searches for new physics in the top quark decay 정 연 세 university of rochester
TRANSCRIPT
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
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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