Top Quark Physics Pierre Savard University of Toronto and TRIUMF APS Meeting Denver May 2004.

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Top Quark PhysicsTop Quark Physics

Pierre SavardPierre Savard

University of Toronto and TRIUMFUniversity of Toronto and TRIUMF

APS Meeting APS Meeting

DenverDenver

May 2004May 2004

History and Theoretical OverviewReview of Experimental Results

Electroweak constraintsRun I Results

Run II ResultsRun II Results

Outlook

OutlineOutline

• Discovery of b quark in 1977 – isospin analysis shows that

b should have SU(2) partner

• Indirect evidence of top through loop contributions:– BB Mixing– Z bb rate

– MW/MZ ratio

• 95 CDF and D0 announce discovery

GeV 180M t

Some HistorySome History

If we assume CKM matrix unitarity and measured mass ~ 175 GeV/c2 , then top properties are well understood within context of SM:

Spin = ½ couplings:

- +2/3e - color triplet - weak (T3)L

- Yukawa coupling ~1 Width: ~ 1.4 GeV lifetime ~ 5 x 10-25 (no top hadrons!)

Top Quark in the SM (I)Top Quark in the SM (I)

Only quark decaying to W

Mt at the ewk scale

Large Mt-Mb difference implies large ewk loop corrections

• Vtb = 0.99• branching ratios:

– t W b BR ~ 1 – t W s BR ~ 10-3

– t W d BR ~ 5 x 10-5 – t c,u BR ~ 10-8

– t Z c,u BR ~ 10-12 • Fraction of longitudinal W

bosons:

Top Quark in the SM (II)Top Quark in the SM (II)

longpol

e-e (1/81)

mu-mu (1/81)

tau-tau (1/81)

e -mu (2/81)

e -tau (2/81)

mu-tau (2/81)

e+jets (12/81)

mu+jets(12/81)

tau+jets(12/81)

jets (36/81)

• Experimental signatures depend on how W decays:

~70% of W bosons longitudinally polarised for Mt of 175 GeV/c2

• Theoretical models proposed to solve problems of SM often have top playing a leading role:

– In supersymmetric models, large top mass causes EWSB:

– In many dynamical symmetry breaking models, top interactions are modified:

• Examples: technicolour models like Top Flavour, Top Seesaw, Topcolour-assisted technicolour

• Need to test all aspects of top production and decay. Experimentally we still know very little about the top quark

MMMqM X

tXhh )ln(8

3)()( 2

Top Quark Beyond the Top Quark Beyond the SM SM

Main Injectorand Recycler

p source

Booster

DD• Run 1, 100 pb-1:

– collisions every ~ 3sec

– beam energy 900 GeV– inst. Luminosity 1031

• Run 2:– collisions every

~400ns– beam energy 980 GeV– inst. Luminosity 1032

• CDF and D0 detectors underwent major upgrades for Run II

Experimental Top Quark Experimental Top Quark PhysicsPhysics

Tevatron Collider is world’s only top quark production machine

First Experimental ResultsFirst Experimental Results

• Samples collected by identifying strong production of pairs of top quarks (have also looked for ewk production)

• To help isolate signal, some analyses look for evidence of a B hadron decay:

– Secondary Vertex Tagging (SVT or SVX)

– Soft Lepton Tagging (SLT)bosons:

Run I Results: Production Run I Results: Production Properties Properties

Test of QCD

• Overall discrepancy could indicate non-SM production mechanisms

• Inconsistencies between channels could indicate non-SM decay mechanisms

• Run I results consistent with SM but with large statistical uncertainties

Run I Results: Top Mass Run I Results: Top Mass

• Top mass important ewk parameter (due to t-b mass difference)

• Uncertainty on top mass currently limiting factor in indirect determination of Higgs mass

• Accurate measurement needed for self-consistency tests of SM

• New Run I D0 l+jet result using matrix element technique

Improved Method by DØ • Use Probability density:

• Background probability – Main component W+jets (85% of background)– Pbkg calculated from leading order matrix element from

VECBOS– 22 events remain: 12 signal, 10 background

• Dominant systematic is jet energy scale: 3.3 GeV/c2

x : reconstructed4-vectors

LO Matrix element + phase space

PDF’s Transfer function: parton values tomeasured quantities

Mt = 180.1 ± 3.6 (stat) ± 3.9 (syst) GeV/c2 =

180.1 ± 5.3 GeV/c2

New Run I Top Mass Result and implications on Higgs

Mass• New DØ combined mass:

– Mt = 179.0 ± 5.1 GeV/c2

• New world average:– Mt = 178.0 ± 4.3 GeV/c2

• Global fit to electroweak data using this top mass– Method of LEPEWWG

(hep-ex 0312023)– Best-fit MH 113 GeV/c2

– 95% C.L. upper limit 237 GeV/c2

Yellow region excluded: MH < 114.4 GeV/c2 @95% CL

Other Run I Results: Single Other Run I Results: Single Top and Branching Ratios Top and Branching Ratios

Br(t q) at c.l. 33% 95%Br(t q) at c.l. 33% 95%

Br(t q) at c.l. 32% 95%.Br(t q) at c.l. 32% 95%.

c.l. 95%at 56.0)Wqt(Br

)Wbt(Br

c.l. 95%at 56.0

)Wqt(Br

)Wbt(Br

Single top D0 cross section (s-channel)

Single top D0 cross section (t-channel)

Single top CDF cross section (s-channel)

Single top CDF cross section (t-channel)

Fraction of longitudinal W bosons (D0):

Fraction of longitudinal W bosons (CDF):

Branching ratios (CDF):

F0 = 0.91 0.37 0.13

F0 = 0.56 0.31 0.04

< 17 pb @ 95% c.l

< 22 pb @ 95% c.l

< 15.8 pb @ 95% c.l

< 15.4 pb @ 95% c.l

• Integrated luminosity between 100 and 200 pb-1

• Focus on new cross section and mass results

• Theoretical cross section: ~5.8-7.4 pb

Run II ResultsRun II Results

Dilepton cross-section: lepton+track (CDF)

• Signature: 1 lepton + 1 isolated track, missing ET , 2 central jets

• Higher acceptance reduced purity relative to Run 1,

• Backgrounds: Z/ * l+l-, WW, WZ, ZZ, W+jets

Measured cross-section for different jet ET and track pT

19 events on 7.1 ± 1.2 background11 e-track, 8 -track

7.0+2.7-2.3(stat) +1.5

-1.3(sys) 0.4 (lumi) pb

# error # error # errorTop dilepton 0.3 0.2 3.4 0.6 11.5 1.5Diboson 21.8 5.2 6.3 1.5 1.2 0.3Drell-Yan 26.5 9.8 16.4 6.0 4.2 1.6Fakes 16.5 2.4 5.0 1.0 1.5 0.5Total bkgnd 64.8 11.3 27.7 6.3 6.9 1.7Total pred. 65.1 11.3 31.1 6.3 18.4 2.3observed 73 26 19

0 jets 1 jet >= 2 jets

Lepton + track Kinematics

Ht:Scalar summed ET of jets, leptons, and missing

RunI: had seen hints of discrepancy in kinematic

distribution: Missing ET

Leptons transverse momentum

With higher statistics in Run II, we observe good agreement with

Dilepton cross-section: ee,,efinal states (CDF)

Different background composition; Lower acceptance, but higher S/B

13 events (1 ee, 3 , 9 e, expect 10.6 SM with 2.4 ± 0.7 events. Result:

Combined result, (hep-ex/0404036, 1st Run II top paper): 7.0+2.4

-2.1(stat) +1.6-1.1(sys) 0.4 (lumi)

8.4+3.2-2.7(stat) +1.5

-1.1(sys) 0.5 (lumi) pb

• Physics background Z/* l+l, W+W- estimated using MC• Instrumental background determined from data:

– Due to fake missing ET in ee channel– Due to isolated fake e/ in all three channels

ee: 156 pb-1 e: 140 pb-1 : 143 pb-1

Dilepton cross-section: ee,,efinal states (DØ)

19.1+13.0-9.6(stat) 3.7

-2.6(sys) 1.2 (lumi) pb

Dilepton cross-section: ee,,efinal states (DØ)

13.1+5.9-4.7(stat) +2.2

-1.7(sys) 0.9 (lumi) pb

11.7+19.7-14.1(stat) +7.9

-5.0(sys) 0.8 (lumi) pb

Kinematic distributions below: Ht (left) and lepton Pt (right)

14.3+5.1-4.3(stat) +1.9

-2.0(sys) 0.9 (lumi) pb

Lepton+jets cross-section using event topology DØ

• Signature: high-pT isolated lepton, missing ET and jets

• Combine topological variables in event Likelihood. Choose variables with

– Good signal-to-background discrimination– Small correlations– Low sensitivity to jet energy scale (e.g. sphericity, energy ratios)

• Fit data to signal and background templates extract tt fraction

muons electronsNev 100 136

fitted NW 74.7 + 12.7 – 12.0 94.6 + 15.8 - 15.0fitted NQCD 7.1 + 0.9 – 0.9 14.1 + 1.2 - 1.2fitted Ntt 17.8 + 9.9 – 8.7 27.5 + 12.7 - 11.7

+jets144 pb-1

e+jets141 pb-1

Lepton+jets cross-section using event topology DØ

8.8+4.1-3.7(stat) +1.6

-2.1(sys) 0.6 (lumi) pb

7.2+2.6-2.4(stat) +1.6

-1.7(sys) 0.4 (lumi) pb

6.0+3.4-3.0(stat) +1.6

-1.6(sys) 0.4 (lumi) pb

Lepton+jets cross-section using SVX tag

CDF• Analysis requirements at least 1 displaced vertex tag (SVX)

• Event b-tagging efficiency ~ 55%, fake tag rate (QCD jets) ~0.5%

• Main backgrounds: W + heavy flavour, W + fake tag, QCD

• Count events with 3 or more jets and Ht > 200 GeV

162 pb-1

(l+jets, SVX) = 5.6+1.2

-1.0(stat) +1.0-0.7(sys) pb

Double Tag Analysis Result:

5.4 2.2(stat) 1.1 (sys) pb

All jets cross-section using SVX Tags (CDF)All jets cross-section using SVX Tags (CDF)

Final state: 6 jets, 2 b-quark jets (top needle in a haystack of QCD)Use dedicated trigger (4 jets >

15 GeV and sumEt >125 GeV)S/B of 1/2500 increased to

1/24 with sumEt> 320 GeV and topo. cuts: aplanarity, centralityRequire 6 to 8 jets, and SVX

tagsDominant systematic

uncertainty due to jet energy scale(l+jets, SVX) =

7.8+2.5-1.0(stat) +4.7

-2.3(sys) pb

All jets cross-section using NN and SVX Tag (DØ)

Final state: 6 jets, 2 b-quark jets

Derive SVX tag rate function in the same multijet events. Apply to untag sample to predict background shape

Three NNs combine various kinematic variables: Ht, sphericity, aplanarity, centrality etc.220 observed with expected

background of 186 5 12(l+jets, SVX) = 7.7+3.4

-3.3(stat) +4.7-3.8(sys) 0.5 (lum)

Run II Top Cross Section Summaries

• Perform kinematic fit:– find top mass that best fits event – loops over jet-parton assignments – Impose constraints: Mt=Mt , M(j,j) =

M(l,) = MW, with inputs: MW, W, t

– loop over two solutions for pz of

– 2-C fit performed

• Perform likelihood fit:– find top mass template

that best fits data with background templates

– background normalisation constrained

b-jetb-jet

5 vertices: 20 constraints

Top mass: l + jets (template)

• Choose events with 4 jets, 1 vertex tag

• 28 events in 162 pb-

1 with estimated bckg of 7.0 ± 0.8

• Syst. uncertainty dominated by jet-energy scale.

Result:

mt = 174.9 +7.1-7.7 (stat) ± 6.5 (sys) GeV

Top Mass: l + jets (DLM)

Dynamic Likelihood Method:

Likelihood defined as d(Mt) per unit phase volume of final partons times the transfer function (jets to partons):

See original paper by K.Kondo J.Phys. Soc. 57, 4126 (1988)use 162 data sample: 22 events with 4.2 ± 0.8 background predicted.

Top Mass: l + jets (DLM)

Result:

mt = 177.8 +4.5-5.0 (stat) ± 6.2 (sys) GeV

Systematic Uncertainties:

Result:

Some other Run II Results:Some other Run II Results:

• Single top cross section (t-channel 162 pb-1)

• Single top cross section (channels combined, 162 pb-1)

• top mass in dilepton channel (126 pb-1)

• cross section ratio ll)/lj) (125 pb-1)

1.45 + 0.83 - 0.55

< 8.5 pb @ 95% c.l

< 13.7 pb @ 95% c.l

175 ± 17stat ± 8sys GeV

0.27 + 0.35 - 0.21

Conclusions and OutlookConclusions and Outlook• We are now improving upon We are now improving upon

many Run I measurements but many Run I measurements but we are still at a very early stage we are still at a very early stage of the Run II top physics programof the Run II top physics program

• ““Precision” top quark Precision” top quark measurements in sight at measurements in sight at TevatronTevatron

• Future looks very bright:Future looks very bright:– Top factory (LHC) will turn on in a Top factory (LHC) will turn on in a

few years. few years. – Fantastic top physics to be done Fantastic top physics to be done

with ATLAS and CMS (e.g. see hep-with ATLAS and CMS (e.g. see hep-ph/0003033) ph/0003033)

Measurement Precision

Top Mass 2-3 GeV/c2

(ttbar) 9%

(ll)/(l+j) 12%

B(t Wb) 2.8%

B(Wlongitudinal) 5.5%

Vtb 13%

B(t c) 2.8 X 10-3

B(t Zc) 1.3 X 10-2

Some measurement targets to aim for in Run II

Top Quark Physics Pierre Savard University of Toronto and TRIUMF APS Meeting Denver May 2004.

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