Top Physics in Hadron Collisions

Transcription

1 Top Physics in Hadron Collisions Dirk Dammann DESY / 44

2 Outline / 44

3 Outline Motivation Top Production Top Decay Top Physics 1 Motivation Top Production Top Decay Top Physics / 44

4 Why is the Top interesting? Motivation Top Production Top Decay Top Physics Features of the Top Quark: most massive observed particle processes can be calculated in perturbative QCD top mass has impact on standard modell (SM) higgs mass measurement of SM parameter V tb possible only quark that does not hadronize studies of production mechanism top is background for several expected beyond SM (BSM) processes search for exotic BSM decays 4 / 44

5 Top Production Motivation Top Production Top Decay Top Physics Gluon-Gluon-Fusion 5 / 44

6 Top Production Motivation Top Production Top Decay Top Physics Gluon-Gluon-Fusion Quark-Quark-Fusion 5 / 44

7 Top Production Motivation Top Production Top Decay Top Physics Gluon-Gluon-Fusion Quark-Quark-Fusion Single Top Production 5 / 44

8 Motivation Top Production Top Decay Top Physics Production: Differences between Tevatron and LHC At Tevatron t t only at high x possible 85% q q At LHC 90% gg-fusion 6 / 44

9 Top Decay Motivation Top Production Top Decay Top Physics almost 100% t b+w top branching ratios given by W s branching ratios 9 decay channels (3 leptonic, 6 hadronic) neglecting masses and higher orders all have same probability t t branching ratios full hadronic: 44% single lepton: each 15% dileptonic: 1% for channels with same leptons, 2% for different leptons 7 / 44

10 Top and Higgs Motivation Top Production Top Decay Top Physics particles get mass by Higgs coupling top is particle with highest known mass top mass important for procceses with Higgs Constraint on Higgs Mass: relation between G F, θ W, m W, m t and m H in high Q 2 collisions: G F = πα 1 2m 2 W sin 2 θ W 1 r with r = α + cot 2 θ W ρ main contributions to ρ caused by top and higgs loops: ρ t 3G F 8 2π 2 m2 t ρ H = 3G F 8 2π 2 (m2 Z + m2 W ) ln( m2 H m 2 W 5 6 ) 8 / 44

11 Top and Higgs Motivation Top Production Top Decay Top Physics relation between W, top and Higgs mass in principle exact measurement of m W and m t fixes SM higgs mass in SuSy modells other corrections contribute and adjust constraint on Higgs mass 9 / 44

12 Top and Higgs Motivation Top Production Top Decay Top Physics relation between W, top and Higgs mass in principle exact measurement of m W and m t fixes SM higgs mass in SuSy modells other corrections contribute and adjust constraint on Higgs mass data seem to prefer SuSy 9 / 44

13 Single Top and CKM-Matrix Motivation Top Production Top Decay Top Physics production via charged current or associated with W small cross section at Tevatron direct measurement of V tb ratio between production channels compared to SM gives hints to different modells of new physics 10 / 44

14 Single Top and CKM-Matrix Motivation Top Production Top Decay Top Physics 2 examples of new physics: 4th generation would decrease V tb lower cross section in all channels new vector bosons W would change cross section in s-channel negligible in t-channel (contribution 1/M W ) 11 / 44

15 Outline Discovery of the Top-Quark Properties Single Top Production 1 2 Discovery of the Top-Quark Properties Single Top Production / 44

16 The Tevatron Discovery of the Top-Quark Properties Single Top Production p p-collider at Fermilab startet in middle of 80s s = 1.8 GeV at beginning since 2001 s = 1.96 TeV 2 all purpose detectors 13 / 44

17 Discovery Discovery of the Top-Quark Properties Single Top Production Both collaborations reported the Observation of the t t-pairs in April 1995 L 50 pb 1 at 1.8 TeV single and dilepton channel used also first values for cross section and mass GeV CDF D 0 events significance 4.8 σ 4.6 σ cross section GeV 6.4±2.2 pb mass 179±8±10 GeV ±22 14 / 44

18 Cross Section and Mass Discovery of the Top-Quark Properties Single Top Production Measurements of mass and σ t t done in all channels: golden channel is semileptonic decay with muon or electron to trigger dileptonic channel has clear signature but low statistics and kinematics are underconstrained full hadronic decay is constrained but has large QCD multijet background 15 / 44

19 Discovery of the Top-Quark Properties Single Top Production Cross Section and Mass in the Single Lepton Channel single lepton decay: branching ratio 15% for each muon and electron on hadronic side 3 jets, fully observable on leptonic side b-jet, lepton and unobserved neutrino further constraints: p T,ν = E T W ± -mass m t = m t kinematic fit possible main background is W+jets CDF results: 7.1 ± 0.4 ± 0.4 ± 0.4 pb (using NN, 2008) ± 1.1 ± 1.1 GeV (2009) D 0 results: ± 0.4 pb ± 0.8 ± 1.6 GeV 16 / 44

20 Discovery of the Top-Quark Properties Single Top Production Cross Section and Mass in Dilepton Channel Dilepton decays: small branching ratio 1% for ee or µµ, 2% for eµ main background is Drell-Yan (DY, q q l l) in ee and µµ-channel also WW, WZ, ZZ 2 invisible neutrinos also with other constraints not solvable: Trick: use quantity which makes problem solvable and which is (almost) mass independent well suited: p z,t t for each event and combination take random values for p z,t t following assumed distribution and calculate top mass for each mass distribution for each event p x,ν + p x, ν = E x p y,ν + p y, ν = E y W ± -mass m t = m t 17 / 44

21 Discovery of the Top-Quark Properties Single Top Production Cross Section and Mass in Dilepton Channel Template Method: compare reconstructed mass shape with generated shapes at differing masses is also applied in other channels Cross Section Constrained Mass Measurement: theoretical cross section falls exponential with top mass acceptance depends also on mass in principle mass measurement solely from cross section possible in upper CDF plot likelihood was used taking into account template method and cross section 18 / 44

22 Discovery of the Top-Quark Properties Single Top Production Cross Section and Mass in Hadronic Channel not as simple as it seems, QCD background b-tagging mandatory no leptons multi-jet trigger required S/B-ratio = O(10 3 ) after trigger high combinatoric background in event reconstruction (90 combinations in 6 leading jets) D 0 publication 2007: background description from events without b-tag σ t t = 8.3 ± ± 0.5 pb m t = ± 2.2 ± 4.8 GeV 19 / 44

23 Cross Section and Mass Summary Cross Section values in all channels are in good agreement with each other also good agreement width theoretical predictions Discovery of the Top-Quark Properties Single Top Production 20 / 44

24 Charge Measurement of the discovered particle s charge Discovery of the Top-Quark Properties Single Top Production top charge of 2 3e has to be proven by experiment exotic 4 3e charge is imaginable as well most challenging problem is jet charge 21 / 44

25 Charge Measurement of the discovered particle s charge Discovery of the Top-Quark Properties Single Top Production top charge of 2 3e has to be proven by experiment exotic 4 3e charge is imaginable as well most challenging problem is jet charge 2007 D0 result used 21 semileptonic t t-events with 2 b-tagged jets estimator q jet = Σ i q i p 0.6 Ti Σ i pti 0.6 with all tracks in jet with p T > 0.5 GeV compared charge distribution with distribution from b(bar)-jets from b b-dijet events with one muon in jet used jet with high p t,rel muon as tag and other jet as probe 4 3e only can be excluded with 92% C. L. contribution of such quark smaller than 80% with 90% C. L. 21 / 44

26 Other Properties Production Mechanism q q or gg? used number of low p t tracks to identify gluon-rich events % from q q QCD predicts 15±10% Discovery of the Top-Quark Properties Single Top Production Forward-Backward Asymmetry A fb N( Y >0) N( Y <0) t t = N( Y >0)+N( Y <0) SM predicts 5% deviation e. g. by Z results compatible with SM but large errors Exotic Decays t Z 0 c in direct search t H + b indirect search no evidence found 22 / 44

27 Discovery of the Top-Quark Properties Single Top Production Confirmation of Single Top Production 23 / 44

28 Discovery of the Top-Quark Properties Single Top Production Confirmation of Single Top Production 0.88±0.14 pb 1.98±0.30 pb 0.08±0.02 pb σ t 60% smaller than σ t t σ tw very small s-channel and t-channel hard to distinguish 23 / 44

29 Discovery of the Top-Quark Properties Single Top Production Confirmation of Single Top Production Discovery: 0.88±0.14 pb first observation with > 3σ by both collaborations in 2007 in s and t-channel used channel with top decaying to e or µ 1.98±0.30 pb 0.08±0.02 pb σ t 60% smaller than σ t t σ tw very small s-channel and t-channel hard to distinguish event selection on: high p t lepton E t 1 or 2 b-jets S/B 1/17 after selection good comprehension of background important used multivariate techniques 23 / 44

30 Single Top Production Discovery of the Top-Quark Properties Single Top Production Latest Results (CDF + D ): significance of 5 σ achieved different multivariate analyses on same data: likelihood, matrix element, boosted decision trees, neural network combined outputs to super discriminator additionally hadronic tau decay included (jets + E t selection) V tb > 0.79 at 95% C. L. no evidence for associated tw production 24 / 44

31 Outline The LHC-Experiments Planned Experiments The LHC-Experiments Planned Experiments 4 25 / 44

32 The LHC The LHC-Experiments Planned Experiments Large Hadron Collider collisions of protons or heavy ions circumference: 27 km s up to 14TeV L = cm 2 s 1 26 / 44

33 Prospects The LHC-Experiments Planned Experiments accuracy of most measurements at Tevatron is still limited by statistics at s = 7 TeV cross section is about 20 times higher than at Tevatron energy higher statistics to decrease errors of existing measurements but also to allow several new measurements delayed start of LHC at lower energies (7 TeV and 10 TeV) gives opportunity to measure cross section at different energies 27 / 44

34 The LHC-Experiments Planned Experiments Mass Measurement Using b Decay Length b decay length is correlated with top mass in top rest frame: p = mt 2 q 1 ((M 2 W m2 b )/m2 t ) 2 4(M W m b /m 2 t ) 2 p L = τ 0βγ = τ 0 m b reconstruct secondary vertex and measure distance to primary vertex systematics independent of those of other methods: b quark fragmentation b hadron lifetimes well suited for crosschecking with other methods systematics expected to be 1.5 GeV statistical error with all Tevatron data 5 GeV while at LHC quickly smaller than 1 GeV 28 / 44

35 Spin Correlations The LHC-Experiments Planned Experiments strong correlation between spins of t and t depending on production mechanism gg t t near threshold gives 1 S 0 state (opposite spin, same helicity) q q t t results in 3 S 1 state (same spin, opposite helicity) states can be distinguished by angular distributions of the decay products (spin analyzing particles) leptons and down-like quarks are best correlated to top spin use dilepton channel (d-type jets difficult to identify) could also be applied at Tevatron but method needs many events deviations from SM predictions could be interesting: assumtion that top does not hadronizes upper limit on top lifetime V tb deviations due to top- or technicolor particles 29 / 44

36 Outline The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction 30 / 44

37 The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction Der CMS-Detektor Compact Muon Solenoid: more compact than Atlas m = t B = 4T good muon identification 31 / 44

38 The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction Der CMS-Detektor magnet s u Compact Muon Solenoid: more compact than Atlas m = t B = 4T good muon identification 31 / 44

39 The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction Der CMS-Detektor magnet pixel detektor % %,Si-tracker, % %,, s %, u %, s, % u s u Compact Muon Solenoid: Pixel Detector: more compact than Atlas silicon pixels m = t Si-Strip Detector: B = 4T silicon strips good muon identification 31 / 44

40 The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction Der CMS-Detektor magnet pixel detektor % %,Si-tracker, % %,, ECAL s %, u %,, s u % u s u s su s u E s u E s u E HO E E ECAL: Compact Muon Solenoid: E Pixel Detector: PbW04 -crystals E more compact than Atlas HCAL:1 440 silicon pixels E HCAL m = t E Si-Strip Detector: sampling calorimeter B = 4T absorbers silicon strips brass good muon identification scintillator 31 / 44

41 The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction Der CMS-Detektor magnet pixel detektor % %,Si-tracker, % %,, ECAL s %, u %, s u, s u % u s u su s E s u s E E µ-detectors HO E E ECAL: Muon System: Compact Muon Solenoid: E Pixel Detector: Tubes PbW0 Drift in4 -crystals barrel E more compact than Atlas 440 silicon pixels HCAL:1 E Catodic Strip Chambers in HCAL m = t E endcaps Si-Strip Detector: sampling calorimeter B = 4T Resistive Platestrips Chambers for absorbers silicon brass good muon identification trigger timing scintillator 31 / 44

42 The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction Aim: top rediscovery in real data (50 pb 1 at 7 TeV in 2010) first cross section measurement concentrate on dimuon channel Signature: 2 muons 2 b-jets E t due to 2 neutrinos Not completly understood detector (Calibration, Alignment) no cut on E t no cut on b-tagging 32 / 44

43 Background The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction Important Backgrounds: DY + jets WW, WZ, ZZ QCD BG e. g. b b also taken into account At first data taking simulation is not fully tuned to reality data driven background description QCD and fake muons from wrong charge events DY from Z 0 -Peak MC for signal and backgrounds at s = 7 TeV pseudo data (realistic mix of all samples) with realistic 50 pb 1 statistics (except QCD) 33 / 44

44 Event Selection The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction trigger on high p t muon S/B 0.7% 2 muons needed that pass kinematic cuts p t > 20 GeV η < 2.4 and quality cuts combined isolation variable basing on activity in η-φ-cone with r = 0.5 around muon in calo and tracker: I Ecal +I Hcal +I Trk p t < / 44

45 Event Selection The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction cuts on invariant µµ-mass veto on Z 0 -mass: (76 GeV < m µµ < 106 GeV) MC can t make detailed predictions about QCD: single events that survive selection have a very high event weight cut away small masses: (m µµ < 20 GeV) 35 / 44

46 Event Selection The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction Jet Selection: 2 jets from b-quarks used anti-kt5 algorithm applied η and p t dependent energy corrections p t > 40 GeV η < 2.3 after selection S/B > 2 for n jets > 2 36 / 44

47 QCD Background Description Wrong Charge Method signal has two oppositly charge muons same is true for DY BG or events with 2 gauge bosons but also events with (almost) uncorrelated charge in QCD and fake muon BG select also events with same-charge muons to describe those BGs The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction determine normalisation factor between like and unlike sign events from data invariant µµ-mass shape asymmetric at low masses cut on low masses 37 / 44

48 QCD Background Description Wrong Charge Method signal has two oppositly charge muons same is true for DY BG or events with 2 gauge bosons but also events with (almost) uncorrelated charge in QCD and fake muon BG select also events with same-charge muons to describe those BGs The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction determine normalisation factor between like and unlike sign events from data invariant µµ-mass shape asymmetric at low masses cut on low masses 37 / 44

49 Wrong Charge Method The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction Plot invariant µµ-mass after cuts in bins of worse isolated muon s isolation separately for opposite and same charge events 38 / 44

50 Wrong Charge Method The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction signal-like processes contribute only in first bin divide number of entries for right and wrong chage in other bins normalisation factor for each bin extrapolate to high isolation 39 / 44

51 Z Background Estimation The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction In Progress: shape of DY spectrum is well described in MC count events in Z-mass region (76 GeV < m µµ < 106 GeV) extrapolate to region outside the peak 40 / 44

52 Kinematic Event Reconstruction System of kinematic equations The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction missing information due to unobserved neutrinos instead of that x- and y- component of their energy is measured as E t using W-mass and equality of top and antitop mass as constraints eliminates three further unknown variables E t,x = p ν,x + p ν,x E t,y = p ν,y + p ν,y E 2 ν = p 2 ν,x + p 2 ν,y + p 2 ν,z E 2 ν = p2 ν,x + p2 ν,y + p2 ν,z m W + = (E l + + E ν) 2 (p l +,x + p ν,x ) 2 (p l +,y + p ν,y ) 2 (p l +,z + p ν,z ) 2 m W = (E l + E ν) 2 (p l,x + p ν,x )2 (p l,y + p ν,y )2 (p l,z + p ν,z )2 m t = (E l + +E ν +E b ) 2 (p l +,x +p ν,x +p b,x ) 2 (p l +,y +p ν,y +p b,y ) 2 (p l +,z +p ν,z +p b,z ) 2 m t = (E l +E ν +E b )2 (p l,x +p ν,x +p b,x )2 (p l,y +p ν,y +p b,y )2 (p l,z +p ν,z +p b,z )2 41 / 44

53 Kinematic Event Reconstruction The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction system of equations can be transformed into a single equation (using simplifying assumptions) 4th order polynomial in one neutrino momentum component, e. g. p ν,x coefficients h i are functions of m t p 4 ν,x + h 3 p 3 ν,x + h 2 p 2 ν,x + h 1 p ν,x + h 0 = 0 equation can be solved analytically for constant h i fourfold ambiguity 42 / 44

54 Kinematic Event Reconstruction The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction loop over muon jet combinations vary top mass parameter in 1 GeV-steps between 100 GeV and 300 GeV compare calculated E ν and E ν to normalized model spectrum from MC probability value from spectrum is taken as weight for each solution take most probable value in each event approximation of the top mass distribution 43 / 44

55 Summary The CMS-Experiment Event Selection Background Description Kinematical Event Reconstruction Analysis General top rediscovery with 4σ with 50 pb 1 at 7 TeV possible new LHC shedule has just been published to collect more lumi at 7 TeV good cross section measurement should be possible with 7 TeV data top is not just another SM particle but strongly connected to Higgs and many BSM modells most experiments still done at Tevatron LHC can improve all values due to much higher statistics 44 / 44