7. Physics of the Top Quark

Transcription

1 7. Physics of the Top Quark 7.1 Introduction (The top quark in the Standard Model) 7.2 Top quark production at the Tevatron 7.3 First results on top quark production from the LHC 7.4 Top-quark mass measurements (Tevatron and LHC) 7.5 Constraints on the Standard Model

2 7.1 Introduction to Top Quark Physics Discovered by the CDF and DØ collaborations at the Tevatron in 1995 Tevatron Run I top physics results are consistent with expectations from the Standard Model (Errors dominated by statistics) Run II top physics program profits a lot from the higher statistics LHC: huge production rates (for s = 7 TeV: about a factor 25 larger cross sections than at the Tevatron) - Better precision - Search for deviations from Standard Model expectations

3 What do we know about the top quark? The top quark is the heaviest know fermion m t ~173 GeV (from experiment) Lifetime ~ 5-25 s (theory, Standard Model decays) no hadronisation, behaves like a quasi-free quark! Predominant decays: t Wb (BR ~0%) largely determined from very small CKM matrix elements V td, V ts Electric charge Q t = +2/3

4 Why is Top-Quark so important? The top quark may serve as a window to New Physics related to the electroweak symmetry breaking; Why is its Yukawa coupling ~ 1?? We still know little about the properties of the top quark: mass, spin, charge, lifetime, decay properties (rare decays), gauge couplings, Yukawa coupling, A unique quark: decays before it hadronizes, lifetime ~ -25 s no toponium states remember: bb, bd, bs.. cc, cs.. bound states (mesons)

5 Pair production: qq and gg-fusion Top Quark Production NLO corrections completely known NNLO partly known approximate NNLO results: qq gg Tevatron 1.96 TeV 85% 15% LHC 14 TeV 5% 95% (pb) 7.0 pb 887 pb For LHC running at s = 7 TeV, the cross section is reduced by a factor of ~5, but it is still a factor 25 larger than the cross section at the Tevatron

6 Single Top Quark Production Electroweak production of single top-quarks (Drell-Yan and Wg-fusion)

7 Top Quark Decays BR (t Wb) ~ 0% Dilepton channel: Both W s decay via W ( =e or μ; 4%) Lepton + jet channel: One W decays via W ( =e or μ; 30%) Full hadronic channel: Both W s decay via W qq (46%) Important experimental signatures: : - Lepton(s) - Missing transverse momentum - b-jet(s)

8 7.2 Measurement of Top Quark production at the Tevatron

9 tt cross section (dilepton) l v Two high p T leptons (opposite charge) ee, eμ, μμ Significant missing transverse momentum 1 jet (eμ), 2 jets (ee, μμ) l W W t t B-jet ee,eμ and μμ combined v B-jet Top quark is needed to describe the b-jet multiplicity distribution in dilepton events

10 Tevatron b-tagging performance Similar for CDF Neural networks are used for optimal combination of tagging information

11 tt cross section (lepton + jets) (including b-tagging) b-tag selection: One high P T lepton (e, μ) Significant E miss T 1 b-tagged jet Kinematic selection: One high P T lepton (e, μ) Significant E miss T 4 jets Likelihood discriminant (tt vs. W+jets) Clear excess above the W+ jet background in events with high jet multiplicity

12 tt cross section summary from the Tevatron Summary of syst. uncertainties Good agreement: - among various exp. measurements (two experiments) - and with NLO + LL QCD prediction - Systematic uncertainties at the % level (luminosity, b-tagging)

13 7.3 First measurements of Top Quark production at the LHC Event display of a top pair e-μ dilepton candidate with two b-tagged jets. The electron is shown by the green track pointing to a calorimeter cluster, the muon by the long red track intersecting the muon chambers, and the missing ET direction by the dotted line on the xy-view. The secondary vertices of the two b-tagged jets are indicated by the orange ellipses on the zoomed vertex region view.

14 First results on top production from the LHC Events ATLAS L = 2.9 pb tagged e/ +jets data tt single top Z + jets W + jets QCD uncertainty Event Selection: Lepton trigger One identified lepton (e,μ) with p T > 20 GeV Missing transverse energy: E T miss > 35 GeV (significant rejection against QCD events) Transverse mass: M T (l, ) > 25 GeV (lepton from W decay in event) One or more jets with p T > 25 GeV and < 2.5 Number of jets

15 Details on the composition of the event samples: ATLAS experiment, L = 35 pb (data from 20)

16 b-tag requirements, higher integrated luminosity Entries/ ATLAS Preliminary + 3 jets data tt W+Jets QCD Other L dt = 35 pb KS test: 0.04 Distribution of the b-tagging discriminant in events with at least 3 jets; 1 tt component dominated the 4 and 5 jet bins w JP Entries/ ATLAS Preliminary + 4 jets data tt W+Jets QCD Other L dt = 35 pb KS test: 0.25 Entries/ ATLAS Preliminary 5 jets + data tt W+Jets QCD Other L dt = 35 pb KS test: w JP w JP

17 Events / (25 GeV) Events / (25 GeV) Description of the invariant mass distributions in the l-had channel ATLAS Preliminary L dt = 35 pb 3 jets / 0 b-tag Data Model [GeV] ATLAS Preliminary m jjj L dt = 35 pb [ +jets] Background [GeV] m jjj [ +jets] 4 jets / 0 b-tag Data Model Background Events / (25 GeV) Events / (25 GeV) 30 ATLAS Preliminary [ +jets] L dt = 35 pb 3 jets / 1 b-tag Data Model Background [GeV] m jjj 50 ATLAS Preliminary [ +jets] L dt = 35 pb 4 jets / 1 b-tag Data Model Background [GeV] m jjj Events / (25 GeV) Events / (25 GeV) ATLAS Preliminary L dt = 35 pb Data Model [GeV] m jjj [ +jets] 3 jets / 2 b-tag Background 22 ATLAS Preliminary [ +jets] L dt = 35 pb 4 jets / 2 b-tag Data Model Background [GeV] m jjj Top fractions increase with number of b-tags Good description for all jet-multiplicity and b-tag combinations Data are consistent with top quark production with mass of 173 GeV

18 CMS tt signals in the di-lepton channel

19 CMS tt signals in the di-lepton channel

20 CMS tt signals in the di-lepton channel

21 Top cross section measurements based on 20 data from ATLAS and CMS [pb] tt 2 NLO QCD (pp) Approx. NNLO (pp) NLO QCD (pp) Approx. NNLO (pp) CDF D0 ATLAS pb (35 pb, Prelim.) CMS pb (36 pb, Prelim.) s [TeV] ) -log ( tt s= 7 TeV 1 L dt = 35 pb 0.5 ATLAS Preliminary tt / SM Best fit (ATLAS) gives a slightly higher cross-section than the expected approx. NNLO QCD value, but consistent within 1 (red: likelihood, stat errors only; blue: stat + syst. uncertainties) Results between the two experiments are consistent Perturbative QCD calculations are in agreement with the obtained results

22 Summary of ATLAS cross section measurements ATLAS Preliminary Data 20, L = 35 pb L+jets w/ b-tagging Theory (approx. NNLO) m t = GeV Multivariate Top mass profile fit Top mass standard fit Counting (stat) (syst) (lumi) tt [ pb ] Perturbative QCD calculations (approx. NNLO) describe the data well

23 7.4 Top-quark mass measurement

24 Top mass measurements Top mass determination: No simple mass reconstruction possible, Monte Carlo models needed template methods, matrix element method Most precise single measurements: m top = ± 0.9 (stat) ± 1.3 (syst) GeV/c 2 (CDF) m top = ± 0.8 (stat) ±1.6 (syst) GeV/c 2 (DØ) Reduce jet energy scale systematic by using in-situ hadronic W mass in tt events (simultaneous determination of m t and energy scale) full hadronic channel

25 Example: template method Calculate a per-event observable that is sensitive to m t Make templates from signal and background events Use pseudo-experiments (Monte Carlo) to check that method works Fit data to templates using maximum likelihood method

26 Summary of present results and future prospects Expected LHC precision for fb : (Combination of several methods, maybe somewhat conservative) < ~ 1 GeV/c 2

27 Top-Quark Mass [GeV] CDF ± 1.2 D ± 1.7 Average ± 1. 2 /DoF: 6.1 / LEP1/SLD LEP1/SLD/m W / W m t [GeV] July 20

28 First top quark mass measurements from CMS CMS, Use lepton + jet channel Full 20 data set Number of Events / 20 GeV candidate events selected CMS Preliminary, L = 36 pb Data tt W l Single-Top + - Z/ * l l QCD e + jets channel Already impressive precision reached at that early stage of the experiment! Fitted Top Mass [GeV] Top quark mass after the fit of the e+jets selected sample for an integrated luminosity of 36/pb after applying the event selection and requesting at least one solution with chi2<.

29 Estimated error compared to expectations from Monte Carlo simulations:

30 Other top properties f 0 f + Tevatron Result luminosity (fb ) Mass ± 1.1 GeV ~ 5.0 W helicity CDF: f 0 = 0.66 ± 0.16, f + = ± 0.07 DØ: f 0 = 0.49 ± 0.14 f + = 0.11 ± Charge Lifetime V tb BR(t Wb) / BR(W Wq) BR (t Zq) rule out Q = +4/3 (90.% C.L.) t < 13.1 GeV (95% C.L.) V tb > 0.89 (95% C.L.) R = 0.97 (+0.09) (-0.08) < 3.7% (95% C.L.) 1.5 ~

31 7.5 Constraints on the Standard Model July 20 Theory uncertainty (5) had = ± ± incl. low Q 2 data m Limit = 158 GeV m W [GeV] July 20 LEP2 and Tevatron (prel.) LEP1 and SLD 68% CL 1 Excluded Preliminary m H [GeV] 80.3 m H [GeV] m t [GeV]

32 Best estimate for the Higgs boson mass from the different electroweak observables: July 20 Z 0 had R 0 l A 0,l fb A l (P ) R 0 b R 0 c A 0,b fb A 0,c fb A b A c A l (SLD) sin 2 lept eff (Q fb ) m W * W * Examples: R l and m W Ratio of Hadronic to Leptonic Width Mass of the W Boson (preliminary) Experiment R l = had / l Experiment M W GeV ALEPH ALEPH DELPHI DELPHI L L OPAL OPAL / dof = 3.5 / 3 2 / dof = 29.2 / 35 LEP LEP common error Q W (Cs) sin 2 (e e MS ) sin 2 W ( N) g 2 L ( N) g 2 R ( N) *preliminary M H GeV S = linearly added to M t = GeV M H GeV M W GeV (5) had = linearly added to M t = GeV 2 3 R l M H [GeV]

33 experimental errors 68% CL: LEP2/Tevatron (today) Tevatron/LHC ILC/GigaZ light SUSY M W [GeV] M H = 114 GeV SM M H = 400 GeV SM MSSM both models MSSM heavy SUSY Heinemeyer, Hollik, Stockinger, Weber, Weiglein m t [GeV] Predictions for future precision (including LHC), compared to the Standard Model and its Minimal Supersymmetric Extension (MSSM) Ultimate test of the Standard Model: compare direct prediction of Higgs mass with direct observation