Top Quark Physics at D0

Yi Jiang University of Science & Technology of China Introduction Top quark production cross section Top quark mass measurement Single top physics Spin correlation Summary

Tevatron Collider in Run II The Tevatron is a proton-antiproton Collider with 980 GeV/beam

s

=1.96TeV in RunII (1.8TeV in RunI) 36 P and Pbar bunches

a

396 ns between bunch crossing Increased from 6X6 bunches with 3.

5m

s in Run I Increased instantaneous luminosity Run II goal Current: ~

Run II D0 Data Taking Status 85~90%

D0 Detector (Run II)

Silicon Microstrip Detector (SMT)

PVrt/IP~ 15 m m

Vertex resolution: ~10

m

m (design) Primary Vertex vs. Impact parameter

Center Fiber Tracker (CFT) SMT combines vertex and tracking capabilities and provides good primary and secondary vertex resolutions.

The Calorimeter y

q j

x Z Resolution: s/E ~ 15%/√E(GeV) “fine” EM 50%/√E(GeV) “coarse” jet sMET ~ a + b*ST + c*ST2 ST scalar sum of ET a ~1.89GeV, (run1) b ~6.7E-3, c ~9.9E-6/GeV

Muon Detector

J/Psi: Local / Global

D0 Detector Performance

Motivation for the Top Quark Studies (I)

•

Top quark has been discovered by CDF and D0 in 1995;

•

Top quark mass ~ 175GeV and strong Yukawa coupling ~1 ;

- Study of the top quark provides an excellent probe of the electroweak symmetry breaking mechanism; - New physics may be discovered in either its production or decays; - Top quark spin can be directly observed.

•

Tevatron is the only palce to study top quark properties before LHC operation.

Motivation for the Top Quark Studies (II) Top Mass, W Mass Measurement

Top Physics Understanding Program Top production & decay Tools Cross section Mass Single top Spin correlation W helicity

Top Quark Production at Tevatron Top-antitop quark Pair Production (mainly)

s

(pb) RunI RunII 4.87(10%) 90% 6.70(10%) 85% 10% 15% Single top quark production (not yet observed) RunII

s

(pb) 0.9(10%) 2.0(10%)

Top Quark Decay In the standard model, the top quark is short lived and decay almost exclusively to W and b quark

Methodology& tools Full characterization of the chosen final state signature in term of SM background processes (control region)

[

Optimize signal for best measurement precision How to separate signal from background:

a

Top events have very distinctive signatures

8

Decay products (leptons, neutrinos, jets) have large PT

8

Event topology: central and spherical

8

Heavy flavor content: always 2 b jets in the final state Tools (need multipurpose detectors)

8

Lepton ID: detector coverage and robust tracking

8

Calorimetry: hermetic and well calibrated

8

B identification: algorithms pure and efficient

8

Simulation: essential to reach precision goals

Production cross section

s (

t t

)  6 .

5   1 1 .

7 .

4

pb

s (

t t

)  5 .

7  1 .

7

pb

RunI~100 events

Top cross section: dilepton channels

CDF & D0: dilepton channels

------------------------------------------------------

Top cross section: lepton+jets “Golden” mode for top studies: ~ 30% yield and relatively clean

Lepton+jets channel: topological analysis

      

Preselect a sample enriched in W events Evaluate QCD multijet background from data for each jet multiplicity bin using “matrix” method e+jets: due to fake jets (

p o

and

g

)



m+jets : due to heavy flavor decays Estimate real W+4 jets contribution with scaling law Additional topological cuts: ≥ H Aplanarity> H 4 jets T T > 180 GeV (e) (jets,p T 0.06

(W))> 220GeV (μ) “Matrix” method N loose N tight = N W =



sig



+ N QCD N W +



qcd



N QCD

D0: b tagging Soft lepton tag b tagging efficiency

Lepton+jets: topological cuts and SLT

Cross section from topological analyses

D0: lepton+jets channels with b-tagging

CDF: lepton+jets channels with b-tagging

D0: e+jets channels with matrix element method use the signal and background process matrix elements to calculate the observation probability function; for each pre-selected event(e+X), calculate the probability of being the signal and background; fit the data with the discriminator plot to extract the probability of signal and background; use likelihood function to extract the signal event fraction of the total pre-selected events.

simulation result:

Discriminator:

D

(

x i

) 

P s

(

x i P s

) ( 

x i P b

) (

x i

)

P s

(

x

)

signal probability

P b

(

x

)

background probability

W

background

(

e

 ) 

jets

signal D(x)

Run II cross section summary

Cross section √s dependence

First Run II look at all jets channel Challenging signature:

9

Very low S/B !

cross section & mass measured in Run I (CDF, D0) Tools needs: kinematical quantities, neural networks, b-tagging … D0 Run I all hardonic channel

Top mass measurement

Lepton + Jets mass method Additional complications from background events detector effect (mismeasurement + resolution) initial and final state radiations

Lepton + Jets mass method

Mass from lepton + jets (Run I)

Mass from alljets (Run I)

Dilepton mass method The final state momentum and angular information is sensitive to the top quark mass.

Dilepton mass method D0: Run I CDF: Run I

First look at top mass in Run II (CDF)

Single top physics Run I results:

Search for single top in Run II

Spin correlation

Spin correlation

Spin correlation D0 Run I Result:

W boson helicity if b quark mass=0, W polarizations can be analyzed from the angular or PT distributions of the charged leptons.

W boson helicity

Summary The Tevatron is the top quark factory until LHC: First Run II results cover a variety of channels and topics CDF and D0 are exploiting their upgraded detector features Several top properties studied using Run I data (limited statistic) There is a big potential to improve crucial aspects of physics analyses (tracking in jets, physics object identification, b-tagging optimization and many others).

A very rich top physics program is underway: let’s see what the top quark can do for us!