Searches for new physics in the top quark decay
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Searches for new Physics in the Top quark decay. 정 연 세 University of Rochester. Outlook. 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(t H + b)

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Searches for new Physics in the Top quark decay

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Searches for new physics in the top quark decay

Searches for new Physics in the Top quark decay

정 연 세

University of Rochester


Outlook

Outlook

  • 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(tH+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

What is Charged Higgs boson?


Physics at tevatron

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


Charged higgs production @ tevatron

Charged Higgs Production @ Tevatron

Prog.Part.Nucl.Phys.50:63-152,2003

  • Direct production cross-section

    • ~ fb level.

    • ~1/1000 of top pair production

  • Associated production with top quark

    • Light Higgs: M(H+)<M(t)-M(b)

    • Heavy Higgs: M(H+)>M(t)+M(b)


Charged higgs in top quark decays

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

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


Charged higgs in top quark decays1

Charged Higgs in Top Quark Decays

  • 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)

q(c)

W+(H+)

b

t

proton

anti-proton

W-

e-,μ-

ν


Tevatron at fermilab

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

CDF


Tevatron performance

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)


Searches for new physics in the top quark decay

CDF II detector

Electromagnetic Calorimeter

Muon Chambers

Beamline

Hadron Calorimeter

39ft.

48ft.

Tracking Chamber

Weight 5000t

1.4 T Solenoid

Silicon Vertex Detectors

Interaction points


Event selection

Event Selection

  • Usethe most energetic four jets (leading jets)

  • Veto events including

    • Z boson (l+l-)/two leptons/Cosmic muon/Conversion(γe+e-)


Event reconstruction

Event Reconstruction

  • 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

Mt=175GeV

Mt=175GeV

MW=80GeV

MW=80GeV

Γ : natural width of W and top quark


Event reconstruction1

Event Reconstruction

  • Look at dijet invariant mass of the final state hadronic boson

  • Search for a second peak in the dijet mass spectrum

  • 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

Mt=175GeV

Mt=175GeV

MW=80GeV

MW=80GeV

Γ: natural width of W and top quark


Energy loss from radiation

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

Pythia MC

Final State

Radiation (FSR)

Initial State

Radiation (ISR)


Angular distribution of the 5 th jet

Angular distribution of the 5th jet

BLEP

BHAD

  • 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

H+

Initial quark

Close to b-jet

Close to Higgs jet


Merging the 5 th jet

Merging the 5th jet

ΔR

  • 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 ttbarreconstruction

    • 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

Reconstruct di-jet mass after merging nearby 5th jet


Dijet mass spectrum 2 2 fb 1

Dijet mass spectrum (2.2 fb-1)

Estimated using Pythia Monte Carlo simulation samples and data

Observed 200 candidates


Extracting b t h b

Extracting B(tH+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(tH+b) using N(W) and N(H+)

    • B(tH+b)+B(tW+b)=1.0

    • B(H+csbar)=1.0

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


Systematic uncertainty

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


Likelihood smearing

Include the systematic uncertainty on B(tH+b)

The likelihood is smeared out according to Gaussian statistics

The total systematic uncertainty is used for the Gaussian error.

Likelihood Smearing

Maximum Likelihood

Observed B(tH+b)

before/afterLH smearing

B(tH+b)


Upper limits on b t h b

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

A negative branching ratio (B(tH+b)<0) is unphysical

use positive branching ratio region

With 95% confidence level (C.L.) the observed B(tH+b) is less than the upper limit

Upper Limits on B(tH+b)

95%

5%


Upper limits on b t h csbar b

Upper Limits on B[tH(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


Expanding searches

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)


Analysis with udbar decays

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%

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


Model independent upper limits

Model Independent Upper Limits

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

  • Assumptions:

    • Spin-0 particle

    • Hadronic decays

    • Narrow width


Summary

Summary

  • The MSSM charged Higgs has been searched in lepton+jets top quark decays

    • This is the first attempt of Hcsbarsearch 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(tH+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


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

Prospect for future H+ analysis


Flavor changing neutral current

Flavor Changing Neutral Current

Search in the Z+4jet channels

B(tZq) < 3.7% @ 95% C.L.

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


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