search for b with semiexclusive reconstruction c.cartaro, g. de nardo, f. fabozzi, l. lista...

Search for B with

SemiExclusive reconstruction

C.Cartaro, G. De Nardo, F. Fabozzi, L. ListaUniversità & INFN - Sezione di Napoli

04/10/03 F.Fabozzi 2

Searches for B at BaBar• Two analyses

– based on 1999-2002 data sample (81.9 fb-1)

– First: reconstruct one B meson• Semi-Exclusively

• In a DlX mode (X = , 0, nothing)

– Then: the recoil is analyzed to search for a B decay

– decays are reconstructed in the following modes:

SemiSemiLeptonic TagsLeptonic Tags (e, (e, ) ) (e, (e, ) )

SemiSemiExclusive TagsExclusive Tags (e, (e, ) ) (e, (e, ) )

((, , 00, , ) )

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

04/10/03 F.Fabozzi 3

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 BB 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

sidebandsideband peakpeak

• mES distribution fitted as Argus Crystal ball

• B sample counting:– NNBBBB = (1.67 = (1.670.09)0.09)101055

04/10/03 F.Fabozzi 4

Events preselection• Preselections for the interesting channels

– 1 GTL & 0 0

– 1 GTL & 1 0

– 3 GTL & 0 0

• 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, 1GTL & 00

mESdata, 3GTL & 00

mESdata, 1GTL & 10

04/10/03 F.Fabozzi 5

Selections of tau decays• Optimized for the best upper limit

eventseventsdecay proceeds via two intermediate resonances, an a1 and a

• 3 tracks and 0 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%

1-prong events1-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%

00 events eventsdecay 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%

• Rejecting events with pmiss in the beam pipe?

04/10/03 F.Fabozzi 6

Nominal Int. Pur. = 50 %

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

04/10/03 F.Fabozzi 7

Selection efficiencies

0.6%

0

0.1%

0

0

0

6.6%

0

1.2%

0

0

0

0.8%

0

0

0

0

00

0.4%21.6%2.7%0.1%

6.8%000

0 0.1%1.0%0.3%0

00.5%7.4%0

00.6%022.9%e

e

• Total efficiency = 11.3%

04/10/03 F.Fabozzi 8

Selected eventsBB

BB

Wrong sign control sample

Unblinded data candidates

04/10/03 F.Fabozzi 9

Background from generic BB

• Mostly from semileptonic Vcb– Can we fight this background looking at the charged tracks

list?

1-prong lepton 1-prong pi rho-channel

Semilept.Vcb

Semilept.Vub

Semilept.Vcb

Semilept.Vcb

HadronicVcb

HadronicVcb

only 3 events selected2 semilept. Vcb1 hadronic charmless

3-prongs channel

04/10/03 F.Fabozzi 10

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 events• Tag one (-3prongs) and look the neutral energy

distribution associated to the other decay– Data-MC comparison

04/10/03 F.Fabozzi 11

Systematic error (II)• Uncertainty on the expected background (bi) estimate

– Peaking background: from generic BB 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 b i: 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

04/10/03 F.Fabozzi 12

Branching fraction extraction• Likelihood ratio estimator Q

– Value of branching fraction Minimum of -2logQ

)(

)(

bL

bsLQ

)(

)(

bL

bsLQ

channels iichannels iii n

i i

ni

bn

i i

nii

bs

n

bebL

n

bsebsL

11

)(

!)( ,

!

)()(

channels iichannels iii n

i i

ni

bn

i i

nii

bs

n

bebL

n

bsebsL

11

)(

!)( ,

!

)()(

iiB

iii

fBBrN

fss

)(

iiB

iii

fBBrN

fss

)(

Br(B )= 1.1 104 +3.81.1

• -2logQmin gives the statistical significance of the signal– ~ 0.1 significance

Unphysical region

04/10/03 F.Fabozzi 13

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 )< 7.7 10-4 (90%C.L.)

QQ

QQ

b

bss

b

bs

N

N

LC

LCLC

..

....

QQ

QQ

b

bss

b

bs

N

N

LC

LCLC

..

....

Uncertainty on bi included

COMBINED with semi-lept analysis:Br(B )< 4.1 10-4 (90%C.L.)

04/10/03 F.Fabozzi 14

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

distribution

04/10/03 F.Fabozzi 15

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 BB MC available– Better understanding of neutral systematics– Try to see if margins to improve background

rejection• Semileptonic Vcb is the main source