Mixing and Lifetime from Partially Reconstructed B0 D* l ν

1)Analysis Strategy; 2)Tagging Lepton Sample description: Prompt Leptons; Cascade Decay Side; Cascade Tag Side 3)Preliminary BLIND Results; Comparison w.r.t. Similar Analyses 4)Study of the Analysis Bias 5)Conclusions

M. Margoni, Time Dependent Analysis Forum, 5/5/20041

Analysis Strategy

Signal Vertex:

Tag Vertex:

lπ* + beam spot (x,y)

l (Elbatag) + beam spot (x,y)

l+Signal SideTag Side

π∗−

l−

2

Fit to ∆t to determine simultaneously τ, ∆m and dilution D, constrained to the fraction of mixed events:

Nmix

/Ntot

= χd D + (1−D)/2; χ

d =x2/(1+x2)2; x=∆m τ

Tag Lepton alone:No D0 bias in tag Vertex

The Signal PDF

Tagging Lepton Sample:

b−> lb−>c−>lD0 −> l

{ From tag BFrom decay B

Cascade Lepton Tag Side

Cascade Lepton Decay Side

Tag B Decay B

π∗Prompt Lepton

Decay Lepton

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D0

Prompt Leptons"MC: D

P~ 0.98

Floated in the fit;"DT: D

P~0.96

"bN,

bW

compatible with zero

τΒ

= 1.539± 0.003 ps

∆m = 0.465± 0.001 ps−1

FU/MP= e−|∆t|/τ (1 ±D

P cos(∆m∆t))

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Unmixed

Mixed

Cascade Decay Side

l+Signal SideTag Side

π∗−

l−

D0−>l−

Time independent dilution D0 leptons mostly populate unmixed sample; No information on mixing

τDeffective

free parameter

in the PDF

∆z meas

Cascade Decay Side Cascade Fraction α(θ) for B0 and peaking B+ determined in three steps as a function of the angle (tag lepton−π∗) in the Υ frame:1) Float the relative amount of cascade/prompt for the B0 sample with fixed B+;2) Rescale the B+ cascade/prompt yields according to the corrections obtained for the B0;3) Fix the B+ and float again the B0

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B0 Btag→leptB0 D0→leptB+ Btag→leptB+ D0→leptcombinatoriccontinuum

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alpha B0

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:4/01/26 16.39

α(cos(θ))

" ρ = Fraction of mixed /(mixed+unmixed) Cascade ~ 5%

6cos(θ) cos(θ)

Cascade Decay Side

"τD" = 0.35± 0.01 ps

bN=b

W = −0.25± 0.01 ps

FU/MD

=e −|∆t|/τD

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Cascade Tag Side

∆z = Zreco

− Ztag

Signal Lepton

π*

∆z true

∆z meas.

If ∆z > 0: ∆zmeas

< ∆ztrue

Signal LeptonTag lepton

Tag lepton

∆z true

∆z meas.π*

If ∆z < 0: |∆zmeas

| > |∆ztrue

|

Btag

c Btag

c

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"bN, b

W

FU/MC= e−|∆t|/τ (1 −+D

C cos(∆m∆t))

"DC : due to wrong flavor charm from upper vertex + hadron misidentification

"PDG gives:

"MC truth : DC= 0.536

"Correction added o(%) due to fake leptons (as for the prompt lepton component)

Cascade Tag Side

Fixed in the fit on Data

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Cascade Tag Side

τΒ

= 1.555± 0.045 ps

∆m = 0.431± 0.022 ps−1

DC= 0.522± 0.016

bN = −0.35± 0.15 psbW=−3 ± 4 ps

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Unmixed

Mixed

B0 Signal: Prompt+Cascade (Tag+Decay)

τ=1.548± 0.003 ps; ∆m=0.466± 0.001 ps−1 ; ρ=−4%11

Preliminary ResultsAdding in the PDF:"Signal"B+: pure lifetime;"BB Bkg: empirical function (oscillating+lifetime+cascade tag+decay tag)"Continuum: pure lifetime;"Sample Composition from Mν2 fit; "Resolution: 2 Gaussians with floating offsets and pulls+outliers

From ~65 K evts: τ=xxx ±0.010± 0.016 ps; ∆m=xxx± 0.005 ±0.008 ps−1

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Mν2 (GeV2/c4)

entri

es /

0.1

data

B0

other

B+

combinatoric

continuum

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Comparison with other similar analyses"Old" τB0 analysis (Run1, no tagging), use now only lepton−tagged events:

−Determination of fr(B+) from the global fit (B+ events do not mix); −Improved signal/Bkg ratio; −Tag−lepton alone in the Tag Vertex: no D0 bias; Dilepton analysis:

−Lower Efficiency... but: −Separation between Decay Side (no dilution) and Tag Side; −Smaller Bkg Fraction; −∆t distribution of Combinatorial using also side−bands in the fit; Simultaneous meas. of τB0 and ∆m

Exclusive D* l ν: − Bigger amount of combinatorial Bkg; − Higher Efficiency: 65K evts in 81 fb−1 vs 14K evts in ~20 fb−1 ; − Not sensitive to K or NN tags; ∆m τB0

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Study of the Analysis Bias"Old" Problem:From the fit to the MC Pure Signal we observe a bias (BAD 287, v11):

δ∆m=−0.006±0.001 ps−1

The mixed event fraction is underestimated: δχ

d = −0.003±0.001

But......The fraction of MC truly mixed events when just a single π*l pair/event is reconstructed is correct!

χd=0.1744 ±0.0005 (w.r.t. 0.174 MC truth)

Bias induced by the events with more than one π∗l pair...

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L−

Decay B

Tag B

...Why?

π*+π*+

In the case of mixed events with two D* from different Bs, a second π* l "true" pair can be reconstructed with the Right Charge Correlation.If the 2nd pair is chosen by the selection algorithm, the event can fall in the Side Band region and it is classified as "Combinatorial Background"

Reduction of the measured mixed event fraction.

1th Pair

2nd Pair

Tag Lepton

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Mult

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"Number of π*l candidates / event (Signal MC) Right Charge Correlation R.C + Wrong Charge Correlation

R. C. Mixed event sample shows higher fraction of multiple candidatesFraction of Mixed Events:

Strong χd dependence vs number of reconstructed candidates for R.C. Correl.

Mult

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Mixed All

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Mixed Event Fraction χd vs Event Tag

Event Tag:1: just one π* l candidate;2: one additional π∗ l candidate (π∗ not from D*);3: one additional π*l candidate (π* from D*);4: two or more π* l candidates (at least one from D*)Right Charge Correlation R.C + Wrong Charge Correlation

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How to manage this effect on the data?Three possible strategies:

1) Use only the event sample with just one π*l candidate, ( ε~80% for R.C+W.C);

2) Determine the fraction of events with more then one D* in Data and MC, tune the simulation and compute the expected bias;

3) Use two separate analysis streams for the two subsamples: −single candidate; −two candidates from D* from different Bs : "golden events" with two B D* l ν and lowest dilution

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... Approach to be chosen...

Work going on...

04/05/04 19.54

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2) Rescale the MC to the DATA result3) Compute the expected bias.

R=N(D* (Kπ)π∗) Side Band, Wrong Charge

/N(D* (Kπ)π∗) Mass Band, Right Charge

independent from efficiency/ mixing effects

D* (Kπ) π∗

Mass BandRight Charge

Mass BandWrong Charge

Side BandRight Charge

Side BandWrong Charge

Strategy n. 2: Determination of the Fraction of Events with more then one D* (Data vs MC)

1) Compute the ratio

19Μ

Conclusions

"Systematic errors still preliminary"Statistical errors to be validated (Toy) .... but a very promising measurement...

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