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Standard Model @ Hadron Colliders X. Top Quark: production ( cont .)

Standard Model @ Hadron Colliders X. Top Quark: production ( cont .). A semileptonic tt event. Is the top quark a normal fermion ?. Weak t coupling (V-A) CKM – elements Electric charge Top mass. gtt couplings spin correlations tt - resonances. Production of top quarks.

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Standard Model @ Hadron Colliders X. Top Quark: production ( cont .)

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  1. Standard Model @ HadronCollidersX. Top Quark: production (cont.) Peter Mättig, Scottish Summer School

  2. A semileptonicttevent Peter Mättig, Scottish Summer School 2012

  3. Isthetopquark a normal fermion? Weak t coupling (V-A) CKM – elements Electric charge Top mass gttcouplings spincorrelations tt - resonances Peter Mättig, Scottish Summer School 2012

  4. Production of topquarks What x requiredfortopproduction? 0.18 at Tevatron 0.05 at LHC (0.025 @ 14 TeV) Dominant at LHC forlowMtt Suppressed @ Tevatron Relevant at LHC for high Mtt Dominant @ Tevatron Peter Mättig, Scottish Summer School 2012

  5. How to measurett cross section (Whyshouldwe?): Sensitive to gluon –ttcouplings Test of QCD with massive quarks • Selectevents: • 4 jetswithpT> 25 GeV • isolatedelectron,muon pT>20 GeV • missingtransverseenergy > 20 GeV Luminosity: Howmanyproton-collisions? Whatfraction of tteventsareretainedafterselection Peter Mättig, Scottish Summer School 2012

  6. Cross sectiondetermination Experimental precisiondepends on how well - background, efficiency, luminositycanbecontrolled Key issuedetermineefficiency Log s • Largestuncertainties: • Jet energyscale • bottomidentification • Background yield • Jets from QCD • selectionefficiency • e, m, ..... ObservedpT wrongenergyscale SelectedpTrange TruejetpT Jet pt Experimental uncertainty ~ 9% Luminosityuncertainty ~ 4.4 % Peter Mättig, Scottish Summer School 2012

  7. Background estimatation • Dominant background: W + 4 jets same final objects • assume QCD generators to becorrect, i.etemplates • datadrivenmethod (ATLAS): • tt – events: samenumber of W+, W- • W+jetsmethod: more W+than W- rMC = NW+/NW- • Furtherstep: estimate W+b(b)+2 jetsfractionbased on • bottomtagging in W+2jets  extrapolated to 4 jets via MC • Otherbackground: QCD withbleptonwith high xFeynman • Estimatefrom ‚non – isolated‘ leptons Peter Mättig, Scottish Summer School 2012

  8. Background in semileptonictt Contribution to sample no b – tag S/B ~ 1/3 W+Jets/tt ~ 1.4 Contribution to samplewith b – tag S/B ~ 4 W+Jets/tt ~ 0.15 price: somewhatreducedstatistics Wb+jetsmoreuncertain Peter Mättig, Scottish Summer School 2012

  9. Dileptons + fullyhadronic Dileptonic: Very pure tt – sample Note: forX-section no need to useanyotherproperty ... Butloss in statistics Fullyhadronic: Huge QCD background Advantage: M(t), M(W)  Kinematic fit Peter Mättig, Scottish Summer School 2012

  10. Summary of Xsection Dileptonic and semi-leptonicmeasurementssimilarprecision All hadronic larger errors Experiments havesmalleruncertaintythantheoreticalcalculation Peter Mättig, Scottish Summer School 2012

  11. Cross sectionmeasurement Theoreticaluncertainty 7-10% partly NNLO Theory & experiment uncertaintyabout equal Very good agreementbetweendata and expectation Peter Mättig, Scottish Summer School 2012

  12. Tevatronfwd-bkwasymmetry ‚Forward‘ hemisphere ‚Backward‘ hemisphere • Count • topquarks in forwardhemiNfwd • topquarks in backwardhemiNbwd Peter Mättig, Scottish Summer School

  13. Standard Model: smallasymmetry • Dominant production @ Tevatron • chargedirection ‚lost‘ • LO: no asymmetry in Standard Model C= -1 C = +1 NLO: Interference small AC Standard Model: (4.8+-0.5)% Peter Mättig, Scottish Summer School

  14. Tevatron: larger asymmetry Moreeventswith qtop· ytop > 0 Low mass: consistentwith Standard Model Masses > 450 GeV 3 – 4 sdeviationfromexpectation Peter Mättig, Scottish Summer School

  15. AFBvsmtt +2.5 ± 3.1 19.8 ± 4.3 Note:Theseareearlier CDF results!! Low mass: consistentwith Standard Model Masses > 450 GeV 3 – 4 sdeviationfromexpectation (CDF) Peter Mättig, Scottish Summer School

  16. A glimpse of multi – TeVphysics? BSM interpretation: asymmetrydue to interference high massparticle + Standard Model Type 1: Gluonwith axial coupling Type 2: t –channel Colour neutral vector with FCNC Type 3: t –channel colouredscalar with FCNC Such massive particlesshouldbecomevisible @ LHC Peter Mättig, Scottish Summer School

  17. tt – asymmetries @ LHC Differences: pp – collider, Tevatron LHC symmetricinitialstateqqtt to ggtt Tevatron LHC 8 TeV Enhanceqqproductionby large Dy high x valencequark on low x seaanti-quark Peter Mättig, Scottish Summer School

  18. tt – asymmetries @ LHC Peter Mättig, Scottish Summer School

  19. Interpretation in models For concretemodel: compareTevatron & LHC varymass and couplings of newparticles Mtt > 450 GeV Many of the ‚Tevatron‘ allowedmodelsdisfavouredby ATLAS (and CMS) Peter Mättig, Scottish Summer School

  20. Jets in topevents QCD effectsimply a potential strongbias to studies Jet multiplicities: Deficiencies at higherNjets Productionproperties: pT and y of tt – system Good descriptionby QCD calculations Peter Mättig, Scottish Summer School

  21. Massspectrum of tt - events pT of topquarks & mass of tt – pairspredictedby QCD ‚Resolved‘ fourjet (+ lepton, n) Standard topselection High massregion: Boostedtops mergedjets Appropriatealgorithmsrequired Peter Mättig, Scottish Summer School

  22. Productionproperties: e.g. Mtt ‚Fatjet‘ Closerlookshowssubstructure Peter Mättig, Scottish Summer School

  23. Fatjet & substructure Highlyboostedtops: closebyjets ‚Fatjet‘ of R = 1.0 • Require: Mfatjet> 100 GeV • Nextstep: lookforsubstructure • use kTjetfinder to ‚uncluster‘ • dij > (40 GeV)2 • Requireoppositejet/leptonsystem Non-boostedboosted Possibleimprovements: trimming of jets: RejectanysubjetwithsomepT,i/pT,jet < fcut Peter Mättig, Scottish Summer School

  24. Models predictresonancesXtt Peter Mättig, Scottish Summer School

  25. Models predictresonancesXtt Highermasses: longtailsdue to gg/qqluminosity Example: 5 dimtheories, Randall – Sundrum etc. predict Kaluza – Klein gluons No significantresonance  MKK > 1.5 TeV Peter Mättig, Scottish Summer School

  26. tt + Z/W events Measurement of Ztt & Wttcoupling Possibleresonancesearchorheavyquark Importantbackgroundfor SUSY searches Searchfor (Zll)+l+b(b),  ttZ equalchargelepton pair + b(b)  ttW/Z Expectedlow X- section, fair agreementwithexpectation Peter Mättig, Scottish Summer School

  27. Isthetopquark a normal fermion? Weak t coupling (V-A) CKM – elements Electric charge Top mass Peter Mättig, Scottish Summer School

  28. Mass of thetopquark A fundamental parameter of the Standard Model A broadspectrum of decays and methods Note: first time a quarkmasscanbe measureddirectly (Lighterquarks to beinferredindirectlyfromhadronmasses) Peter Mättig, Scottish Summer School

  29. Top massfroml+jetdecays Favouredtopology: tt 4 Jets (2 b –jets) + e/m + n • Theproblems: • How to getthe z – component of n • Out of 4 (ormore) jets: whichjetbelongs to whichtop? • Whatistheenergyscale of jets (and electrons) Peter Mättig, Scottish Summer School

  30. Problem 1: pz(n) Constraintfrom W - mass Note: n – masscompletelynegligible Quadraticequation 2 solutions physics: in 70% thesolutionwithsmallerpzcorrect Peter Mättig, Scottish Summer School

  31. Problem 2: whichjets? • Twofacettes: • ifmorethan 4 jets (initialstate rad.) • mostlyjetswithhighestpT • ifexactly 4 jets: whichbelongs to • whichtopquark? • 4 jets 4 possibleassigments • (jAjBJC/jD, jAjBjD/jC, ....) • Note: if b – jetsidentified, reduced to 2 possibilities • Importantconstraints • mass (jjj) = mass(jln) (= Mt) • mass (jj) = MW Peter Mättig, Scottish Summer School

  32. Problem 3: jetenergyscale Measuresignals in calorimeter derivejetenergy Impliesuncertainty!  relatesdirectly to topmass Top – quarksoffer ‚selfcalibration‘ M(jj) has to beequal MW  change JES such thatfulfilled Still the (slightly) dominant uncertainty of Mt Peter Mättig, Scottish Summer School

  33. Most precise: matrixmethod Theoreticalpredwith M1(top) w1 • probabilitydensityfor M1 • use 24 integration variables Theoreticalpredwith M1(top) w2 Nextstep: convolute with exptl. effects • Assignweight • to eachevent Example: energyresolution Peter Mättig, Scottish Summer School

  34. Likelihoodfrom different masses wAwBwCwD Sumover all events and find combineweights ...... Find M(top) withmaximumweight Peter Mättig, Scottish Summer School

  35. Top massfromdileptons & hadronic • Dileptons: • No directmasspeakvisible • useenergies of electrons (& bottomjets) • using MET adjustneutrinoenergies to • yieldsame MW and Mtop • All hadrons: • Fight hugebackground • suppressbyneuralnetwork Peter Mättig, Scottish Summer School

  36. Measurements of Mtop A lot of measurements, a lot of methods all decaychannelsbynowbetterthan 2 GeV! Combination 173.2±0.6±0.8 GeV Peter Mättig, Scottish Summer School

  37. How to interpretresult? For Standard Model fit  ‚pole mass‘ required Instead: all methodsbased on simulation of QCD effects of mass ‚topquarknottotallyfree‘: colourflow - howdoesthisaffectmassdetermination? Different modelsmassdifferences of a fewGeV e.g. colourreconnection Skands&Wicke Peter Mättig, Scottish Summer School

  38. Top massfrom cross section • Massmeasurementsbased • on MC simulation •  not well defined • QCD corrections • Difficult to interpret in • Electroweakfits • pole massfrom NNLO • calculations on Xsection Peter Mättig, Scottish Summer School

  39. Currentresults Theoreticallybettermotivated Buterrors of ~ 5 -8 GeV mostlydue to theoryuncertainty note: MS – massaround 160 GeV! Peter Mättig, Scottish Summer School

  40. Speculationsaboutthetopmass Top mass and the 1018GeVscale • Naturalnessproblem: • RenormalisingtheHiggsmass • Contributions to DmH • ‚most relevant‘ • compensatetop • Higgs potential: • l(mH) = 0.125 (+uncertainties) • l(Q2) • Ifl < 0  universeunstable Nice to speculate ..... Butcanwereallyextrapolatesafelyover 14 orders of magnitude? Peter Mättig, Scottish Summer School

  41. t - quark Helicity structure of topdecay Isthetop a normal weaklydecayingparticle? Note: first time helicitystructure of quarkcanbedetermined b b W+ W+ allowed forbidden W – polarisationagainstdirection of t – quarkmomentum Longitudinal polarosation also possible Polarisation reflected in decay angle of fermions Peter Mättig, Scottish Summer School

  42. Helicity structure of topdecay Ratherstraightforwardfor e, m For W  qqidentify q vs. q Challenging! angle related to leptonenergy, Mbl, ..... Peter Mättig, Scottish Summer School

  43. Measurements Agreement with NNLO expectation: ‚no‘ right handedW‘s, mostW‘sare longitudinal Peter Mättig, Scottish Summer School

  44. Limits on additional couplings Several BSM models deviations General approachEffectiveLagrangian: Parametrisationintohigherdimensionoperators gL, gR: left/righthandedcoupling of dim-6 operator Peter Mättig, Scottish Summer School

  45. Top spincorrelations @ LHC ‚Bare‘ quark directinformation on spinconfiguration Spin correlationsoffer test of production of tt – pairs Potentiallyimportanttool to identifynewparticles Close to threshold: High ttmasses: S = 0 state, gluonhelicitieslikegg tt: helicityconservation Top spinsaligned Top spinsopposite Useleptons to identifyspindirections and correlations Dileptondecayneeded restsystemcannotbedetermined Peter Mättig, Scottish Summer School

  46. Experimental method Definequantisationaxis, e.g. beam a*Signaltemplates + b* backgroundtemplate = DATA Peter Mättig, Scottish Summer School

  47. Spin correlations @ Tevatron Note: Tevatrontops via qq – scattering! f = 0.85 ± 0.29 Tevatron no or marginal evidenceforspincorrelations Peter Mättig, Scottish Summer School

  48. Spin correlations @ LHC MeasureDf of leptons in transverse plane Note experimental distortion: Alignmentmeans on leptonlowenergetic alignedopposite Peter Mättig, Scottish Summer School

  49. Comparisonwith SM expectation First significantevidence of spincorrelations Agreement with Standard Model Study of spincorrelations: A method to separate newresonancesfrom QCD continuum (?) Peter Mättig, Scottish Summer School

  50. Single topproduction toppairsdue to strongcoupling, weakcoupling singletopquarks Dominant s(7 TeV) = 65 pb (half of tt – Xsection) Remember: W±couples to fermiondoublets Peter Mättig, Scottish Summer School

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