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3 Single Top Physics Observation of single top allows direct access to V tb CKM matrix element cross section ~ |V tb | 2 study top-polarization and EWK top interaction Probe b-quark PDF (t-channel) Test non-SM phenomena heavy W’ boson anomalous Wtb couplings FCNC couplings like t Z/ c 4 th generation Potentially useful for Higgs searches single top has same final state as Higgs+W (associated) production Why look for single top ?
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Single Top Production Search at CDF
Frontiers in Contemporary Physics III
May 23-28 Vanderbilt University, Tennessee
CDFJulien Donini
University of Padova and INFN
On behalf of the CDF collaboration
CDF
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Search for Single Top
Top has been discovered at Tevatron in tt pairs
Single top production predicted by theory: Wtb coupling
s-channelTev = 0.88 ± 0.11 pb
t-channelTev = 1.98 ± 0.25 pb
Wt-associatedTev< 0.1 pb
negligible at Tevatron
Z. Sullivan hep-ph/0408049, T. Tait hep-ph/9909352
Observation of single top: cross section is about 40% of ttbar more difficult to observe due to large background
- important W+jets background
(for Mtop = 175 GeV)
CDF
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Single Top Physics
Observation of single top allows direct access to Vtb CKM matrix element cross section ~ |Vtb|2
study top-polarization and EWK top interactionProbe b-quark PDF (t-channel)Test non-SM phenomena
heavy W’ boson anomalous Wtb couplings FCNC couplings like t Z/ c 4th generation
Potentially useful for Higgs searches single top has same final state as Higgs+W (associated) production
Why look for single top ?
CDF
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Run I Results
Tevatron Run I results at s = 1.8 TeVTheoretical cross-sections were 30% smaller
t = 1.40 pb s = 0.76 pb
Single top has not been observed: both CDF and DØ set 95% limits t-channel: 22 pb (DØ) 13 pb (CDF) s-channel: 17 pb (DØ) 18 pb (CDF) combined: 14 pb (CDF)
Run II higher beam energy (1.96 TeV) higher rate more luminosity: CDF/DØ have more than 800 pb-1 on tape better detector acceptance
CDF
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Signal Topology
General strategy: Separated search of signal of s and t-channels single top production in 1-tag and 2-tag samples Combined search of s+t signal
Event selection: we look for W+2 jets channel one high pT isolated lepton (from W): Et > 20 GeV, || < 1 veto Z, dilepton, convertion events missing Et ( from W): MET > 20 GeV exactly two jets w/ Et > 15 GeV, || < 2.8 ask for at least 1 b-tag
Note: s and t-channel both produce 2 b-jets but bbar from t-channel usually lost into beam pipe
Additional cuts cut on mt mass window: 140 < Mlb < 210 GeV for 1-tag sample, leading jet Et > 30 GeV
CDF
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Invariant Mass Mlb Calculation
b choosing in 1-tag events, choose b-tag jet as b from top in 2-tags evt, choose tight jet with larger Q. as the b from top
Recipe is 97% successful for t-channel 53% (75%) successful for s-channel 1-tag sample (2-tags)
Choosing correct neutrino solution Et() = MET pz () is obtained from the constraint Ml = MW:
Two solutions: pick the one with lower |pz ()|
i.e most central solution.Successful matching rate: 74% t-channel, 71% s-channel
CDF
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Three methods are currently used and well tested in high pt physics
Soft lepton tagging Use semi-leptonic decay of b-quarks: b lc (BR ~ 20%), b c ls (BR ~ 20%). Leptons with softer pt spectrum than W/Z, less isolated.
Secondary vertex identification Iterative fit on tracks with significant impact parameter Efficiency is 45-50% for central top b-jets Mistag rates are kept typically at 4-5%
Jet-probability Probability P that tracks associated with a jet come from primary vertex
Heavy flavors: P tends to 0.
b-tagging at CDF
CDF
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Signal and Background Modeling
Signal modeling Pythia (or Herwig) do not reproduce correct NLO pt distribution for the b jets in the t-channel Use MadEvent generator
generate b+q t+q’ and g+q t+b+q’ separately and merge them to reproduce the b pt spectrum from NLO calculations (ZTOP)
Background modelingttbar and non-top background Top pairs: Pythia, (tt) = 6.7 pb. Non-top:
W + heavy flavor (62%): Alpgen mistags (25%) and non-W (10%): estimated from data diboson (WW, WZ, ZZ) production (3%): Pythia
CDF
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Signal and Background Modeling
Expected number of signal and background events compared with observation for L = 162 pb-1
Combined and 1-tag signal mostly dominated by t-channel (65-70%) 2-tags signal: only s-channel Total background is dominated by non-top events (89%) Observations are in good agreement with predictions
Published CDF results: Phys. Rev. D 71, 012005 (2005)
CDF
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Separate Search for Single Top
Maximum likelihood technique to extract the signal content from the dataPerform search to separate s-channel and t-channel events: use variables that help discriminate the two channels.
For t-channel use kinematical boost In proton-antiproton collisions:
top production: light quark jet goes in p direction anti-top: light quark jet goes in p-bar direction
Correlation between pseudorapidity of untagged jet lepton charge Q
Q. distribution asymmetric for t-channel
s-channel Count double b-tags
g
u
d
b
W+t
z+ direction
CDF
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Q. Distributions for Separate Search
Data and stacked MC templates weighted by the expected number of events in the 1-tag sample.
CDF
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Likelihood Function
Joint likelihood function for Q. distribution in the 1-tag sample and number of events in 2-tag samples:
Poisson
bg gauss. constraints Syst. Uncert.
4 processes: t-channel (j=1), s-channel (j=2), ttpair and non top (j=3, 4) k: mean number of events in bin k of Q. distribution d: mean number of events in the 2-tags sample nk, nd: observed number in data the background is allowed to float but is constrained to SM predictions 7 sources of systematic uncertainties
t or s-channelk: bin indexj: process index
CDF
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Systematic Uncertainties
Systematic acceptance uncertainties for t-channel, s-channel and combined signal
Main systematics:
• b-tagging (7%)• luminosity (6%)• top mass (4%)• JES (4%)
CDF
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Expected Sensitivity for Separate Search
Perform N MC experiments to estimate expected sensitivity: For each exp. we integrate out all nuisance parameters (all variables except sig) from Lsig, and calculate the upper limit at 95% C.L. Calculate the median of N individual upper limits
t-channel s-channel
CDF
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Posterior probability densities for datat-channel s-channel
The maxima of the probability densities give the most probables values for the cross sections:
Since all these results are compatible with zero, we set upper-limits (at 95% CL):
t-ch = 0.0+2.4-0.0 pb
s-ch = 4.6 ± 3.8 pb
t-ch < 10.1 pbs-ch < 13.6 pb
CDF
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Combined Search for Single Top
Measurement of the combined t-channel plus s-channel signal in the data use kinematic variable that discriminates s+t channels from the background total transverse energy: HT distribution (scalar sum of MET, Et of lepton and jets)
same HT distribution for s and t-channels different distributions for ttbar and non-top background
Make a likelihood function similar to that in separate search
Use smoothed HT distributions
CDF
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HT Distributions for Combined Search
Data Distributions versus SM Expectation for combined sample
CDF
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Limits for Combined Search
Expected sensitivity Probability density from data
s+t (MPV) = 7.7 +5.1-4.9 pb
s+t (95% CL) < 17.8 pbs+t (a-priori) < 13.6 pb at 95% CL
CDF
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Summary
No significant evidence for single top quark production set first limits on single top cross section compared with Run I improvement of upper limits of 28% (t-channel) and 20% (s-channel)
Technical improvements of the analysis improved MC modeling for single top events full Bayesian treatment of systematic uncertainties
95% CL limits and most probable x-sections values (pb), with 162 ± 10 pb-1
CDF
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Multivariate Analysis
New analysis improvements Resolution of Mlb :
Factors contributing to poor resolution: Jet energy, b-jet choosing, neutrino choosing, lepton energy.
use a kinematic fitter to find the neutrino solution most consistent with measured values of the b-jet and MET
Multivariate likelihoods use a combination of variables to optimize signal and background separation variables include Q*, dijet invariant mass, cos lepton – other jet, HT, ET
likelihood function computed from distributions of each variable compute sensitivities (i.e how much luminosity for discovery ?)
CDF
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Conclusion
CDF accumulated more events than in entire Run ISingle top search at the Tevatron is more challenging than anticipated
CDF prepares advanced analysis techniques multivariate (neural-network) analysis
CDF expects 3 signal evidence with 1.5 fb-1
Many improvement are needed to observe single top in the next few years but it is feasible in Run II !
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Backup slides
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CDF
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