A TEVATRON eredményei

Transcription

1 A TEVATRON eredményei A hadronütköztetők általában Jelen és jövendő hadronütköztetők - Tevatron és LHC A Standard modell ellenőrzése - QCD: Jet, W/Z, top-keletkezés - A W és a top-kvark tömege A Higgs-bozon keresése Új fizika keresése Pásztor Gabriella helyett beugorva: Horváth Dezső (ahogy azt a webről össze tudta lopkodni) Fő forrás: K. Jakobs (Freiburg), CERN, 2006

2 Key Questions of Particle Physics 1. Mass: What is the origin of mass? - How is the electroweak symmetry broken? - Does the Higgs boson exist? 2. Unification: What is the underlying fundamental theory? Motivation: Gravity not yet included; Standard Model as a low energy approximation - Is our world supersymmetric? - Are there extra space time dimensions? - Other extensions? 3. Flavour: or the generation problem - Why are there three families of matter? - Neutrino masses and mixing? - What is the origin of CP violation?

3 The role of Hadron Colliders 1. Mass The link between SUSY and Dark Matter? - Search for the Higgs boson 2. Unification - Test of the Standard Model - Search for Supersymmetry - Search for other Physics Beyond the SM 3. Flavour - B hadron masses and lifetimes - Mixing of neutral B mesons M. Battaglia, I. Hinchliffe, D.Tovey, hep-ph/ CP violation Energy Explore the TeV energy domain Experiments must also be prepared for the unexpected Precision Further tests of the Standard Model

4 Where do we stand today? e+e- colliders LEP at CERN and SLC at SLAC + many other experiments (Tevatron, fixed target.) have explored the energy range up to ~100 GeV with incredible precision However: The Standard Model is consistent with all experimental data! Light Higgs boson favoured No evidence for phenomena beyond the SM

5 Why a hadron collider? e+e- colliders are excellent machines for precision physics!! - e+ e- are point-like particles, no substructure clean events - complete annihilation, centre-of-mass system, kinematic fixed

6 A proton-proton ütközések bonyolultak

7 A tárológyűrűs e+e- ütközőnyalábok fő problémája a szinkrotron-sugárzás - Körönkénti energiaveszteség (E energia, R görbületi sugár, m tömeg): - Proton és elektron energiavesztesége: Megoldás: LHC (protongyorsító LEP-alagútban) ILC (International Linear Collider)

8 Important components of the accelerator superconducting dipole magnets - challenge: magnetic field of 8.33 Tesla - in total 1232 magnets, each 15 m long - operation temperature of 1.9 K LHC is the largest cryogenic system in the world Eight superconducting accelerator structures, acceleration gradient of 5 MV/m

9 FERMILAB

10 The Tevatron Collider at Fermilab Proton antiproton collider 2 Experiments: CDF and DØ : Run I, s = 1.8 TeV 6 x 6 bunches, 3 µs spacing L dt = 125 pb : upgrade programme Accelerator: new injector (x5) antiproton recycler (x2) 36x36 bunches, 396 ns spacing + Detectors March 2001 Feb 2006: Run II a, s = 1.96 TeV, 1.2 fb-1 July Run II b, s = 1.96 TeV, 5-8 fb : Real Data

11 Physics at Hadron Colliders Protons are complex objects: Partonic substructure: Quarks and Gluons Hard scattering processes: (large momentum transfer) quark-quark quark-gluon scattering or annihilation gluon-gluon However: hard scattering (high PT processes) represent only a tiny fraction of the total inelastic pp cross section Total inelastic pp cross section ~ 70 mb (huge) Dominated by events with small momentum transfer

12 Variables used in the analysis of pp collisions p θ pt Transverse momentum (in the plane perpendicular to the beam) pt = p sinθ θ = 90o η=0 θ = 10o η 2.4 θ = 170o η -2.4 θ= 1o η 5.0

13 Hard scattering process: Proton beam can be seen as beam of quarks and gluons with a wide band of energies The proton constituents (partons) carry only a fraction 0 < x < 1 of the proton momentum x1 p s The effective centre-of-mass energy x2 p is smaller than s of the incoming protons To produce a mass of: 100 GeV: 5 TeV: LHC x ~ x ~ 0.36 Tevatron

14 From where do we know the x-values? The structure of the proton is investigated in Deep Inelastic Scattering experiments: Today s highest energy machine: the HERA ep collider at DESY/Hamburg Scattering of 30 GeV electrons on 900 GeV protons: Test of proton structure down to m HERA ep accelerator, 6.3 km circumference

15 How do the x-values of the proton look like? Parton density functions (pdf): u- and d-quarks at large x-values Gluons dominate at small x!! Uncertainties in the pdfs, in particular on the gluon distribution at small x

16 Calculation of cross sections σ= dx a dx b f a x a, Q 2 f b x b, Q 2 σ ab x a, x b a,b Sum over initial partonic states σ ab a,b hard scattering cross-section fi (x, Q2) parton density function Example: W-production: (leading order diagram) u W+ σ (pp W) ~ 150 nb ~ σtot (pp) d + higher order QCD corrections (perturbation theory)

17 Luminozitás Várható esemény-gyakoriság adott fizikai folyamatra: N dimenzió: s-1 = ε L = σ cm-2 s-1 cm2 ε = hatásfok L = luminozitás σ = hatáskeresztmetszet Luminosity depends on the machine: important parameters: number of protons stored, beam focus at interaction region,. In order to achieve acceptable production rates for the interesting physics processes, the luminosity must be high! L = cm-2 s-1 L = 1033 cm-2 s-1 L = 1034 cm-2 s-1 design value for Tevatron Run II planned for the initial phase of the LHC (1-2 years) LHC design luminosity, very large!! (1000 x larger than LEP-2, 50 x Tevatron Run II design) One experimental year has ~ 107 s Integrated luminosity at the LHC: 10 fb-1 per year, in the initial phase 100 fb-1 per year, later, design

18 What experimental signatures can be used? Quark-quark scattering: q q p No leptons / photons in the initial and final state q q p If leptons with large transverse momentum are observed: interesting physics! Example: Higgs boson production and decay q q W W H p ν ν q q p Important signatures: Leptons and photons Missing transverse energy

19 The CDF-Experiment New in Run II : Tracking system Silicon vertex detector (SVXII) Intermediate silicon layers Central outer tracker (COT) End plug calorimeter Time of flight system Front-end electronics Trigger and DAQ systems

20 The DØ Experiment 19 countries, 83 institutions 664 physicists New for Run II Inner detector magnetic field added Preshower detectors Forward muon detector Front-end electronics Trigger and DAQ

21 Tevatron Luminosity Goals ~ 8 fb-1 We are here ~ 5 fb-1 Additional improvements in shutdown 2006 (electron cooling in the recycler) Final performance depends on antiproton stacking rate in the accumulator (at present 20 ma/h = pbar /h )

22 QCD processes at hadron colliders Hard scattering processes are dominated by QCD jet production Originating from quark-quark, quark-gluon and gluon-gluon scattering Due to fragmentation of quarks and gluons in final state hadrons Jets with large transverse momentum PT in the detector Cross sections can be calculated in QCD (perturbation theory) Comparison between experimental data and theoretical predictions constitutes an important test of the theory. Deviations? Problem in the experiment? Problem in the theory (QCD)? New Physics, e.g. quark substructure?

23 A two jet event at the Tevatron (CDF) Dijet Mass = 1364 GeV/c2 ET = 666 GeV η = 0.43 CDF (φ-r view) ET = 633 GeV η = -0.19

24 A two jet event in the DØ experiment φ η Mjj = 838 GeV/c2 pt(1) = 432 GeV/c pt(2) = 396 GeV/c

25 Test of QCD Jet production Data from the DØ experiment (Run II) Inclusive Jet spectrum as a function of Jet-PT very good agreement over many orders of magnitude! within the large theoretical and experimental uncertainties

26 Similar data from the CDF experiment contributions of the various sub-processes to the inclusive jet cross section Data corresponding to ~1 fb-1 Double differential distributions in PT and η

27 Main experimental systematic uncertainty: Jet Energy Scale Jet response correction in DØ: measure response of particles making up the jet use photon + jet data - calibrate jets against the better calibrated photon energy q g γ q

28 Comparison with Theory Fully corrected inclusive jet cross section Systematic uncertainties: - jet energy scale (red band) - parton density functions - theory: renormalization scale

29 Di-jet angular distributions: reduced sensitivity to Jet energy scale sensitive to higher order QCD corrections Good agreement with next-to-leading order QCD-predictions

30 Test of W and Z production Number of detected W-bosons: p p q q l W(Z) ν (l) Drell-Yan production process (leading order) Tevatron: expected rates for 2 fb-1: LHC: expected rates for 10 fb-1: 3 Mio W ℓ ν events 60 Mio W ℓ ν events

31 Z ℓℓ cross sections Good agreement with NNLO QCD calculations C.R.Hamberg et al, Nucl. Phys. B359 (1991) 343. Precision is limited by systematic effects (uncertainties on luminosity, parton densities,...)

32 W ℓν Cross Section M TW = 2 P Tl P νt 1 cos Δφ l,ν Note: the longitudinal component of the neutrino cannot be measured only transverse mass can be reconstructed Good agreement with NNLO QCD calculations C.R.Hamberg et al, Nucl. Phys. B359 (1991) 343. Precision is limited by systematic effects (uncertainties on luminosity, parton densities,...)

33 Comparison between measured W/Z cross sections and theoretical prediction (QCD) C. R. Hamberg, W.L. van Neerven and T. Matsuura, Nucl. Phys. B359 (1991) 343

34 QCD Test in W/Z + jet production Z + 1 jet Z + 2 jets Z + 3 jets Compare # of W/Z + n jet events (data versus QCD Monte Carlo Models -SHERPA, ALPGEN-)

35 Why is Top-Quark so important? The top quark may serve as a window to New Physics related to the electroweak symmetry breaking; We still know little about the properties of the top quark: mass, spin, charge, lifetime, decay properties (rare decays), gauge couplings, Yukawa coupling,

36 Top Quark Production Pair production: qq and gg-fusion qq gg σ (pb) Run I 1.8 TeV Run II 1.96 TeV LHC 14 TeV 90% 10% 85% 15% 5% 95% 5 pb 7 pb 600 pb Electroweak production of single top-quarks (Drell-Yan and Wg-fusion) σ (qq) (pb) σ (gw) (pb) σ (gb) (pb) Run I 1.8 TeV Run II 1.96 TeV LHC 14 TeV

37 Top Quark Decays BR (t Wb) ~ 100% Both W s decay via W ℓν (ℓ=e or µ; 5%) dilepton dilepton channel One W decays via W ℓν (ℓ=e or µ; 30%) lepton+jets lepton + jet channel Both W s decay via W qq (44%) all hadronic, not very useful Important experimental signatures: : - Lepton(s) - Missing transverse momentum - b-jet(s)

38 DØ top candidate event with two leptons pt(e) = 20.3 GeV/c pt(µ) = 58.1 GeV/c ETj = 141.0, 55.2 GeV ET=91 GeV

39 tt cross section (lepton + jets) (topology, no b-jet identification) 1 high-pt isolated lepton 3 jets Large missing ET l v W W t t B-jet B-jet HT = scalar sum of all high PT objects (jets, leptons, ETmiss) Before b-tagging: background from W+jet events clearly dominates

40 Tagging of b-quarks Soft lepton tagging Search for non-isolated soft lepton in a jet Silicon Vertex tag B mesons travel ~ 3 mm before decaying: Search for secondary vertex

41 tt cross section (lepton + jets) (including b-tagging) 1 high-pt isolated lepton, at least one b-tagged jet Large missing ET hep-ex/ secondary vertex tag L=230 pb-1 Control region Signal region Excess above the W+ jet background in events with high jet multiplicity

42 tt cross section (lepton + jets) (including double b-tag) 1 high-pt isolated lepton + Two b-tagged jet Large missing ET Very clean top sample

43 tt cross section summary (preliminary) QCD prediction: - Cacciari et al., hep-ph/ Kidonakis et al., hep-ph/ Good agreement among various exp. measurements and with QCD prediction (similar results for DØ)

44 Precision measurements of mw and mtop Motivation: W mass and top quark mass are fundamental parameters of the Standard Model; The standard theory provides well defined relations between mw, mtop and mh Electromagnetic constant measured in atomic transitions, e+e- machines, etc. mw = πα EM 2 G F Fermi constant measured in muon decay 1/2 1 sin θ W 1 Δr weak mixing angle measured at LEP/SLC GF, αem, sin θw are known with high precision radiative corrections r ~ f (mtop2, log mh) r 3% Precise measurements of the W mass and the top-quark mass constrain the Higgsboson mass (and/or the theory, radiative corrections)

45 mw = πα EM 2 G F 1/2 The W-mass measurement 1 sin θ W 1 Δr mw (from LEP2 + Tevatron) = ± GeV mtop (from Tevatron) = ± 2.3 GeV 1.4% light Higgs boson is favoured by present measurements Ultimate test of the Standard Model: comparison between the direct Higgs boson mass (from observation, hopefully) and predictions from rad. corrections.

46 Technique used for W-mass measurement at hadron colliders: Event topology: DØ Z ee Observables: PT(e), PT(had) PT(ν) = - ( PT(e) + PT(had) ) long. component cannot be M WT = 2 P Tl P νt 1 cos Δφ l,ν measured In general the transverse mass MT is used for the determination of the W-mass (smallest systematic uncertainty).

47 Shape of the transverse mass distribution is sensitive to mw, the measured distribution is fitted with Monte Carlo predictions, where mw is a parameter Main uncertainties: mw= 79.8 GeV mw= 80.3 GeV result from the capability of the Monte Carlo prediction to reproduce real life: detector performance (energy resolution, energy scale,.) physics: production model pt(w), ΓW,... backgrounds mtw (GeV) Dominant error (today at thetevatron, and most likely also at the LHC) : Knowledge of lepton energy scale of the detector!

48 Signature of Z and W decays Z l+l W lν

49 Top mass measurements Top mass calculation: Kinematic fit under (tt) hypothesis compute likelihood for observed events as a function of the top quark mass Maximum likelihood mtop l v W W t Bjet t Bjet Lepton+jets ( 1 b-tag) Most precise single measurements: mtop = ±3.5 (stat+jes) ± 1.3 (syst) GeV/c2 (CDF) mtop = ±4.4 (stat+jes) ± 1.4 (syst) GeV/c2 L=230 pb-1 (DØ) Reduce JES systematic by using in-situ hadronic W mass in tt events (simultaneous determination of mt and JES from reconstructed mt and MW templates)

50 Future Prospects for the top quark mass measurement 1. Channel dependence? still statistically consistent results; full hadronic channel is difficult 4. Expected Tevatron precision (full data set): ± 1.5 GeV/c2 6. Expected LHC precision for 10 fb-1: < ~ 1 GeV/c2 (Combination of several methods, maybe somewhat conservative)

51 Konklúzió Rengeteg értékes eredmény a TEVATRON-tól: (top, W, protonszerkezet) Minden egyezik a Standard modellel Útmutatás LHC-fizikának Higgs-bozon, új fizika meglelésére kicsi az esélye LHC nagyságrenddel nagyobb felfedezési potenciállal rendelkezik