Su-Jung ParkII. Physikalisches Institut,

Georg-August-Universität Göttingen

on behalf of the DØ Collaboration

106/17/2008

Measurement of the Forward-Measurement of the Forward-Backward Charge Asymmetry in Top-Backward Charge Asymmetry in Top-

Quark Pair Production at DØQuark Pair Production at DØ

- SUSY Conference, Seoul 2008 -

06/17/2008 Su-Jung Park 2

OverviewOverview

• Introduction• Analysis

• Event Selection• Sample Composition and Asymmetry• Acceptance and Reconstruction Effects

• Sensitivity to New Physics• Summary

tt

W+

-

b

b

-W-

µ+

qq’

νµ

06/17/2008 Su-Jung Park 3

IntroductionIntroduction• Top-quark pair production at the Tevatron:

• Initial pp state not a charge conjugation C eigenstate:

• Final state not expected to be symmetric under C

85% qq annihilation 15% gluon fusion

-

-

€

≠p

p p-

p-C

06/17/2008 Su-Jung Park 4

Forward-Backward AsymmetryForward-Backward Asymmetry

• Not enough statistics for differential measurement => measure total forward-backward asymmetry Afb:

α

Forward-backward asymmetryCharge asymmetry

CP symmetry, not at LHC

f bfb

f b

N NA

N N

−=

+Nf = # “forward” events (cos α>0 ~> y>0) Nb = # “backward” events (cos α<0 ~> y<0)

€

y = y t − y t = 2 ⋅atanhcosα

1+ 4mt2 /(ˆ s − 4mt

2)

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟ (LO; invariant to boosts)

€

A(cosα ) =N t (cosα ) − N t (cosα )

N t (cosα ) + N t (cosα )=

N t (cosα ) − N t (−cosα )

N t (cosα ) + N t (−cosα )≠ 0

06/17/2008 Su-Jung Park 5

Theoretical PredictionsTheoretical Predictions• At LO top-quark pair production is charge symmetric• At NLO asymmetry arises from interferences between diagrams• Total asymmetry of (4-5)% (NLO) expected [Bowen et al.; Kühn et al.]

• Different for 2->2 and 2->3 processes:

• Interesting to measure separately

Negative contribution to AfbPositive contribution to Afb “4 jets” “≥5 jets”

6.4% (NLO) -(7-8)% (NLO)(-3-0)% (NNLO) [Dittmaier et al.]

06/17/2008 Su-Jung Park 6

Analysis StrategyAnalysis Strategy

• Select sample enriched in top pair events• Reconstruct full event using kinematic fitter• Fit for sample composition and asymmetry simultaneously

• Acceptance effects:no correction: simple description of accepted phase space

• Reconstruction effects:no correction: dilution given to be used with any model

06/17/2008 Su-Jung Park 7

Final State and SelectionFinal State and Selection

tt

W+

-

b

b

-W-

µ+

qq’

νµ

Decay channels classified by W decay:• All jets• Lepton + Jets• Dilepton

Primary Vertex of good quality

≥4 jets in ||<2.5, pT>20 GeV,

Leading jet pT>35 GeV

Isolated lepton with track match, electron pT>15 GeV, ||<1.1 or muon pT>18 GeV, ||<2

Missing transverse energy MET>15 GeV

≥1 b-tags

06/17/2008 Su-Jung Park 8

Kinematic FitterKinematic Fitter

tt

W+

-

b

b

-W-

µ+

qq’

νµ

• Varies 4-momenta of objects within their resolutions• Minimizes 2 statistic with:

{constrain to W mass 80.4 GeV

{

constrain to W mass 80.4 GeV

use b-tag information to reduce jet combinatorics

constrain both to top quark mass 170 GeV

Lepton charge determines t or t

-

• Gives twice the sensitivity than lepton charged-signed direction

06/17/2008 Su-Jung Park 9

Background EstimationBackground Estimation• Largest backgrounds: W+jets and multijet production• W+jets: Define discriminant L based on variables, which

• well-modeled• good separation between signal and W+jets • do not bias |y|

• Asymmetry of W+jets is suppressed by kinematic fit, estimated by simulation (ALPGEN+PYTHIA)

• Size and asymmetry of multijet background from data (sample with non-isolated lepton)

• Other background sources in systematic uncertainties• Maximum likelihood fit of L distributions;

simultaneously for sample composition and sign of y

06/17/2008 Su-Jung Park 10

AsymmetryAsymmetry• Fitted distribution for ≥4 jets in 0.9 fb-1 data and simulation:

yreco > 0 yreco < 0

DØ data MC@NLO

≥4 jets Afb = 12 ± 8(stat) ± 1(syst) % Afb = 0.8 ± 0.2(stat) ± 1.0(acc) %

=4 jets Afb = 19 ± 9(stat) ± 2(syst) % Afb = 2.3 ± 0.2(stat) ± 1.0(acc) %

≥5 jets Afb = -16 (stat) ± 3(syst) % Afb = -4.9 ± 0.4(stat) ± 1.0(acc) %+15- 17

L L

06/17/2008 Su-Jung Park 11

Acceptance EffectsAcceptance Effects• Integrated asymmetry strongly depends on region of phase

space probed

• Strongest effect: requiring ≥4 jets above pT threshold => generated asymmetry as function of 4th highest particle-jet pT

MC@NLO

• Exact structure in SM unknown => no correction of acceptance• Analysis designed so that selection can be described by simple

particle-level cuts (change asymmetry <2%)

06/17/2008 Su-Jung Park 12

Reconstruction EffectsReconstruction Effects• Misreconstruction of sign of y dilutes the observed asymmetry

• Misidentification of lepton charge (negligible)• Misreconstructed event geometry

• Probability p for correct sign of y => fraction of asymmetry visible (“dilution factor”)

• Correction for reconstruction effects highly model-dependent => provide parameterization of dilution instead:

€

D(| Δy |) = c0 ln(1+ c1 | Δy | +c2 | Δy |2)

€

D = 2p −1

tt PYTHIA-

06/17/2008 Su-Jung Park 13

Predicted AsymmetryPredicted Asymmetry• To compare predicted asymmetry from MC@NLO with

observed

• MC@NLO generated Afb has to be folded with acceptance and reconstruction effects:

probability density within geometrical acceptance

particle-level Afb (MC@NLO here)

dilution

• Any model can be compared to observed asymmetry• Dilution is provided for ≥4, =4, ≥5 jets

• Particle level cuts: jet pT>21 GeV, ||<2.5, leading jet pT>35 GeV, lepton requirements

06/17/2008 Su-Jung Park 14

Sensitivity to New PhysicsSensitivity to New Physics

• Resonance production of tt can change asymmetry• Axigluons predicted to give negative asymmetries

• Z' production of tt can give large positive asymmetries

• Sensitive to narrow and wide resonances• Complementary to direct Z' search

(P6 [168] by T. Schliephake)• Leptophobic Z' with Z-like coupling

• Predict distribution of Afb as a function of the fraction f of tt events produced via Z'

-

-

-

06/17/2008 Su-Jung Park 15

Limits on Limits on Z'Z'• Ensembles of simulated data sets• Upper limits on fraction f of tt events produced via Z' at 95%

confidence (Feldman/Cousins) for narrow resonances:

Expected limits

Observed limits

Band of ±1 sd expected limits

Band of ±2 sd expected limits

-

06/17/2008 Su-Jung Park 16

SummarySummary

• First measurement of integrated forward-backward charge asymmetry in top pair production

• Measured asymmetries consistent with MC@NLO predictions• Expected behavior for events with =4 jet and ≥5 jet observed• Parameterization of acceptance and dilution allows comparison

to any model• Limits on tt production via Z'

• Published in Phys. Rev. Lett. 100 142002 (2008) arXiv:0712.0851

-

06/17/2008 Su-Jung Park 17

BACKUP SLIDES

06/17/2008 Su-Jung Park 18

Tevatron at FermilabTevatron at Fermilab• pp collisions at center of mass energy √s = 1.96 TeV

Chicago

p

p_

Tevatron

Main Injector

CDF

DØ ~900 pb-1

analyzed

-

4.23 fb-1

3.66 fb-1

06/17/2008 Su-Jung Park 19

DDØ DetectorØ Detector• Multi-Purpose Detector

Acceptance:• Electrons || < 3.0• Muons || < 2.0 • Jets || < 4.2

Tracking System• Tracks • Vertices

Calorimeter• Electrons • Photons

Muon System• Muons• Jets

• Missing ET