s. bolognesi & m.a. borgia for the cp-violation exam

A measurement of the B0B0 oscillation frequency and

determination of flavor-tagging efficiency using semileptonic and

hadronic B0 decays

S. Bolognesi&

M.A. Borgia

for the CP-violation exam

Introduction

The strategy The experimental environment

S. Bolognesi & M.A. Borgia CP-violation exam

The measurement

• top contribute is dominant sensible to Vtd element of CKM matrix

one B reconstructed in a flavour eigenstate (Brec)

one B only tagged as B0 or B0 from its decay products (Btag)

• mixed if same flavor / unmixed if opposite flavor

PDF for the two categories (mixed + / unmixed -)

“dilution factor” due to mistag rate

distance between Brec and Btag decay (≈ B = 1.548±0.032 ps)

if perfect flavour tagging≈•

•

•

• time resolution function with parameters

B0B0 mixing through NLO EW diagrams involving exchange of up-type quarks

S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)

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Likelihood and time independent analysis

Likelihood = sum over all events (mix. & unmix.) and over different tag types (with its own Di)

minimized to extract simultaneously md, Di (and some ai)


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Time-independent analysis = neglecting background and assuming Brec correct identification, the observed time-integrated fraction of mixed events χobs as a function of B0B0 mixing probability χd can be expressed as:

where ω is the mistag rate, and χ d = ½ xd2/(1+xd2) = 0.174 ± 0.009 and xd2 = Δmd/Γ

BaBar detector DCH + SVTdetection and momentum measurement for charged particles

SVTvertex information

DIRC

z ≈ 50 m for Brec

z ≈ 100-150 m for Btag

particle identification (charged hadrons)

DCHparticle identification (dE/dx)

EMC photons, electrons and neutral hadrons

IFR (RPC) muons and neutral hadrons


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8.9 fb-1 @ Y(4s) +

0.8 fb-1 @ 40 MeV below Y(4s)

(10.1 ± 0.4) × 106 BB pairs

Particle identification Electrons

• track + EMC (shower shape, E/p)

• dE/dx in DCH

• Cherenkov angle in DIRC

efficiency 92%

mistag () 0.3%

Muons

• interaction lenghts and # hits in IFR• MIP in EMC

efficiency 75%

mistag () 2.5%

Kaons

15combKcomb

L

L where

comb SVT DCH DIRCL L L L

dE/dx• Cherencov angle• # photonsefficiency 85%

mistag () 5%


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Time resolution(t) dominated by (z) (z) dominated by (zBtag)

pseudo-track extrapolated from the interaction point in the Btag direction

reconstruct Brec

compute the Btag direction from the energy conservation

Btag vertex = intersection of pseudo-track with all the other tracks

Brec

Beam Spotpseudo track (Btag)

z

z ≈ 260 mz) ≈ 180 m

Resolution function is the sum of three gaussians

(3 parameters from MC3 parameters from fit)


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Flavor tagging

4 strategies to define if Btag is B0 or B0 4 tagging cathegories

Lepton tag: presence of a prompt lepton (pCM>1.1 GeV against charm semileptonic decay)

Kaon tag: total kaons charge not 0

2 neural network cathegories:

5 neural network algorithm

4 based on tracks1 exploits the charge of high momentum particles

whose outputs are combained in a single full neural network tagger (xNT [-1,1])

NT1

NT2


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Analysis

Hadronic decay channels Leptonic decay channels


Hadronic decaysB0

rec D* - + ( / + / a1+ )

D0 -

K+ -

K+ - 0

K+ + - -

K0s + -

D - + ( / + / a1+ )

K+ - -

K0s -

B0rec

B0rec J/ K*0

e+ e- / + -

(K0s → + -, 0 → )

Usual cuts on intermediate/final particles: resonances invariant mass (±2) vertex 2 threshold on momenta opening angle between decay products


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B0 candidates characterized by

beam energy sobstituted mass:5.270 < mES < 5.290 (GeV)

EB0 – Ebeam in Y(4s) CM:|E| < 3E where E = E resolution (19 -40 MeV)

Cuts against continuum (e+e- → qq) normalized second Fox-Wolfram moment

(R2=H2/H0) < 0.5

large angle between thrust axis of B0 and of the remaining tracks

Backgrounds (HD*)


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Data extracted from fit to the mES distrbution

* H

D =

Had

roni

c de

cays

Semileptonic decays


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*0 DB

0D

0

K

K

K

D0 candidates: combination with all charged tracks (pTmin 50 MeV/c and charge opposite to that of the candidate K) => D* candidates

Usual cuts on intermediate/final particles: invariant mass (±2) around nominal D0 mass vertex 2 > 1% threshold on momenta

Mass difference: m(D*-)-m(D0) (± 2.5σ) of the nominal value

D* candidates:

D*- , pl > 1.2 GeV back-to-back => cosθ(D*- ) < 0

Neutrino existence consistency:

lDB

lDBlDBlDB pp

EEMMlDBpppp

*

*2

*2

22* 2

2)*,(cos0)(

(solving in the Υ(4s) system frame)

Sample composition


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After cuts, 7517±104 B→D*lν events3101±64 in the mode1986±51 in the mode2430±56 in the mode

KD0

00 KD KD

0

Mass difference distributions for each flavor tagging category

Backgrounds are larger for semileptonic modes than

for hadronic modes

Background

Combinatorial

•Due to falsely reconstructed D* candidates•Estimated by fitting Δm(D*-D0) distributions•Gaussian + threshold function with a sharp rise followed by exponential tailoff•Signal region within ±2.5σ of the peak in Δm(D*-D0) •Combinatorial background control sample provided by the sidebands region

0.150 < Δm(D*-D0) < 0.160 GeV/c2


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Three types of background to B0→D*lν :

• Combinatorial background

• Wrong-lepton background

• B+ background

Wrong-lepton Wrong-lepton:D* combination with wrong lepton

Four potential sources:• ”Fake lepton”

(estimated selecting events in which a track candidate has failed very loose lepton criteria is

substituted for the lepton candidate)

• Real D* from one B + real lepton from the other B (“uncorrelated lepton” bg) (estimated by parity-inversion of the lepton momentum in the Y(4s) frame => control sample)

• Events of the type B0→D*DX in which the D decays semi-leptonically produce

a non-primary lepton (estimated with Monte Carlo, less than 1% => neglected)

• cc events producing real D* and lepton in back-to-back configuration.

(estimated using combinatorial-subtracted off-resonance data)


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B+ background

Due to B-decays which involve additional final state particles (B→D*(nπ)lν)

• B0→D*(nπ)lν that pass selection criteria are considered as signal (they

contribute to the measurement of Δmd and the additional low momentum π

does not affect the tagging algorithm)

• B-→D*+(nπ)l-ν considered as bg: they do not oscillate and must be corrected

for in extracting Δmd and their mistag rate may differ from that of B0 decays

as well


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Results

Likelihood fit results Time integrated method results

Combined results


Backgr. treatment (LM*) PDF must be extended with background contributions

b = background sourcesi = tagging cathegories)

f = fraction of signal or background events

B = empirical description of t distribution in background events

(where

Fit to the background control samples (mES sidebands) to determine time dependence, dilution factor, resolution function:

three components

for each background source

• zero lifetime:

• non zero lifetime, no mixing:

• non zero lifetime with mixing:


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* LM = Likelihood method

t distribution (LM*)


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HADRONIC SAMPLE LEPTONIC SAMPLE

Time dependent asymmetry a(t) (LM*)


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HADRONIC SAMPLE LEPTONIC SAMPLE

Fit results (LM*)


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Identical analysis procedure on MC data with detailed detector simulation:

fit results consistent with a priori insterted value and MC truth information

observed differences applied as a correction to the measured values

Systematic errors (LM*)


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HADRONIC SAMPLE

Statistical error dominant, followed by MC correction uncertainties (t for md)

Systematic error dominant due to big uncertanties in background characteristic (t for md)

LEPTONIC SAMPLE

Time integrate (single bin) method

First aim: measurement of the mistag rate

Main feature: restriction of the sample to events in a single optimized Δt interval (| Δt | < 2.5 ps because of Babar vertex resolution)

Events with | Δt | > 2.5 ps have on average equal numbers of mixed and unmixed events => contribute nothing to the determination of the mistag rate

Considering the different background contribution:

ff ddsobs ))21((

fs, fβ = fraction of signal and background source

χβ = fraction of mixed events in each background source

χobs = Observed fraction of mixed events

χ d = ½ xd2/(1+xd2) and xd2 = Δmd/Γ, while χ’d takes into account the sample restriction


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=>

Results

HadronicSignal region defined as events with mES > 5.27 GeV/c2

Fraction of mixed events in the background determined by tag category using the sideband control sample, mES < 5.27 GeV/c2

Semileptonic - bg evaluated for each tag category and for each D0 decay - mistag fractions calculated individually by tag category and decay

mode using the Eq. shown - combination of the different decay modes, using the statistical errors to weight the individual results


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Systematic errors

• Hadronic

• Semileptonic


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Sources of systematic error for the mistag measurement on the hadronic and semileptonic samples

Comparison between the two methods


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Combining results for the hadronic and semileptonic B samples for the likelihood fit method and for the single-bin method and taking

into account the systematic errors

Preliminary mistag rate

Single bin fit uses a subset of the sample used for the other method

The two sets of results are uncorrelated

Good agreement between the two methods. Final result: Q ≈ 0.28

Final result


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Hadronic sample:

Leptonic sample:

Back-up slides


Fox-Wolfram momentsThe Fox-Wolfram moments , , are defined by

is the opening angle between hadrons and

the total visible energy of the event

are the Legendre polynomials

To the extent that particle masses may be neglected, . It is customary to normalize the results to , i.e. to give .

2-jet events tend to give for even and for odd.


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s. bolognesi & m.a. borgia for the cp-violation exam

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