of lepton in the W rest frame • We find the fraction of RH W’s to be (95% CL) F+ < 0.25 (DØ) ; 0.27 (CDF) CDF finds the fraction of longitudinal W’s to be F0 = 0.74 +0.22 –0.34 (lepton pT andcos * combined) In the SM, F+  0 and F0 ~ 0.7 Longitudinal cos* PRD 72, 011104 (2005) All consistent with the SM

  30. Spin correlations in top pair production • DØ run I analysis, using only 6 events Phys. Rev. Lett. 85 256 (2000) • CDF sensitivity studies… need a few fb-1 before correlations can be seen • At LHC, precision measurements seem possible • Look at dilepton and l+jets events, various bases • Useful tool to study/look for nonstandard production mechanisms (resonances, effects of extra dimensions)

  31. New particles decaying to top? • One signal might be structure in thett invariant mass distribution from (e.g.) X tt • Interesting features in both distributions, but are they consistent ? ?

  32. Alas, with about twice the data, the excess washes out Feb 2006 Update 682 pb-1

  33. Single Top production • Probes the electroweak properties of top and measures CKM matrix element |Vtb| • Good place to look for new physics connected with top • Desirable to separate s and t-channel production • The s-channel mode is sensitive to charged resonances. • The t-channel mode is more sensitive to FCNCs and new interactions.

  34. Single top searches • Much higher backgroundsthan tt production: • Current results: 160 pb-1 230 pb-1 370 pb-1 World’s best limits

  35. Multivariate techniques • Reference case: DØ single top search • Last year, moved from simple cuts to multivariate approach: roughly doubled sensitivity • Likelihood discriminants • Neural networks • Decision trees • less familiar in high energy physics • Some attractive features (“not a black box”) • Boosting algorithms • Used in miniBooNE and GLAST • Boosted DT results for single top soon

  36.  data only e data only Single top searches • Not yet able to see SM rate, but starting to disfavour some models

  37. Single top prospects • with current sensitivity, statistically significant observation will happen in Run II – but improvements still desirable!

  38. Tevatron Performance 2005 1.6 × 1032 cm-2 s-1 2004 2003 2002 The highest luminosity hadron collider ever built

  39. Tevatron Status 8 fb-1 Champagne for 1 fb-1 Electron cooling in Recycler 4 fb-1 2 fb-1

  40. Status of LHC • First collisions in Summer 2007 • Initial measurements 2 years from now? • First precision measurements 3 years from now with 1-10fb-1?

  41. LHC Tevatron Top at LHC • LHC has great potential for Top physics • Enormous cross sections • 1 day at 1033  10 years at Tevatron for SM processes • In many cases, negligible stat uncertainties • Dramatic improvements over statistically limited Tevatron analyses (e.g. spin, polarisation, rare decays) • Improved understanding of top and window on BSM physics • LHC is on the road • Huge amount of work needed prior to measurements • to understand the detectors & control systematics • Early top signals will also be critical to commissioning the detectors • Some of the earliest LHC physics results, and earliest sensitivity to new physics, should come from top physics • Top is also a background to discovery physics at LHC • e.g. H  WW, top, susy

  42. and W+jetsfor 150 pb-1 Tools for top at LHC • Enormous statistics • Even greater emphasis on control of systematics • Worry about issues at < 1% level that are not major concerns at the Tevatron • Jet masses, calibrate parton energy or jet energy • Can afford to talk about strategies like removing events with identified semileptonic b-decay jets • What can be done at the start of the run? • Signal is large enough that clear lepton + jets signal can be seen in 150 pb-1 with HT cuts, no b-tagging • ATLAS studies + ongoing work for CMS Physics TDR

  43. Top mass at LHC • Lepton + jets - golden channel • S/B ~ 30, statistical uncertainty is tiny (100 MeV?) • Can afford to select high-pT sample to reduce combinatorics, if desired • would this help reconstructing t quark C of M for spin studies? • Importance of kinematic fitting • msys at the 1 GeV level (b-jet JES dominates) • Dilepton channel • msys at the 1.7 GeV level • More exotic possibilities … • Measurements in all-jets channel • msys ~ 3 GeV • Leptonic final states with J/: • statistics low, but msys 0.5 GeV?? • Methods have very different sensitivities to systematics • Combining all of the above: measure mt to ~ 1 GeV with 10 fb-1

  44. W: LHC will have sufficient statistics to permit mW = 15 MeV • to reach this precision will be a challenging, multi-year project • will there be a physics need – precision test of SUSY? hep-ph/0307177 + LEPEWWG05 ∆mt=1 GeV/c2 ∆mW=15 MeV/c2 mt and mW at LHC • Top: Measure mass to the 1 GeV level • Dominant systematic is b-jet energy scale

  45. … and at ILC • mt ~ 100 MeV claimed possible • But requires further theoretical progress on higher order calculations • “The tools are there… (just) a lot of tedious work required”

  46. Top + Higgs at LHC • tH+ and tbH+ • discovery modes for charged Higgs • ttH • Verify top Yukawa coupling • fermion mass generation • gt : 20 to 30 % @ 300 fb-1

  47. Single top at LHC • t-channel process • Cross section 120 times higher than Tevatron • ATLAS study • s-channel process • Direct extraction of |Vtb| from ratio of W* (single top) to real W • Harder: cross section only 10 times higher than Tevatron S/B ~ 3 √(S+B)/S ~ 1.4% @ 30 fb-1 After preselection M(tb)

  48. tW production at LHC • Only single top process where we directly observe the W • The t W mode is a more direct measure of top’s coupling to W and a down-type quark (down, strange,bottom). • Tiny cross section at Tevatron, significant at LHC (x 400) • Theoretical definition is “delicate”; new work in progress • Major background from tt • ATLAS study: S/B ~ 1/7 √(S+B)/S ~ 4% @ 30 fb-1

  49. Top spin and polarisation at LHC F0=0.677 ± 0.015 FL=0.309 ± 0.009 FR=0.014 ± 0.009 • High statistics – can significantly improve on the Tevatron • W helicity in top decay • Measure at the 1 – 7 % level • dominated by systematics • Top spin in single top • 90% polarised • Measure at the few % level (fast sim) • Search for CP violation! • Top-antitop spin correlation • A = 0.33 in SM • Measure A to ~ 10% (fast sim) Full simulation (preliminary) 1.5 fb-1

  50. FCNC decays • HERA sets limits on FCNC utZ, ut couplings • Still need to understand the origin of observed isolated lepton events in e+p data in H1 (and not in Zeus) • LHC sensitivities (100 fb-1, 5 significance)

More Related