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Search for B t n with SemiExclusive reconstruction

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Search for B tn with SemiExclusive reconstruction

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

Università & INFN - Sezione di Napoli

- 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:

SemiExclusive Tags

- (e, ) (e, )
- (, 0, )

SemiLeptonic Tags

- (e, ) (e, )

- The samples of the two analyses are statistically independent and the results have been combined

F.Fabozzi

- 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:
- NBB = (1.670.09)105

sideband

peak

F.Fabozzi

mESdata, 1GTL & 00

- 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 & 00

mESdata, 1GTL & 10

F.Fabozzi

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

- events
- decay proceeds via two intermediate resonances, an a1 and a
- 3tracks and00
- 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 10
- 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

- 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

- Different from the integrated purity that we see

Charged B sample

Nominal Int. Pur. = 50 %

F.Fabozzi

e

0

00

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%

- Total efficiency = 11.3%

F.Fabozzi

B

Wrong sign control sample

B

Unblinded data candidates

F.Fabozzi

1-prong lepton

1-prong pi

rho-channel

Hadronic

Vcb

Hadronic

Vcb

Semilept.

Vub

Semilept.

Vcb

Semilept.

Vcb

Semilept.

Vcb

- 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

- 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

- Tag one t (t-3prongs) and look the neutral energy distribution associated to the other t decay

- Better understanding of systematics on neutrals

F.Fabozzi

- 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

- mES fits in bins of a given 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

- Peaking background: from generic B+B- MC

F.Fabozzi

- -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

- 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

- For each channel we fluctuate the observed events according to a Poisson distribution

F.Fabozzi

- 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