Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://qfthep.sinp.msu.ru/talks/2010_09_14_Perfilov_FCNC1.pdf Äàòà èçìåíåíèÿ: Tue Sep 14 16:09:58 2010 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 19:54:20 2012 Êîäèðîâêà:

Search for Flavor Changing Neutral Currents in Single Top Quark Production -1 using 2. fb at D 3
Maxim Perfilov Moscow State University on behalf of the D0 Collaboration

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Tevatron

-396 ns between bunches - - has delivered about 9.2 fb since 2002

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FCNC

models with soliton structure of the top quark

Flavor Changing Neutral Currents
change flavor of quarks without changing the charge models with top quark as a composite object

models with new dynamical interactions of the top quark

models with multiple Higgs doublets, such as SUSY
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FCNC: exchange photons and Zbosons


FCNC through photons and Zbosons:
CDF constraints (95% CL): (Phys. Rev. Lett. 80, 2525 (1998))

L3 (LEP) direct constraints on FCNC parameters:
(Phys. Lett. B459, 290 (2002))

ZEUS (HERA) constraints (95% CL):
(Phys. Lett. B559, 153 (2004))

CDF recent constraints (95% CL):
(Phys. Rev. Lett. 101, 192002 (2008))

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FCNC: exchange of gluons


Vertex tgc (tgu)



TEVATRON searches for FCNC through gluons:
previous D0 results ( 2 3 pb ; 2006) 0
(Phys. Rev. Lett. 99, 191802 (2007))
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CDF results (2.2 fb ; 2008)
k g / 0.105 TeV
c -1

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, k g / 0.025 TeV

u

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(Phys. Rev. Lett. 102, 151801 (2009)) 5


FCNC search: CompHEP samples


For this search we used our experience of D0 FCNC search analysis, 2006 We generated CompHEP MC signal FCNC samples for p q q with subsequent top decays , p t t





Samples: tgc tgu QFTHEP2010, 2010/09/14
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FCNC search: selection and S/B


Event selection
W from top decays leptonically exactly one of the jet is required

Phys. Rev. Lett. 103,
092001 (2009)

is identical to the D0 Observation Single Top Analysis



to be btagged (in contrast with SM)

SM



Signal modeling
we model the signal kinematics and obtain the acceptance for the signal sample at only one value of the FCNC coupling (0.03); then we scale the CS correspondingly

FCNC



Background modeling
SM single top processes considered as background double top production, W+jets...

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FCNC search: multivariate analysis


Bayesian Neural Networks Discriminating variables



Variables considered for the observation analysis 18 28 per channel

+

Variables used in the previous FCNC analysis in each analysis channel 10 variables



Stable list of final variables: ~24 per channel «Object Kinematics», «Jet Widths», «Angular correlations»,
«Event Kinematics», «Top Quark Reconstruction»

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FCNC analysis: discriminating variables

Plots compare the observed data to the background for some discriminating variables

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FCNC search: BNN training


Kinematics of the FCNC processes are similar



two FCNC signal processes (tgc, tgu) combined into a single signal for training the BNN Separate BNNs are trained for each choice of lepton flavor (e or mu), jet multiplicity (2,3 or 4) and data taking period (RunIIa and RunIIb) -- 12 in total



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FCNC search: BNN discriminant plots

Comparison between background and data for all twelve BNN discriminant combined for
(left) whole discriminant range (right) high discriminant region

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FCNC search: systematics and setting limits


Systematic uncertainties
mainly from JES, btagging, normalization when setting limits on FCNC coupling additional signal CS uncertainty is included

Relative Systematic Uncertainties Components for Normalization Integrated luminosity 6.1% cross section top pair 12.7% single top cross section 8.4% Z+jets cross section 5.8% Diboson cross sections 5.8% Branching fractions 1.5% Parton distribution functions 3.0% (signal acceptances only) Triggers 5.0%

. . .


(D0 Collaboration, Phys. Rev. D 78, 012005 (2008))

The observed data are consistent with the background expectation
we proceed to set limits on the FCNC couplings

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FCNC search: statistical analysis


We determine the FCNC limits for up and charm quark couplings using a Bayesian approach
first we obtain a 2D posterior as a function of both cross sections and as a function of both couplings, using the BNN distribution for data, background and signals

The 2dimensional posterior probability density as a function of
(left) FCNC cross sections (right) the squared FCNC couplings

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FCNC search: limits


Onedimensional posterior densities as a function of CS are derived from the general 2dimensional posterior by integrating over the axes

The 1dimensional posterior probability as a function of (left) squared tgu coupling (right) squared tgc coupling


Limits on the couplings can be translated into decay branching fraction limits QFTHEP2010, 2010/09/14
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FCNC search: results


Observed 95% C.L. limits on cross sections, couplings and branching fractions:

q



for comparison
previous D0 results (2006): CDF results (2008):

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FCNC search: conclusion


We presented a search for FCNC interactions through gluons Using 2.3 fb of integrated luminosity recorded by the D0 detector at Fermilab we set limits on the couplings, branching fractions and cross sections
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q



This branching fraction and FCNC couplings limits are the most stringent at the moment The paper: arXiv:1006.3575 We could set stronger limits with new D0 data





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