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 taggingť
•
•
• time resolution function with parameters
B0B0 mixing through NLO EW diagrams involving exchange of up-type quarks
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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
S. Bolognesi & M.A. Borgia CP-violation exam
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*)
S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)
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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
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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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* LM = Likelihood method
HADRONIC SAMPLE LEPTONIC SAMPLE
Time dependent asymmetry a(t) (LM*)
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* LM = Likelihood method
HADRONIC SAMPLE LEPTONIC SAMPLE
Fit results (LM*)
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* LM = Likelihood method
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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* LM = Likelihood method
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
S.Bolognesi & M.A. Borgia CPV exam (27 July 2007)
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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
S. Bolognesi & M.A. Borgia CP-violation exam
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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