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Search for B  t n with SemiExclusive reconstruction. C.Cartaro, G. De Nardo, F. Fabozzi, L. Lista Università & INFN - Sezione di Napoli. Searches for B  t n at BaBar. Two analyses based on 1999-2002 data sample (81.9 fb-1) First : reconstruct one B meson Semi-Exclusively

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search for b t n with semiexclusive reconstruction

Search for B tn with SemiExclusive reconstruction

C.Cartaro, G. De Nardo, F. Fabozzi, L. Lista

Università & INFN - Sezione di Napoli

searches for b t n at babar
Searches for B tn at BaBar
  • Two analyses
    • based on 1999-2002 data sample (81.9 fb-1)
    • First: reconstruct one B meson
      • Semi-Exclusively
      • In a DlnX mode (X = g, p0, nothing)
    • Then: the recoil is analyzed to search for a B tn decay
    • t decays are reconstructed in the following modes:

SemiExclusive Tags

  • (e, ) (e, ) 
  • (, 0, ) 

SemiLeptonic Tags

  •  (e, ) (e, ) 
  • The samples of the two analyses are statistically independent and the results have been combined

F.Fabozzi

b sample counting
B sample counting
  • In Semi-Exclusive analysis we fully reconstruct a B meson in a B  DXhad mode
  • For the BR determination we need the number of B+B- events with a fully reconstructed B
    • determined with a fit to mES
    • for the systematics we fit also with a Gaussian (4.5% less events)
      • Too conservative? We’ll try to fit with a double Gaussian
  • mES distribution fitted as Argus  Crystal ball
  • B sample counting:
    • NBB = (1.670.09)105

sideband

peak

F.Fabozzi

events preselection
Events preselection

mESdata, 1GTL & 00

  • Preselections for the interesting channels
    • 1 GTL & 0 p0
    • 1 GTL & 1 p0
    • 3 GTL & 0 p0
  • Then fit to the mES distributions
    • Crystal Ball + Argus
  • Fits after the preselection used for
    • data-MC comparison
    • expected background prediction
  • The shape of the Argus function after the preselection is in agreement with the shape after the full selection

mESdata, 3GTL & 00

mESdata, 1GTL & 10

F.Fabozzi

selections of tau decays
Selections of tau decays

1-prong events

  • 1 track, 0 0
  • No KS
  • Neutral Energy <110MeV
  • Neutral Bumps < 1
  • Pmissing > 1.2 GeV/c
  • Kaon veto
  • Particle ID
  •  only
      • Lepton veto
      • pc.m.s. > 1.2 GeV/c
  • SemiExcl purity mode > 50%
  •  events
  • decay proceeds via two intermediate resonances, an a1 and a 
  • 3tracks and00
  • Pmissing > 1.2 GeV/c
  • Neutral Energy < 100 MeV
  • Neutral Bumps < 1.5
  • 600 MeV < m(0) < 950 MeV
  • 1.1 GeV < m(+) < 1.6 GeV
  • |p1 + p2 +p3 |> 1.6 GeV/c (c.m.s.)
  • Lepton & kaon veto
  • SemiExcl purity mode > 30%

 0 events

decay proceeds via an intermediate 

  • 1 track and 10
  • pmissing> 1.4 GeV/c
  • Neutral Energy < 100MeV
  • 0.55 GeV < m(0) < 1 GeV
  • SemiExcl purity mode > 50%
  • Optimized for the best upper limit
  • Rejecting events with pmiss in the beam pipe?

F.Fabozzi

integrated purity cut
Integrated purity cut
  • For each selection we apply a cut on the “nominal”integrated purity of the Breco side
    • Different from the integrated purity that we see
      • from the fit to mES distribution
    • Not relevant from the point of view of the analysis but generated confusion during review

Charged B sample

Nominal Int. Pur. = 50 %

F.Fabozzi

selection efficiencies

e







0

00

 0

e

22.9%

0

0.6%

0

0

0

0



0

7.4%

0.5%

0

0

0

0



0.1%

2.7%

21.6%

0.4%

1.2%

0

0.1%



0

0

0

6.8%

0

0

0

0

0

0.3%

1.0%

0.1%

6.6%

0.8%

0.6%

Selection efficiencies
  • Total efficiency = 11.3%

F.Fabozzi

selected events
Selected events

B   

Wrong sign control sample

B   

Unblinded data candidates

F.Fabozzi

background from generic b b

1-prong lepton

1-prong pi

rho-channel

Hadronic

Vcb

Hadronic

Vcb

Semilept.

Vub

Semilept.

Vcb

Semilept.

Vcb

Semilept.

Vcb

Background from generic B+B-
  • Mostly from semileptonic Vcb
    • Can we fight this background looking at the charged tracks list?

3-prongs channel

only 3 events selected

2 semilept. Vcb

1 hadronic charmless

F.Fabozzi

systematic error i
Systematic error (I)
  • Uncertainty on NBB
  • Uncertainty on signal selection efficiencies
    • Better understanding of systematics on neutrals
      • Now: smearing procedure on the photon energy
      • Cut on neutral energy in the EMC
        • Sensitive to the Data-MC agreement in the low-energy region (ex. simulation of machine background in the EMC)
    • Developing a module to study t+t- events
      • Tag one t (t-3prongs) and look the neutral energy distribution associated to the other t decay
        • Data-MC comparison

F.Fabozzi

systematic error ii
Systematic error (II)
  • Uncertainty on the expected background (bi) estimate
    • Peaking background: from generic B+B- MC
      • More generic MC is available  will reduce main systematic error
    • Continuum+combinatorial background: from data sideband
      • scaling to signal region based on the fitted Argus shape
      • Possible dependencies of the fitted Argus shape on the variables used in the selection taken into account
        • mES fits in bins of a given variable
          • a correction factor for each variable
      • Total correction as the product of the individual corrections
        • assumption that variables are uncorrelated
        • the effect of correlation between variables is small (total error on bi: 4.9  5.0, since the statistical error on bi is dominant
      • We may think to remove highly correlated variables if they do not add in signal-to-background discrimination

F.Fabozzi

branching fraction extraction
Branching fraction extraction
  • -2logQmin gives the statistical significance of the signal
    • ~ 0.1s significance
  • Likelihood ratio estimator Q
    • Value of branching fraction  Minimum of -2logQ

Unphysical region

+3.8

Br(B-  t- n )= 1.1  10-4

-1.1

F.Fabozzi

upper limit determination
Upper limit determination
  • Determination of C.L. and upper limit with a Toy MC
    • 10000 random experiments generated for 400 values of the branching fraction

Br(B-  t -n )< 7.7  10-4 (90%C.L.)

Uncertainty on

bi included

COMBINED with semi-lept analysis:

Br(B-  t -n )< 4.1  10-4

(90%C.L.)

F.Fabozzi

expected sensitivity
Expected sensitivity
  • For each channel we fluctuate the observed events according to a Poisson distribution

F.Fabozzi

conclusions
Conclusions
  • Conference paper will be made public soon
    • Some delay due to discussion on if/how to quote a central value
  • Another iteration of the analysis is starting
    • More signal and generic B+B- MC available
    • Better understanding of neutral systematics
    • Try to see if margins to improve background rejection
      • Semileptonic Vcb is the main source

F.Fabozzi