Searches for new Physics in the Top quark

decay

정 연 세 University of Rochester

Outlook2

Introduction to charged Higgs search Tevatron & CDF detector Event Selection & Reconstruction Dijet mass improvement study H+ search in CDF II data Placing upper limits on B(tH+b) Extending limits to generic charged boson Summary & Prospect; FCNC in Top decay

What is Charged Higgs boson?

In standard model, a scalar complex doublet field is responsible for the electro-weak symmetry breaking (EWSB) Results in massive weak bosons and predicts a neutral scalar Higgs

boson No observation of a SM Higgs boson to date

Extend Higgs sector beyond the SM The simplest extension of the Higgs sector: two-Higgs-doublet model

(2HDM) Predicts two charged (H±) and three neutral (h0, H0, A0) Higgs bosons

Minimal Supersymmetric Standard Model (MSSM): Employs type-II 2HDM

One complex doublet couples to up-type fermions and the other to down-type fermions

Higgs bosons expressed by two key parameters: tanβ – ratio of vacuum expectation values of two doublets m(H+) – mass of the charged Higgs

Discovery of the charged Higgs is evidence for new physics beyond SM No analogous particle in the SM

2 2 221 2

1

vtanβ= ,v +v =(246 GeV)

v

3

Physics at Tevatron

Measured cross sections agree with SM theory prediction

Direct charged Higgs production cross section is much smaller than other SM processes Impossible to

separate such small signal in an enormous number of SM events

10-3 + - + -qq H H ,bb H W

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Charged Higgs Production @ Tevatron•Direct production cross-section

•~ fb level. •~1/1000 of top pair production

•Associated production with top quark•Light HiggsLight Higgs: M(H+)<M(t)-M(b)•Heavy HiggsHeavy Higgs: M(H+)>M(t)+M(b)

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Charged Higgs in Top Quark Decays

Charged Higgs search is doable in high and low tanβ assumption in MSSM

For tan β > 7, H dominant challenging to reconstruct τ

For low tan β< 1, dominant for H+

mass ≤130 GeV full reconstruction is

possible

+H cs

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Phys. Rev. Lett. 96, 042003 (2006)

Charged Higgs in Top Quark Decays

tttt

tprotonproton anti-protonanti-proton

WW--WW--b

bb

q(cq(c))

ee--,,μμ--

q(s)

νν

WW++(H(H++))

Top quarks are mainly produced in pairs in proton-antiproton collisions.

Use lepton+jets channel Decays to lepton,

neutrino, 4 quarks Best signal to background

ratio Expect a charged Higgs

boson in place of the hadronic W in the final state

W and H+ are separated by the invariant mass of the two jets (dijet mass)

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Tevatron at Fermilab

The most powerful Collider to data

ppbar collision at center of mass energy of 1.96 TeV

Two Collision points (D0 and CDF)

Deliver 60 pb-1/week Delivered 6.8 fb-1

Acquired 5.6 fb-1

This analysis uses 2.2 fb-1

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CDCDFF

Tevatron Performance

Used 2.2 fb-1 data taken by 08/2007

Cross section σ: 60 mb at 1.96 TeV (1b = 10-24 cm2)

Muon ChambersElectromagnetic Calorimeter

Hadron Calorimeter

1.4 T SolenoidSilicon Vertex Detectors

Tracking Chamber

Beamline

39ft.

48ft.

Weight 5000t

Interaction points

CDF II detector

Event Selection

Use the most energetic four jets (the most energetic four jets (leading jets)leading jets)Veto events including

Z boson (l+l-)/two leptons/Cosmic muon/Conversion(γe+e-)

Final State Object Requirement

A lepton (e/μ) pT≥20 GeV/c, |η|<1

Missing transverse energy

≥20 GeV

Number of Jets ≥4

Jet ET≥20 GeV, |η|<2

Number of b-jets ≥2

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Event Reconstruction

Mt=175GeVMW=80GeV

Γ : natural width of W and top quark

, , 2 , , 22

2 2,4 ,

22 2

2 2 2

( ) ( )

( )( ) ( )

i fit i meas UE fit UE measT T T T

i l jets j x yi j

bjj tl W bl t

W t t

p p p p

m mm m m m

Mt=175GeVMW=80GeV

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Assign Selected objects to ttbar decay particles Kinematic fitter minimizes the 2 by

Constraining top quark and leptonic W masses Varying jet energies in the fitter Unclustered energies to have corrected missing ET

Event Reconstruction

Mt=175GeVMW=80GeV

Γ: natural width of W and top quark

, , 2 , , 22

2 2,4 ,

22 2

2 2 2

( ) ( )

( )( ) ( )

i fit i meas UE fit UE measT T T T

i l jets j x yi j

bjj tl W bl t

W t t

p p p p

m mm m m m

Mt=175GeVMW=80GeV

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Assign Selected objects to ttbar decay particles Kinematic fitter minimizes the 2 by

Constraining top quark and leptonic W masses Varying jet energies in the fitter Unclustered energies to have corrected missing ET

•Look at dijet invariant mass of the final state hadronic boson•Search for a second peak in the dijet mass spectrum

Energy Loss from Radiation

Low mass tails From final state gluon

radiation An extra jet in addition to the

four leading jets radiation jet from Higgs boson

Final State Radiation (FSR)

Initial State Radiation (ISR)

Pythia MC

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Angular distribution of the 5th jet

Use the 5th energetic jet with ET>12 GeV and |η|<2.4

Calculate angular distance between the 5th jet and closest leading jet

The radiation jet (5th jet) is close to its mother

BLEP

Initial quarkH+

BHAD

Close to b-jet

Close to Higgs jet

2 21 2 1 2ΔR= (η -η ) +(φ -φ )

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Merging the 5th jet

Merge the 5th jet to the closest leading jet if ΔR < 1.0 The leading jet recovers its energy by merging with

the hard radiation jet Merging is done before ttbar reconstruction Better energy fit in the ttbar final state Higgs boson mass resolution is improved in events

with more than four jets About ¾ among 5th jets added to the Higgs turn out to be

a real FSR from Higgs 103.3±21.8 GeV 105.7±20.8 GeV in events with more

than four jets true mass = 120 GeV in MC

ΔR

Reconstruct di-jet mass after merging nearby 5th jet

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Dijet mass spectrum (2.2 fb-

1)

Dijet composition Fraction (%)

(assuming =6.7 pb)

91.6

Non- Events 8.4

W + heavy flavor jets 4.5

W + light flavor jets 1.1

Single Top 1.0

Non-W (QCD) 0.9

Diboson (WW/ZZ/WZ) 0.4

Z + light flavor jets 0.3

Estimated using Pythia Monte Carlo simulation samples and data

tttt

σ

tt

Observed 200 candidates

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Extracting B(tH+b) Use maximum binned

likelihood method Nbkb (Non-ttbar) is

constrained using Gaussian statistics

Event composition in dijet mass is estimated using templates : W, H+, non-ttbar

distribution Maximum LH returns

N(W), N(H+), N(non- ) Calculate B(tH+b) using

N(W) and N(H+) B(tH+b)+B(tW+b)=1.0 B(H+csbar)=1.0

N(W)=N(H+)=N(non-ttbar)

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Systematic Uncertainty

Dominant systematic errors come from dijet mass shape perturbation due to: Jet Energy Scale Correction Monte Carlo Program

(Pythia vs. Herwig) More/Less Initial/Final state

radiation W+jet production scale (Q2)

Negligible uncertainties from top mass constraints, b-jet tagging efficiency

Individual systematic uncertainties are combined in quadrature

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Likelihood Smearing

Include the systematic uncertainty on B(tH+b) The likelihood is

smeared out according to Gaussian statistics

The total systematic uncertainty is used for the Gaussian error.

B(tH+b)

Maximum LikelihoodObserved B(tH+b)

before/after LH smearing

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Upper Limits on B(tH+b)

The upper limit is placed where the integration reaches 95% of positive branching ratio area

A negative branching ratio (B(tH+b)<0) is unphysical use positive branching

ratio region With 95% confidence

level (C.L.) the observed B(tH+b) is less than the upper limit

95% 5%

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Upper Limits on B[tH(csbar)b]

Observed limits from 2.2 fb-1 data agree with the SM expectation

SM expected upper limits are obtained from 1000 SM pseudoexperiments

M(H+)>80 GeV from LEP data

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Expanding searches

The charged Higgs search is performed independent of any model specific parameters

Assumptions for charged Higgs (H+): Scalar charged boson Narrow width

Which is predicted for the light H+ in MSSM H+csbar 100%

The search is extended to Mass below W boson (down to 60 GeV) Same analysis is performed for another

two jets decay mode (udbar)

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Analysis with udbar Decays

Dijet mass of udbar decays is narrower than that of csbar decays

Better mass separation between W and charged boson (udbar) results in lower upper limits by ~10%

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The upper limits for decays can be used conservative limits for any generic non-SM scalar charged boson in top quark decays

cs

Model Independent Upper Limits

Applicable to generic non-SM scalar charged boson in top quark decays

•Assumptions:•Spin-0 particle•Hadronic decays •Narrow width

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Summary

The MSSM charged Higgs has been searched in lepton+jets top quark decays This is the first attempt of Hcsbar search using fully

reconstructed mass No evidence of the charged Higgs in 2.2 fb-1 CDF II data

Based on simulation, upper limits on B(tH+b) can be used as model independent limits for generic scalar charged boson production in top quark decays

This analysis is about to be submitted to PRL soon The binned likelihood method using kinematic

distribution for branching ratio measurement (or ratio of decay

branching ratio) of new particles especially in early LHC data analysis

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Prospect for future H+ analysis

With doubled CDF II data the limits would be improved by 25%.

LHC is a top factory (Expect X100 more top pair production)

The precision study of top quark decay branching ratio available

In the first 200 pb-1 data at 10 TeV energy beam, the limits will be around 0.04~0.14 assuming similar systematic uncertainty with CDF

Sizable cross section for the direct charged Higgs production associated with top quark at LHC

Dominant decay is predicted H+tb for heavier H+ than top quark

Top quark is still the key to the charged Higgs search at LHC

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Flavor Changing Neutral Current

Search in the Z+4jet channels

B(tZq) < 3.7% @ 95% C.L.

World’s best limit. Improved previous limit (13.7% @ L3) by a factor of 3.5