John Womersley 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

John Womersley 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 by exchanging vector bosons (photons, W ± and Z, gluons) Force carriers If this was all we were talking about, I dont think wed be here

John Womersley a revolução está vindo! *

John Womersley * revolution is coming * 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 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… But (like capitalism!) it contains the seeds of its own destruction

John Womersley 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 wed 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!

John Womersley 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! WIMP Mass and cross section EW symmetry breaking

John Womersley What does any of this have to do with top? We know theres new physics at the electroweak scale We really dont 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: QCD Electroweak physics Higgs or new physics top

John Womersley 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

John Womersley tt final states tt final states Standard Model: t Wb dominates 21% 15% 1% 3% 1% 44% tau+X mu+jets e+jets e+e e+mu mu+mu all hadronic bbbbb qq l - W-W- t bbbbb qq l + qq l + W+W+ t 30% e/ + jets 5% ee/e /

John Womersley How to catch a Top quark t t W b W b Muon Neutrino

John Womersley So… Top requires an excellent understanding of the whole detector and of QCD –Triggering, tracking, b-tags, electrons, muons, jets, missing E T –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

John Womersley 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 m W calibration 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 p T balance & Z bb soon.

John Womersley 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 E T and η

John Womersley Event Generators Significant progress on event generators: top-quark production with spin correlations single top production including 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 TopReX Alpgen NLO calculation Additional final state jets Polarised top decays Full 2 6 process Pythia + work on b-quark fragmentation in top decays

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

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

John Womersley 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

John Womersley 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 E T or dependence, or on b-jet scale DØ lepton + jets matrix element

John Womersley Mass in dilepton events Reduced statistics, but less sensitivity to JES –On the other hand, cant 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

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

John Womersley Prospects With plausible (but not easy to achieve) assumptions about evolution of systematic errors hep-ex/ = ~1.1 GeV

John Womersley 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-tagging using neural networks Improvement in dijet mass resolution using track momentum as well as calorimetry

John Womersley W mass Tevatron goal: improve on LEP2 –will require ~ 1fb -1 or more Strategy: extract mass from kinematic quantities –Transverse mass –(Lepton p T ) –(Missing E T ) Overall scale is set by Z (using LEPs mass measurement) CDF analysis with ~ 200pb -1 m W = 76 MeV m W still blinded

John Womersley W mass prospects

John Womersley What this would mean A % measurement of the Higgs mass

John Womersley 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 DØ Run II Preliminary Lepton+jets (~230 pb -1 ) Lepton+jets and dilepton (~160 pb -1 )

John Womersley Top charged Higgs If M H <m t - m b 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%?)

John Womersley 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 p T 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)

John Womersley Spin in Top decays 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 Ws to be (95% CL) F + < 0.25 (DØ) ; 0.27 (CDF) CDF finds the fraction of longitudinal Ws to be F 0 = –0.34 (lepton p T and cos * combined) In the SM, F + 0 and F 0 ~ 0.7 All consistent with the SM L=230 pb -1 PRD 72, (2005) Left-handed Right-handed Longitudinal cos *

John Womersley Spin correlations in top pair production DØ run I analysis, using only 6 events Phys. Rev. Lett (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)

John Womersley 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 ? ?

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

John Womersley Single Top production Probes the electroweak properties of top and measures CKM matrix element |V tb | 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.

John Womersley Single top searches Much higher backgrounds than tt production: Current results: s (pb) t (pb) s+t (pb) SM (NLO prediction)0.88 ± ± 0.21~ % CL upper limits Observed(expected) 13.6(12.1)10.1(11.2)17.8(13.6) MPV 68% CL 4.6 ± –4.9 95% CL upper limits Observed(expected) 6.4(5.8)5.0(4.5) MPV 68% CL % CL upper limits Observed(expected) 5.0(3.3)4.4(4.3) Worlds best limits 230 pb pb pb -1

John Womersley 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

John Womersley Single top searches Not yet able to see SM rate, but starting to disfavour some models e data only data only

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

John Womersley Tevatron Performance × cm -2 s -1 The highest luminosity hadron collider ever built

John Womersley Tevatron Status Electron cooling in Recycler 8 fb -1 4 fb -1 2 fb -1 Champagne for 1 fb -1

John Womersley 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 ?

John Womersley Top at LHC LHC has great potential for Top physics –Enormous cross sections 1 day at 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 LHC Tevatron

John Womersley 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 H T cuts, no b-tagging ATLAS studies + ongoing work for CMS Physics TDR and W+jets for 150 pb -1

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

John Womersley m t and m W at LHC Top: Measure mass to the 1 GeV level –Dominant systematic is b-jet energy scale hep-ph/ LEPEWWG05 m t =1 GeV/c 2 m W =15 MeV/c 2 W: LHC will have sufficient statistics to permit m W = 15 MeV –to reach this precision will be a challenging, multi-year project –will there be a physics need – precision test of SUSY?

John Womersley … and at ILC m t ~ 100 MeV claimed possible But requires further theoretical progress on higher order calculations –The tools are there… (just) a lot of tedious work required

John Womersley Top + Higgs at LHC tH + and tbH + –discovery modes for charged Higgs ttH –Verify top Yukawa coupling –fermion mass generation g t : 20 to fb -1

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

John Womersley tW production at LHC Only single top process where we directly observe the W –The t W mode is a more direct measure of tops 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 ~ 30 fb -1

John Womersley Top spin and polarisation at LHC 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 F 0 =0.677 ± F L =0.309 ± F R =0.014 ± 0.009

John Womersley 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)

John Womersley More complementarity with HERA First experimental determination of b-quark distribution –Needed for single top Z + b-jet … and for backgrounds, like W/Z + heavy flavour

John Womersley Top properties scorecard Is it a standard model up type quark? Electric charge +2/3Known not to be 4/3 Colour tripletYes? (production cross section) Spin ½not really tested – spin correlations Isospin ½Yes? (decay to W + down type quark) V – A decayat 20% level BR to b quark ~ 100%at 20% level FCNCprobed at the 10% level Top width?? test with single top ?? Yukawa couplingtest at 20-30% level at LHC Top mass ± 2.9 GeV Higgs mass< 186 GeV

John Womersley What do we know? At the 20% level, top seems to behave like an up type quark which just happens to have an extraordinarily large mass That mass has been measured very precisely –Thus constraining the Higgs sector –We do not yet know if this mass arises (as in the SM) from a Yukawa coupling or from something more interesting Searches for single top have sufficient sensitivity to see this process soon Starting to make interesting measurements of spin in top decays, searching for non-standard production and decay mechanisms Fascinating results from the Tevatron with much more to come Can look forward to a step-change in statistics at LHC, and sensitivity to new observables –CP violation in top?!

John Womersley Conclusion Top is a unique window on particle physics QCD Electroweak physics Higgs or new physics top

John Womersley B S Mixing New DØ result at Moriond 2006, 1 fb -1 of data: Likelihood minimum at m S = 19 ps < m S < 21 ps -1 (90% CL) –3.8% probability for an oscillation with frequency above the region of sensitivity (> 22 ps -1 ) to give a minimum this significant anywhere in the range ps -1